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Hydrocarbon-potential-of-the-Zechstein-Main-Dolomite-in-the-western-part-of-the-Wielkopolska-platfor

Hydrocarbon potential of the Zechstein Main Dolomite in the western part of the Wielkopolska platform,SW Poland:New sedimentological and geochemical data

Pawe1Kosakowski*,Marcin Krajewski

AGH University of Science and Technology,Faculty of Geology,Geophysics and Environment Protection,al.Mickiewicza30,30-059Kraków,Poland

a r t i c l e i n f o

Article history:

Received21February2013 Received in revised form

5September2013

Accepted3October2013 Available online16October2013

Keywords:

Upper Permian

Zechstein main dolomite Paleogeography

Facies model

Geochemistry

Source rocks data a b s t r a c t

The results of identi?cation of facies and microfacies of highstand systems tract(HST)deposits in the Zechstein Main Dolomite(Ca2)are presented followed by a critical analysis of the existing views on the paleogeography and facies architecture of the Wielkopolska Platform(eastern part of the Fore-Sudetic Monocline,SW Poland).The authors discuss and question,the existence in zones that have been treated as basinal?at in the analysed area,and an extensive platform margin ooidal barrier.In general, the facies architecture of the study area resembles the block built of the Paleozoic basement.

On this background analytical geochemical results are presented and their source nature characterized in the context of a new revised model of the Ca2basin paleogeography.A new facies classi?cation of carbonates,which has not been applied for the Polish part of the Ca2basin yet,was presented.The geochemical characteristic of Ca2rocks indicates that good oil-prone source rocks are present only locally and are connected with calcisiltites and dolomudstones of facies II and dolobindstones of facies IV. These facies are mainly present in the slope and toe-of-slope zones.The microbial-algal facies,abundant in the carbonate platform,have a weaker residual potential.However,the thermal maturity stays within the entire range of the“oil window”,the rocks should be re-labeled as good source rocks.The presence of oil-prone kerogen Type II,and locally even Type I,makes the Ca2beds in this part of the Wielkopolska Platform a very ef?cient source rock,the character of which is con?rmed by numerous hydrocarbon accumulations,related to the course of tectonic zones and morphology of the basement.The proposed characteristic of source rocks in the new paleogeographic-facies system,enables their better petroleum evaluation and shall enable future prospecting of the perspective zones.

1.Introduction

The Southern Permian Basin(SPB)is a huge basin that extends in the entire area between Lithuania and Western Belarus in the east and the onshore part of the United Kingdom in the west (Fig.1A;van Wees et al.,2000;Geluk,2007).The study area com-prises the Polish southwestern part of the SPB(Fig.1A),and is located within the Fore-Sudetic Monocline that belongs to the least researched regions as regards facies and microfacies of the Zech-stein Main Dolomite(Ca2)strata and their petroleum characteris-tics.The Zechstein as a whole is attributed to the Late Permian, although the lack of index fossils makes precise dating dif?cult (S1owakiewicz et al.,2009;Peryt et al.,2012).It is certain that the PZ1cyclothem rocks reveal early(but not the earliest)Lopingian age(Fig.2;Korte et al.,2005;S?rensen et al.,2007;S1owakiewicz et al.,2009;Peryt et al.,2010a).

At the same time,in this region a number of hydrocarbon shows and accumulations have been found,a considerable part of which are of great economic importance(Fig.1B).The previous views on the development and sedimentary evolution of the Polish part of the Ca2basin were presented with relation to the Gorzów Platform (Jaworowski and Miko1ajewski,2007;S1owakiewicz and Miko1ajewski,2009,2011)and Grotów Peninsula(Kotarba and Wagner,2007;S1owakiewicz and Miko1ajewski,2009)(Fig.1B). As a result of the occurrence of the greatest Polish oil and gas?elds (Czeka n ski et al.,2010),these areas were well researched consid-ering facies and they became model areas for other Ca2occurrence in Poland.In case of the Fore-Sudetic Monocline,microfacies studies were sporadically published based on few wells in the second half of the20th century(Peryt,1978).Since that time,in-formation on geochemistry,microfacies and sedimentary facies, derived from wells drilled in the1980s,1990s as from more recent

*Corresponding author.Fax:t486172431.

E-mail address:kosak@https://www.sodocs.net/doc/eb7055240.html,.pl(P.

Kosakowski).Contents lists available at ScienceDirect

Marine and Petroleum Geology journal h omepage:ww w.elsevi https://www.sodocs.net/doc/eb7055240.html,/locate/marp

etgeo

https://www.sodocs.net/doc/eb7055240.html,/10.1016/j.marpetgeo.2013.10.002

Marine and Petroleum Geology49(2014)99e120

ones,has not been published and veri ?ed,and most of drill cores

were cast off.When analysing unpublished materials (well docu-mentation and microscope thin-sections),it can be clearly seen that the facies subdivision and paleogeographic maps were done based on very simpli ?ed criteria,i.e.subdivision of rocks into mud-supported facies that represent low-energy environments of ba-sin plain or inner platform ?at and grain-supported facies repre-sents high-energy environments of outer oolithic barrier and a number of inner oolithic barriers of the platform.However,in some cases we observed that even this simpli ?ed criterion was not used either and the way of attributing the areas to particular paleogeo-graphic zones is not clear.

The previous views on the development and paleogeography of the Ca2strata in the Fore-Sudetic Monocline were presented on a paleogeographic map (Fig.1B;Kotarba and Wagner,2007;S 1owakiewicz and Miko 1ajewski,2011).As early as during pre-liminary analysis of this map a number of doubts arouse as regards the position and extent of particular paleogeographic zones,which resulted in update.However this updated map is unavailable.Therefore throughout this paper we refer to the most recent pub-lished map (Kotarba and Wagner,2007;S 1owakiewicz and Miko 1ajewski,2011).Macroscopic examination of cores and microscopic observations con ?rmed a number of discrepancies between the observations and the existing view on the facies and paleogeography differentiation of the Ca2basin in the Fore-Sudetic Monocline.The above presented doubts have been con ?rmed by sedimentological studies of the Zechstein Main Dolomite rocks.Also proposed changes in the paleogeographic setting have been presented.

Geochemical studies and modelling of hydrocarbon generation and expulsion processes considerably improved our knowledge of the value of the Zechstein Main Dolomite in terms of petroleum prospects,and in the corrected paleogeographic setting they enabled new characterization of the distinguished zones:basin plain,carbonate platform slope and carbonate platform with the oolitic barrier.Similarly,facies identi ?cation based on the new methodology allowed to verify views on the source potential of individual facies types of the Zechstein Main Dolomite.Particularly important changes in this characterization concern the Kargowa e Babimost Bay,previously located in the zone of the basin plain and now in the zone of the carbonate platform slope.In the latter,a few hydrocarbon accumulations occur,namely the Kargowa,Babimost and Wilcze deposits.In this zone,the Zechstein Main Dolomite rocks reveal signi ?cant source potential and are characterized

Figure 1.Paleogeographical map of the (A)Southern Permian Basin (Geluk,2007),(B)Zechstein Main Dolomite of the western part of the Fore-Sudetic Monocline (after S 1owakiewicz and Miko 1ajewski,2011),with location of analysed wells and geological cross sections,and (C )new interpretation of paleogeographical location of analysed wells.

Wells:1a e Bedów 1,1b e Bedów 2,2e Babimost 1,3e Babimost 3,4e Babimost 4,5e Babimost 5,6e Babimost 6,7e Chro s nica 1,8e Gry _zyna 1,9e Gry _zyna 2,10e Gry _zyna 3,

11e Gry _zyna 4,12e Jab 1onna 2,13e Jab 1onna 3,14e Jastrzebsko 1,15e Jastrzebsko 3,16e Kargowa 1,17e Kargowa 2,18e Kargowa 3,19e Kargowa 4,20e Kargowa 5,21e

Kargowa 6,22e Kargowa 7,23e Kije 4,24e Kije 9,25e Kosobudz 1,26e q agów 1,27e Mozów 1,28e Myszecin 1,29e O 1obok 1/1K,30e Papro c 21,31e Papro c 29,32e Radnica 1,33e Radoszyn 1,34e Radoszyn 3,35e Radoszyn 4K,36e Skape 1,37e Stare Karamsko 1,38e Staropole 1,39e Staropole 8,40e Struga 1,41e Templewo 1,42e

Wilcze 2,43e Wilcze 4,44e Wilcze 5,45e Wilcze 7,46e Zawisze 1,47e ,Zawisze 1K,48e Zbaszy n

3,49e Zbaszy n 5.P.Kosakowski,M.Krajewski /Marine and Petroleum Geology 49(2014)99e 120

100

signi ?cant undiscovered petroleum potential.Also other zones of the analysed area of the Fore-Sudetic Monocline demonstrate essential changes in the characteristics of the Zechstein Main Dolomite source potential and possibilities of hydrocarbon generation.

2.Geological background

During the Late Permian the SPB extended over an area of approximately 600000km 2(van Wees et al.,2000;García-Veigas et al.,2011).The Polish part of the Zechstein Basin,with thickness of the strata exceeding 1500m (Wagner,1994;Peryt et al.,2010b ),was situated in the eastern part of the SPB (Fig.1A).Tectonic and paleogeographic factors caused a speci ?c distribution of facies and thickness of the Polish Zechstein (Wagner et al.,1978;Wagner,1994).Successive transgressions of the Zechstein Sea led to depo-sition of evaporate cyclothems PZ1to PZ4,which have been correlated with the German cycles:Werra e Z1,Stassfurt e Z2,Leine e Z3,and Aller e Z4(Fig.2;Wagner,1994;Wagner and Peryt,1997).The rocks of the Zechstein Main Dolomite (Ca2),discussed in this paper occur at the base of the PZ2cyclothem.The overlying sequence is similar to that of the remaining cyclothems,carbonates pass into anhydrites and salts e the Basal Anhydrite (A2),the Older Halite (Na2),and locally the Older Potash (K2)(Fig.2).The salinity of the sea-water was higher than in the PZ1cycle (Wagner,1994;Kiersnowski et al.,1995).It is evidenced,among others,by scarce bioturbation and limited fauna.

The geometry and paleogeography of the Zechstein Basin show relationship with the morphology and tectonics of the post-Carboniferous basement and early-Permian volcanism (van Wees et al.,2000;Kotarba and Wagner,2007;Kwolek and Miko 1ajewski,2007;Geissler et al.,2008).It is particularly visible in the ?rst cyclothem PZ1.The study area is located in the western part of the Polish Zechstein Basin and covers the western part of the

Wielkopolska Platform and eastern part of the Rzepin Bay (Fig.1B).Two zones with different origin can be distinguished here:the northern zone and the southern zone.In the northern zone,on the sulphate platform of the PZ1cyclothem,a marginal oolitic-oncolitic barrier with a steep slope developed,whereas in the southern zone we can observe vast low-energy platform ?at that grades eastwards into the gentle slope with the Zechstein Main Dolomite thickness decreasing in the same direction (Peryt,1978;Wagner,1994,1997).In the southwestern part of the analysed area,one of characteristic elements of the paleogeographic setting of the Zechstein Main Dolomite basin occurs,namely the Kargowa-Babimost Bay (Fig.1B).This bay is characterized by very large thickness of the carbonates reaching 190m.The carbonates are developed mainly in the mud dominated facies,more rarely in the wackestone facies with sub-ordinate admixture of packstones.Most likely this zone was formed in an area of a tectonic graben (Kiersnowski et al.,2010)and reveals in ?uence of the Variscan tectonics substratum on the paleogeo-graphic setting of the Zechstein Main Dolomite basin and the dis-tribution of the hydrocarbon accumulations.3.Samples and methods

The study applied classical methods of sedimentological anal-ysis to unpublished data from several tens of wells located in the northern part of the Fore-Sudetic Monocline.372core samples from 47wells were collected from which thin-sections and pol-ished slabs were prepared.Moreover,archival thin-sections belonging to the Polish Oil and Gas Company (POGC)and data from geological documentation of wells were utilized.In total,917thin-sections (from the AGH-UST and POGC collections)were used for microfacies studies.On the basis of the assembled analyses,fundamental components and textures were determined following the standard criteria and classi ?cation published by Dunham (1962),and modi ?ed by Embry and Klovan (1972).The facies types together with interpretation of the paleogeography are pre-sented in Table 1.On the basis of the obtained data,new sedi-mentological sections of the Ca2strata from the study area were constructed.The gathered materials allowed to compare them to earlier materials considering both the general paleogeographic and facies architecture,the data available in the existing geological documentation of wells.Also 2D seismic data were utilized,on their 2D modelling was conducted which allowed to reconstruct the morphology of the carbonate platform and pre-Zechstein basement (Figs.3,4).

To assess the character of the Zechstein Main Dolomite as source rock,the Rock-Eval pyrolysis,biomarkers,elemental analysis methods,and organic sulphur content in kerogen were used.Additionally,to determine types of kerogen and its thermal maturity,the maceral composition and vitrinite re ?ectance mea-surements were commonly used.

The measurements for the Rock-Eval Model II pyrolysis were conducted following the work ?ow described by Espitaliéet al.(1985).The Rock-Eval analysis provides information about the amount,quality,type,and maturity of organic carbon in a sample.The results of the Rock-Eval analyses for 466core samples collected in 23wells are summarized in Tables 2e 4.Vertical distribution of basic geochemical parameters is presented in Figure 6.

The saturated hydrocarbons,extracted with dichlor-omethane:methanol (93:7),were analysed by the GC e MS for biomarker determination.The analyses were carried out with the Agilent 7890A gas chromatograph equipped with the Agilent 7683B automatic sampler,an on-column injection chamber and a fused silica capillary column (60m ?0.25mm i.d.)coated with 95%methyl/5%phenylsilicone phase (DB-5MS,0.25m m ?lm thickness).Helium was used as the carrier gas.The GC oven was programmed:

L O P I N G I A N

C H A N G H S I N G I A N

W U C H I A P I N G I A N

P Z 1(W E R R A )

T1 Kupfershiefer

Ca1 Zechstein Limestone A1d Lower Anhydrite Na1 Oldest Halite A1g Upper Anhydrite Ca2 Main Dolomite A2 Basal Anhydrite Na2 Older Halite K2 Older Potash Na2r Screening Older Halite A2r Screening Anhydrite T3 Grey Pelite

T4 Aller Pelite Na4 Aller Halite Ca3 Platy Dolomite A3 Main Anhydrite Na3 Younger Halite P Z 3(L E I N E )

P Z 4(A L L E R )

P Z 2(S T A S S F U R T )260

251

254

4 cm

Figure 2.Litostratigraphic pro ?le of the Upper Permian and core photographs of platform slope deposits from Wilcze 7well earlier interpreted as marginal platform barrier:A e crinkled lime-mudstones and bindstones with sediment deformations,small synsedimentary normal fault and slump-folding.B e Erosional contact between A1g and Ca2at the platform slope zone.

P.Kosakowski,M.Krajewski /Marine and Petroleum Geology 49(2014)99e 120101

80 C held for1min,then increased to120 C at the rate of20 C/ min,then increased further to300 C at the rate of3 C/min,and ?nally held for35min.The gas chromatograph was coupled with the5975C mass selective detector(MSD).The MS was operated with an ion source temperature of230 C,ionisation energy of 70eV,and a cycle time of1s in the mass range from45to500Da. The aromatic hydrocarbon fraction was analysed by the GC e MS for phenantrene,dibenzotiophene and their derivatives determina-tion.The analysis was carried out using the same equipment as for the saturated hydrocarbon fraction.The GC oven was programmed from40to300 C at the rate of3 C/min.The MS was operated with a cycle time of1s in the mass range from40to600Da.

The elemental analysis of the isolated kerogen(C,H,N and S) was made with the Carlo Erba EA1108elemental analyser.The quantity of pyrite contaminating the kerogen was analysed as iron on the Perkin e Elmer Plasma40ICP-AES instrument after digesting the ash from the burned kerogen(815 C,30min)with hydro-chloric acid.The organic sulphur content in kerogen was calculated as the difference of the total and pyritic sulphur.The oxygen con-tent was calculated as the difference to100%,taking into account the C,H,N,S,moisture and ash contents.

The measurements of re?ectance of vitrinite-like macerals(R o) were carried out with the use of the Zeiss-Opton microphotometer at546nm wavelength,in oil.The standards used were0.496%, 0.921%,1.141%and 1.662%re?ectance.Sample preparation and point counts were carried out in accordance with the ICCP proce-dure(Taylor et al.,1998).

4.Paleogeography and facies architecture versus new facies and microfacies data

For the Zechstein Main Dolomite basin in Poland,the following paleogeographic and facies zones are distinguished by the previous authors:basin(deep and shallow basin including bays),ramp, platform slope,toe-of-slope apron,and carbonate platform(outer and inner barriers,restricted lagoons,tidal?at,channel,and supratidal sabkha;e.g.Peryt,1986;Wagner,1994;Jaworowski and Miko1ajewski,2007;Czeka n ski et al.,2010;S1owakiewicz and Miko1ajewski,2011).Sedimentation of the Zechstein Main Dolo-mite took place mainly in the HST and the lithologic variability re?ects mainly the platform morphology and sea-level oscillations (Strohmenger et al.,1996;Kaiser et al.,2003;S1owakiewicz and Miko1ajewski,2009).In general,the facies types observed on the analysed cores and the basic components(peloids,coated grains, organisms)are similar to those observed in adjacent areas of the Zechstein Basin(e.g.Clark and Tallbacka,1980;Paul,1980;Peryt, 1978,1986;Holltingworth and Tucker,1987;Mausfeld and Zankl, 1987;Steinhoff and Strohmenger,1996;Strohmenger et al.,1996; Wagner,1994;Kaiser et al.,2003;Flügel,2004;Jaworowski and Miko1ajewski,2007;S1owakiewicz and Miko1ajewski,2011),

Table1

Characteristic of the facies of the western part of Wielkopolska Platform;for older interpretation see Figure1C.

HST microfacies association Facies interpretation Brief description

Type I Lime dolomudstones-wackestone Basin,toe-of slope facies with pelagic

microfosils Dark rhythmically laminated calcareous laminites,planar or irregular mm-bedding,rare bioclasts

Type II Microbio clastic-peloid calcisilte,dolopackstone

and dolomudstones Allochtonous and autochtonous lower-

middle slope and toe-of slope facies

Fine-grained turbidites with shallow-water

grains,massive,graded-bedding,slumping

Type III Microbreccia,lithoclastic dolopackstone,

grainstone,dolo?oatstone,dolomudstone Upper-middle steep slope facies slump-

breccias,debrites,unites of?ne

grainstone,packstone and lime-

mudstones

Flaser,wavy-bedding,rare cross bedding,

brecciation,boudinage,slumping,faulting,

stromatactis

Type IV Aminated peloidal dolopackstone,

dolobindstone,dolomudstone Low-angle platform slope(ramp)with

microbial-peloidal mud-mound facies

Massive,rare deformation,wavy-bedding mm/

cm bedding

Type V Ooid dolograinstone-packstone High energy shallow subtidal platform

margin bar/back-bar and inner platform

oolitic shoal,upper platform slope Well sorted oolite sands(diameter0,3

e1.3mm),rare biolamination,cross-

strati?cation,abundant high oomoldic porosity

Type VI Micritic ooid dolograinstone-packstone Moderate energy shallow subtidal inner

platform oolitic shoal facies Massive,biolamination,wavy cm/mm bedding, oolite diameter0,2-0,4mm,low-porosity

Type VII Microbial dolobindstone-microframestone,

dolopackstone Shallow subtidal margin and inner

platform microbial reef facies

Massive carbonates with botroids,stromatactis,

stromatolites(SH-LLH type),thrombolites,

algae,bryozoa,tubiphytes,crinkly bedding

Type VIII Oncoid dolopackstone-?oatstone Shallow subtidal open marine inner

platform lagoon facies Massive,oncoids with irregular coatings,oncoid range0,4-2mm,microbial lamination,low-porosity

Type IX Aggregate-grain,lump,algal dolopackstone-

grainstone,dolobindstone Shallow subtidal open/restricted inner

platformwide facies,microbial-algal

mounds

Massive,crinkly bedding,rare cross-bedding,

microbial lamination,irregular composite

grains,algae,bryozoans,low-porosity

Type X Bioclastic-peloidal dolowackestone-mudstone Shallow subtidal protected inner

platform(restricted lagoon)facies with

moderate-low water circulation Massive,cm bedding,small-scale cross-strati?cation,bioclasts,low porosity

Type XI Fenestral microbial dolobindstone,

dolomudstone,dolopackstone Inner platform tidal?at facies Crinkly bedding,?aser-planar lamination,

desiccation cracks,fenestral fabric,

microteepees,rare fauna,stromatolites and

mud,pellets

Type XII Pisoid dolorudstone-grainstone Meteorically affected(supratidal)shoal

and bar facies Massive,coarse grained,graded bedding,cross-bedding,abundant high porosity,pisoids, gravitational cements

Type XIII Lithoclastic dolorudstone-?oatstone,

dolopackstone Inner platform lag facies in tidal

channels and?ats

Massive,coarse grained,graded bedding,cross-

bedding,abundant high porosity,pisoids,

gravitational cements

Type XIV Poorly laminated micrite,microsparite with

evaporitic minerals Salina or evaporitve tidal?ats facies Massive,cm/mm bedding,mud cracks,

bioturbation,anhydrite nodules

Type XV Laminite evaporite-carbonate dolomudstone

and dolobindstone Intertidal-supratidal sabkha facies Crinkly-bedding,micoteepees,stromatolites,

nodular chicken-wire anhydrites

P.Kosakowski,M.Krajewski/Marine and Petroleum Geology49(2014)99e120 102

therefore in this paper they are not characterized in detail.Char-acteristic for Polish Zechstein Main Dolomite microfacies types with facies interpretation are described in Table 1.A fundamental problem presented in this chapter is veri ?cation of previous views on paleogeographic and facies position of particular areas (Table 1,Fig.5).The present views on the paleogeographic and facies ar-chitecture of the northern part of the Fore-Sudetic Monocline is outlined (Fig.1B;Kotarba and Wagner,2007;S 1owakiewicz and Miko 1ajewski,2011).The views were then compared with results of the present study and presented in the next chapter.4.1.Basin

In previous papers concerning the discussed part of the Fore-Sudetic Monocline,the basinal zone is located in three areas:(i)q agów-Kosobudz-Staropole (abbreviated to LKS;Rzepin Bay on Fig.1B),(ii)Be ?dów-Sycowice (BS;Be ?dów Bay on Fig.1B)and (iii)Kargowa-Babimost-Mysze ?cin (KBM;Kargowa-Babimost Bay,on Fig.1B).

Due to scarce archival materials (unpublished reports),paleo-geographical analysis of the LKS area should be based partly on indirect premises such as signi ?cant thickness variations in the Zechstein Main Dolomite deposits from this area and 2D seismic interpretations (Fig.3).Thickness known from the q agów area amounts to 30m and is much greater than in zones regarded as the basin (e.g.Wagner,1994;Strohmenger et al.,1996;Jaworowski and Miko 1ajewski,2007).Analysis of thin-sections from the q agów 1well demonstrated that the rocks are mainly represented by strongly recrystallized microbioclastic calcisiltites (Type II;Table 1),described in numerous wells located on the platform slope and toe-of-slope apron (Table 1,Figs.1B,C,5A,B).The Zechstein Main Dolomite thicknesses in the Staropole area,which has been hith-erto regarded in part as a carbonate ramp separating two basinal zones (Fig.1B),are as small as a few metres (6m in the Staropole 1well and 8m in the Staropole 8well).In the carbonate ramp zone much greater thicknesses could be expected,which would corre-spond with thicknesses on the adjacent carbonate platforms,reaching usually several tens of metres (e.g.Peryt,1978,1986;Jaworowski and Miko 1ajewski,2007).The signi ?cantly smaller thicknesses in the Staropole area compared to adjacent areas may be related to such paleogeographic zones as:(i)steep slope,(ii)basin plain or (iii)inner parts of the platform highs with peritidal ?at environment.The last possibility seems to be most likely.The 2D seismic data in the Staropole area document occurrence of a considerable paleohigh in the Carboniferous basement (Fig.3).In the literature it was already emphasized that the block-faulted structure of the basement determined the morphology of the Up-per Permian platforms in the Polish Zechstein Basin (e.g.Wagner,1994;Kiersnowski et al.,2010).Much greater thicknesses and the microfacies Type II of the Zechstein Main Dolomite rocks in areas bounding the Staropole high from the west and east in the q agów and Mysze ?cin areas (30m in the q agów 1well and 122m in the Mysze ?cin 1well;Fig.1B)indicate that they represent platform slope deposits.It may be supposed that on the Staropole high a

1000-200-400-600-800NE

A ’

Figure 3.G eological cross-section Struga e Kosobudz e Staropole (location see Fig.1)based on seismic pro ?le.

P.Kosakowski,M.Krajewski /Marine and Petroleum Geology 49(2014)99e 120103

shallow-water microplatform originated.Absence of the accom-modation space in such areas as Staropole and highstand shedding (Schlager et al.,1994)by redeposition of sediments down the slope to adjacent basin were direct reasons for much smaller thicknesses than in adjacent areas.

In case of the BS area (Fig.1B)it is impossible to verify the archival data because its wells were thrown away.Only from the Be ?dów part (the Be ?dów 1and 2wells)thin-sections have been preserved.Sediments are represented by laminated fenestral dolobindstones,dolomudstones and poorly laminated dolomicrite and microsparite (Type XIV;Table 1).Also the thickness of the Zechstein Main Dolomite is much lower than the thickness of the corresponding facies,noted in the adjacent zones and does not exceed several metres (14m in the Be ?dów 1well and 11m in the Be ?dów 2well).It should be expected that the existence of such narrow basinal zone in the immediate vicinity of extensive shallow-water platforms as it was previously presented,is not

realistic (Fig.1B,C).On account of material supply from the nearby platform,the Zechstein Main Dolomite thickness in these wells should be much greater than in the adjacent areas,as it is seen in the KBM area (Fig.6).The deposits recognized in the nearby wells (Radnica 1and Struga 1;Kosakowski et al.,2009)located to the west of the study area represent facies typical of slope sedimen-tation with calciturbidites,microbreccias and grain-?ow deposits (Table 1,Figs.1C,5A,C).On the basis of the analogy to the Staropole area it seems likely that both the Staropole and Be ?dów areas represent similar paleogeographical position and facies types.The 2D seismic section in the vicinity (in the Zawisze area)indicates occurrence of a platform high on which,in the SE extension,the Be ?dów area is located (Figs.1,4A).In central parts of these highs,peritidal mud-dominated sediments (Type XIV)were deposited,which probably grade laterally to grain-dominated shoals and bars (Type IX),as it can be observed in the Ska ?pe and Zawisze areas (Table 1,Fig.4A).

In the Kargowa-Babimost-Mysze ?cin (KBM)area (Fig.1B),the thickness of the Zechstein Main Dolomite acquires maximum values in the Polish part of the Zechstein Basin and reaches nearly 200m (Fig.6).As regards the paleogeography and facies,they were previously characterized in the same manner as other basin plain zones,including the LKS and BS areas where the Zechstein Main Dolomite thickness does not exceed several metres.The sedimen-tary sequences observed in the KBM area represent both proximal and more distal settings of the platform slope facies (Table 1,Figs.1C,6).Two basic types of sediments are distinguished there:(i)autochthonous and (ii)allochthonous (Type II;cf.Flügel,2004;Mawson and Tucker,2009).The allochthonous sediments are rep-resented mainly by microbioclastic-peloidal calcisiltite (Table 1).Millimetre-scale lamination with normal grading of micrograins is frequently observed (Fig.5A).Moreover,at a smaller scale,cross lamination is https://www.sodocs.net/doc/eb7055240.html,mination is often contorted and wavy.Sediments of this type represent both the upper and lower slope apron (cf.Strohmenger et al.,1996;Mawson and Tucker,2009)and completely ?ll the narrow zone between the two zones interpreted as the shallow-water platform.In a number of wells (Kargowa 1,5,Babimost 1,4,5,and Mysze ?cin 1)within some intervals these sediments underwent aggrading neomorphism and now most of the original textures are obliterated.With regard to the platform zones surrounding the KBM area it may be expected that the ma-terial supply related to the highstand shedding (Fig.4B;Schlager et al.,1994)was coming from different directions.Sedimentary sequences that represent the upper slope can be observed in the Babimost 3,6and Kargowa 5,6and 7wells where besides calci-turbidites also redeposited ooid sediments (Types V and VI)are common,which may correspond with types II and III more rarely type IV,observed in the German part of the Zechstein Main Dolo-mite platform by Steinhoff and Strohmenger (1996),as well as breccias (Table 1,Fig.5C,F,H).Taking the above observations into account it may be suggested that in case of the KMB in the study area the basin plain zone,as characterized in the previous studies does not occur (Fig.1C).The KBM area as a whole is a zone ?lled with platform slope facies (Fig.4B).The sediments of this area are developed identically,as those described from platform slope facies in the adjacent areas (cf.Strohmenger and Strauss,1996;

Jaworowski and Miko 1ajewski,2007;Czeka n ski et al.,2010).4.2.Slope and toe-of-slope apron

Sediments representing the platform slope in the study area

have been hitherto recognized in the Radoszyn,O 1obok and Gry-_zyna areas (Fig.1B).In case of the Radoszyn area,the platform slope was interpreted from the basin plain side as well as from the inner platform side (Fig.1B).In the central and eastern parts of the study

Zawisze 1

km 500-200-400-600-800N

500-200-400-600-800W

B’

C’

Figure 4.Geological cross-section (A )Zawisze e Skape and (B )Babimost e Zbaszy n

(location see Fig.1)based on seismic pro ?le.

P.Kosakowski,M.Krajewski /Marine and Petroleum Geology 49(2014)99e 120

104

Figure 5.The most important microfacies of the Main Dolomite HST deposits from Fore-sudetic Monocline.In the brackets older paleogeographic interpretations (Fig.1).A,e thinly laminated lime-dolomudstones and normally graded calciturbidites represents calcisiltite (very ?ne grainstone)e microfacies Type II,well Wilcze 4(2116m),B e laminated

dolobindstone and calcisiltite (Type II and IV)with sediment deformations,small synsedimentary normal fault (broken line and arrows)and slump-folding,well Gry _zyna 4

(2505m),C e clast-supported microbreccia,lithoclastic dolopackstone-?oatstone (Type III),well Babimost 3(2205e 2208m),D e breccia;visible redeposited organic reef

boundstone (Type III),with fragments of macrofauna (?calcisponge e Sp),tubiphytes and bryozoans (arrow),well Gry _zyna 1(2416m),E àmicrobial-peloidal bindstone (Type VII),

with small,originally aragonitic botryoidal fans (arrows),well Gry _zyna 2(2434m),F e ooid dolograinstone (Type VI),well Kargowa 5(2156e 2171m),G e concentric ooid

grainstone with oomoldic porosity (Type V),well Wilcze 5(2092m),H e concentric ooid dolograinstone with oomoldic porosity (Type V),Kargowa 6(2101e 2104m),I e breccia (Type V/VI),with redeposited lithoclasts,aggregate grains and ooids,well Stare Kramsko 1(2142e 2160m),J e dolorudstone-grainstone (Type XII),with small and large pisoids,well Radoszyn 3(2380m),K e Bryozoa (?Acanthocladia)dolopackstone-bindstone (Type IX),with fragments of zoaria nad foraminiferal encrustations around bryozoans,well Jab 1onna 3(Ca1,2398e 2417m),L e dolopackstone-bindstone with poorly preserved bryozoans (Type IX),well Templewo 1(2710e 2727m),M e microbial-algal dolopackstone (Type IX),well Templewo 1(2693e 2710m),N e algal-lump dolopackstone (Type IX),well Brzozowa 1(2830-2840m),O e microbial-lump dolobindstone (Type IX),well Zawisze 1(2233e 2248m).

P.Kosakowski,M.Krajewski /Marine and Petroleum Geology 49(2014)99e 120105

area,in the Stare Kramsko,Wilcze and Zba ?szy n

areas,the platform slope facies have not been interpreted so far and the marginal ooidal platform facies pass directly into the basin plain facies (Fig.1B).

The obtained results allowed us to con ?rm the paleogeographic

position of sediments in the Gry _zyna and O 1obok areas,which

represent the platform slope facies (Fig.1B).Nevertheless,positions of individual wells within the slope can be particularized (Fig.1C).

The Zechstein Main Dolomite rocks in the Gry _zyna area are char-acterized by signi ?cant thickness differences,from a few metres in case of steep upper slope (2.5m in the Gry _zyna 3well and 7.5m in

the Gry _zyna 4well)up to several tens of metres in case of the lower

slope (37m in the Gry _zyna 1well and 40m in the Gry _zyna 2well).

In wells that represent the steep upper slope,dolograinstones,breccias (Type III)and laminated ?ne-grained calcisiltites (Type II)dominate over laminated peloidal dolobindstones ((Type IV;Table 1).Furthermore,numerous synsedimentary deformation

textures small faults (Fig.5B),neptunian dykes and stromatactis-like cavities occur.The stromatactis-like cavities show smooth ba-ses and digitate roofs and presence of ?oating allochems often occurring in various depositional systems on slopes of reefs or platforms (Bathurst,1982;Wallace,1987;Matyszkiewicz,1993;Olchowy,2011).The development of these cavities is still contro-versial and is linked with various biological and/or physical pro-cesses,including the cavitational erosion and synsedimentary tectonics (e.g.Flügel,2004;Olchowy,2011).The slope deposits representing the lower slope and toe-of-slope facies that were

found in the Gry _zyna 1and 2wells (Fig.1B,C)are formed mainly of

caltisilt stabilized by microbial crusts (Table 1).It is interesting that

in the Gry _zyna area no well representing the upper slope facies on

the margin of the rimmed platform encountered redeposited oolitic grainstones,which would be expected in the supposed immediate vicinity of the oolitic barriers (see Fig.1B,C).In case of the upper slope facies observed in other areas,components of this type are commonly noted (e.g.Steinhoff and Strohmenger,1996;Strohmenger et al.,1996;Jaworowski and Miko 1ajewski,2007;Kaiser et al.,2003).Within the slope facies,microbial-algal-lump dolobindstones are frequently observed (Type VII;Table 1).They represent reworked reef blocks from the steep platform margin.Besides the microbial crusts and algae,also poorly preserved macrofauna (calcisponge?)can be observed,as well as benthic foraminifers,bryozoans and microencrusters like tubiphytes (Fig.5D).These organisms represent an assemblage typical of Late Paleozoic reefs and frequently constitute a microframework with marine-phreatic cements (e.g.Weidlich,2002;Flügel,2004;Peryt et al.,2012).Botryoid cements (Fig.5E)observed within the dolo-bindstones are most often linked with rapid cementation of the steep slope of reefs (e.g.Flügel et al.,1984a;Peryt,1986;Tucker and Hollingworth,1986;Flügel,2004).Moreover,synsedimentary car-bonate cements were often formed on the shelf slope of the seaward platform margin.The development of the botryoid ce-ments together with the algal structures and microbial textures allow to classify these buildups as inorganic/organic reefs.There-fore it may be presumed that in the Gry _zyna area no signi ?cant

ooidal barrier,envisaged by the earlier authors,is present (see Fig.1B),but reef complexes on the platform margin.In case of the

Table 2

The results of Rock-Eval and bitumens analyses in the slope and too-of-slope.Index

III

VII

XIII

Total organic carbon (TOC)(wt.%)0.01to 3.360.21

(183)(13)0.00to 3.870.83

(34)(8)0.00to 0.770.19

(30)(7)0.04to 0.110.08(10)-(2)0.01to 0.040.02((10)-(1)0.01to 0.830.19(20)(3)0.01to 0.770.27

(7)(1)0.01to 0.110.03(5)(1)-S 1tS 2(mg HC/g rock)0.18to 3.12

0.86

(38)(5)0.08to 10.712.01

(8)(4)0.11to 0.980.65(8)(5)e e 0.51to 2.51

1.24

(5)(1)0.33to 2.261.25

(4)(1)e Hydrogen index (HI)(mg HC/g TOC)

25to 225

75(38)(5)67to 166116

(8)(4)56to 1083

(8)(5)e e 52to 170

109

(5)(1)90to 145119

(4)(1)e Oxygen index (OI)

(mg CO 2/g TOC)

6to 219103(38)(5)5to 18074

(8)(4)8to 204113

(8)(2)e e 93to 604

280

(5)(1)69to 223159

(4)(1)e T max (o C)

434to 510454

(31)(5)435to 462449

(6)(3)420to 456436

(8)(5)e e 433(1)433to 442e

(2)(1)e e Production index (PI)0.30to 0.89

0.50

(38)(5)0.22to 0.530.31

(8)(4)0.36to 0.560.45

(8)(5)e e 0.38to 0.69

0.58

(5)(1)0.33to 0.870.56(4)(1)e e Bitumens (ppm)210to 2800

1000

(19)(5)190to 23701068

(8)(4)440to 1070653(3)(3)

e e 100to 53802303(3)(1)

1880

(1)

e Aromatics HC (%)13to 46

(10)(5)29and 39e

(2)(2)36e

Saturated HC (%)10to 37

(10)(5)28and 32e

(2)(2)16e e e

Resins (%)3to 22

(10)(5)20and 21e

(2)(2)12e e e

e Asphaltenes (%)14to 49

(10)(5)

9and 22e

(2)(2)

e TOC e total organic carbon;Tmax e temperature o

f maximum of S 2peak;S 2e residual petroleum potential;S 1e oil and gas yield (m

g HC/g rock);PI e production index;HI e hydrogen index;OI e oxygen index.Range of geochemical parameters is given as numerator;median values in denominator,in parentheses:number of samples from wells (numerator)and number of sampled wells (denominator).

Table 3

The results of Rock-Eval and bitumens analyses in the marginal platform barrier.Index

Facies V

XII/V

Total organic carbon (TOC)(wt.%)

0.09to 1.920.83

(3)(1)0.37to 1.130.75

(5)(1)S 1tS 2(mg HC/g rock) 4.50and 9.87e

(2)(1) 4.07to 11.756.62

(5)(1)Hydrogen index (HI)(mg HC/g TOC)147and 164e

(2)(1)147to 207156

(5)(1)Oxygen index (OI)(mg CO 2/g TOC)

24and 94e (2)(1)24to 153156(5)(1)T max (

434

(1)e

Production index (PI)0.68and 0.84e

(2)(1)0.79to 0.860.83

(5)(1)Bitumens (ppm)12440(1)7540to 128009560(3)(1)

Aromatics HC (%)21(1)e Saturated HC (%)45(1)e Resins (%)

7(1)e Asphaltenes (%)

(1)

Abbreviations as in Table 2.

P.Kosakowski,M.Krajewski /Marine and Petroleum Geology 49(2014)99e 120

106

Polish part of the Zechstein Basin,particularly in the Zechstein Limestone(Ca1)horizons,occurrences of similar though more completely developed reefs was linked,among others,with tec-tonic horsts conditioned by the morphology of the Zechstein basement(Kiersnowski et al.,2010;Peryt et al.,2012).

The KBM area is the next example considered here(Fig.1B).If the sedimentary model of a rimmed platform is to be accepted,the absence,in the Mysze?cin,Zba?szy n,Kargowa,Wilcze and Stare Kramsko areas,of a distinct platform slope zone,which separates the basin plain from the platform seems to be doubtful.Another case is represented by the Radoszyn area where peritidal ooidal barriers with horizons of pisoids are dominant(Fig.5J).To the south,this area was separated by the distinct platform slope (Fig.1B)from the inner platform?at where peritidal mud-dominated facies occur.This interpretation is doubtful because in this area there were no wells drilled and the seismic limit of visi-bility in the Zechstein sequences with small thickness of the Zechstein Main Dolomite seems to be insuf?cient for such an interpretation(Kwolek and Miko1ajewski,2010).Taking into ac-count,however,the3D seismic data which were acquired in the Radoszyn area,it cannot be excluded that a distinct slope occurs there.But in such a case the outline of paleogeographic zones should also be changed and the situation from the Radoszyn area resembles rather a microplatform(cf.Peryt and Dyjaczy n ski,1991; Jaworowski and Miko1ajewski,2007)or an off-platform high (Strohmenger et al.,1996)which are frequently observed in adja-cent areas.

4.3.Marginal platform barrier

Analysis of the existing paleogeographic map(Fig.1B)allows to draw a conclusion that the basin plain was separated from the inner parts of the platform by a distinct barrier that is interpreted in the Polish part of the Zechstein Main Dolomite platform as the high-energy oolitic facies(e.g.Wagner,1994;Jaworowski and Miko1ajewski,2007;S1owakiewicz and Miko1ajewski,2011).In the major part of the study area,the barrier facies bound the narrow KBM zone was interpreted so far as the basin plain (Fig.1B).

On the basis of the detailed microfacies analysis,in most wells which were interpreted previously to represent the high-energy oolitic barrier facies characteristic of the rimmed platform margin,facies veri?cation and reinterpretation were carried out in the framework of this study.Among the examined wells included so far to the barrier,only in one well from the Radoszyn area (Radoszyn3well;Table1,Figs1B,C,5H)concentric ooids and aggregate grains could be observed,representing high-energy facies Type V(Table1).In the remaining areas,ooids and aggre-gate grains occur much less frequently and they most often origi-nated by redeposition from adjacent areas,as it can be observed, among others,in the Stare Kramsko area(Table1,Fig.5I).A fundamental difference between the obtained data and the previ-ous view on the facies distribution in the Radoszyn area stems from the fact that the width of the zone with dominating ooidal facies is signi?cantly smaller than it has been considered so far.In the Radoszyn1and4k wells,sediments represent principally microbial-algal-lump facies Type IX with skeletal grains,tubi-phytes,bryozoans,foraminifers,and gastropods(Table1).Most of the lumps occur together with algae and bryozoans,which suggests that a part of them represent a disintegrated and poorly preserved form of algal and bryozoan zoaria.Also in all other examined wells according to the paleogeographic map(Fig.1B),which were to intersect the high-energy ooidal barrier strata in the Zba?szy n,Stare Kramsko,Zawisze,Brzozowa and Templewo areas,we observed mainly the microbial-algal-lump dolopackstone-bindstone microfacies(Type IX;Table1,Fig.5K e O).In case of the Zechstein sediments,they may correspond with the algal-laminated shoal facies described in the territory of Germany(cf.Steinhoff and Strohmenger,1996;Kaiser et al.,2003).Like in other areas,beside microbiolites,lumps and algae(Fig.5N,O),also peloids and?ne ooids and pisoids can be observed.The latter conform to the LST and to subaerial exposure conditions(Peryt,1983;Flügel,2004).In well sections,poorly preserved fragments of calcareous algae (Fig.5H,N)and bryozoans(Fig.5K,L)are frequently observed.In some intervals they are common.Most often the benthic algal textures occur together with microbial textures in stromatolites or single microbial crusts(cf.S1owakiewicz and Miko1ajewski,2011). In consideration of diagenetic processes,detailed characterization

Table4

The results of Rock-Eval and bitumens analyses in the inner platform?at.

Index II IX X XI XIV

Total organic carbon (TOC)(wt.%)0.03to2.95

0.35

(30)

(3)

0.01to1.32

0.23

(86)

(7)

0.03to1.33

0.23

(19)

(3)

0.03to0.71

0.27

(6)

(4)

0.07to0.84

0.27

(8)

(1)

S1tS2(mg HC/g rock)0.58to5.16

2.32(13)

(3)

0.23to11.8

2.32

(30)

(6)

0.52to3.20

1.62

(5)

(2)

0.22to2.21

0.74

(4)

(3)

0.28to1.90

0.70

(5)

(1)

Hydrogen index(HI) (mg HC/g TOC)70to106

(13)

(3)

70to106

(30)

(6)

60to116

101

(5)

(2)

63to180

106

(4)

(3)

72to152

103

(5)

(1)

Oxygen index(OI) (mg CO2/g TOC)17to233

101

(13)

(3)

69to336

172

(30)

(6)

16to153

(5)

(2)

45to261

153

(4)

(3)

43to95

(5)

(1)

T max( C)437to448

445(7)

(2)

438to457

445

(11)

(4)

437to446

442

(4)

(2)

429to444

434

(3)

(2)

429to441

437

(5)

(1)

Production index(PI)0.44to0.94

0.67(13)

(3)

0.27to0.83

0.59

(30)

(6)

0.46to0.77

0.60

(5)

(2)

0.19to0.85

0.56

(4)

(2)

0.33to0.47

0.43

(5)

(1)

Bitumens(ppm)870to5360

2500(11)

(2)

510to15760

2500

(15)

(5)

1950to4440

2950

(3)

(2)

660and1010

(2)

660and1320

(1)

Aromatics HC(%)24and29

e (2)

(2)

19to34

(8)

(4)

2020and28

(2)

Saturated HC(%)39and40

e (2)

(2)

9to69

(8)

(4)

1414and40

(2)

Resins(%)17and17

e (2)

(2)

7to24

(8)

(4)

179and29

(2)

Asphaltenes(%)14and22

e (2)

(2)

3to56

(8)

(4)

2123and37

(2)

TOC e total organic carbon;Tmax e temperature of maximum of S2peak;S2e residual petroleum potential;S1e oil and gas yield(mg HC/g rock);PI e production index;HI e hydrogen index;OI e oxygen index.Range of geochemical parameters is given as numerator;median values in denominator,in parentheses:number of samples from wells (numerator)and number of sampled wells(denominator).

P.Kosakowski,M.Krajewski/Marine and Petroleum Geology49(2014)99e120107

of algal textures is very dif ?cult.In the examined material,frag-ments of the Mizzia green algae were observed,which represent warm,hypersaline environment (Kirkland and Chapman,1990),and Archaeolithoporella frequently seen in the Late Paleozoic and Triassic sediments (Flügel et al.,1984b ;Riding and Guo,1991;Krainer et al.,2003;Flügel,2004).Sediments with predominant algal textures most likely form microbial-algal reefs (mounds;Type IX).These buildups grew in situ by accumulation of disintegrated algal,more rarely bryozoan material,and trapped bioclastic material (Fig.5K e O).As regards the development,these buildups resemble the reef complexes found within the Zechstein Limestone (Ca1)strata (Fig.5K;e.g.Peryt et al.,2012).

A next divergence between the paleofacies image presented in the literature and the facies development recorded in drill cores is represented by incorrect inclusion of a part of wells in the Wilcze and Stare Kramsko areas to the ooidal marginal platform facies (Table 1,Fig.5I).In the Wilcze 7well which represents upper slope (Fig.1B,C),sediments of the redeposited oolitic grainstones or

Figure 6.Geochemical pro ?les of Zechstein Main Dolomite in selected wells in platform slope e Stare Kramsko 1,Wilcze 7and Kargowa 6.

P.Kosakowski,M.Krajewski /Marine and Petroleum Geology 49(2014)99e 120

108

aggregate grains have not been observed.Instead mud-dominated facies with synsedimentary deformations and faults are common, which indicates the steep upper slope(Types II and III;Table1, Fig.2).It is likely that the platform margin zone,so far interpreted in this area as a basin,along the Wilcze-Kargowa-Babimost line (Fig.1B)runs along the tectonic zone that constitutes the south-western extension of the Pniewy-Papro c Fault Zone and Kargowa-Rawicz Fault Zone recognized in the Dolomite basement (Kiersnowski et al.,2010).Along this line several hydrocarbon?elds are located.

To sum up,the currently accepted hypothesis of existence of an extensive rimmed platform with a distinct high-energy oolitic barrier separating the platform from narrow basin zones,isolated from the open sea(Fig.1B),is unlikely.Our examinations of cores intersecting the supposed barrier zones indicate that the oolitic facies occur only in few wells and in some intervals,a few metres thick.These facies likely represent small bars or shoals that boun-ded local highs.In central parts of these highs,laminated or poorly laminated dolomudstones and microbial-algal-lump dolopack-stone-bindstone are dominant.The area of the Radoszyn oil?eld is an example.In the Radoszyn3well,the thickness is as small as a few metres,which corresponds with the earlier described thickness of the Staropole or BS areas.Facies development of the microplat-forms may resemble atolls commonly observed in the adjacent areas(Jaworowski and Miko1ajewski,2007)bounded by ooidal and microbial-algal-lump facies.The remaining zones considered earlier to be ooidal barrier zones developed on the platform margin,most often also represent the microbial-algal-lump shoals or reefs(Types VII and IX;mounds)developed in moderate-energy zones(Flügel,2004).In many upper slope facies(the Gry_zyna areas)near the supposed grain-dominated barrier zones of the platform margin,redeposited detrital ooidal sediments or aggre-gate grains are absent.Probably in the studied zones,transition from the inner platform facies to the basinal facies can be observed more frequently;most likely it occurs on the low-angle slope witin a distally steepened ramp-like system(cf.Mawson and Tucker, 2009).In case of a steep slope,fragments of the redeposited mar-ginal platform reefal and ooidal facies can be observed.

4.4.Inner platform?at

In the study area,sediments interpretated so far as the inner platform?at are located in the south e southeastern part of the area(Fig.1B).Like in the remaining paleogeographic zones,also in this case the veri?cation and reinterpretation of the paleogeo-graphic position of sediments representing the inner platform facies(Table1,Fig.1B,C)were carried out.In some wells from the Ska?pe and Wilcze areas,which have been hitherto located on the inner platform?at,high-energy oolitic facies(Type V;Table1)were found.From the Wilcze1well,only a general section is preserved, in which a several metres thick interval of oolitic facies was found (see Peryt,1978).More microfacies data come from the Wilcze5 well where oolitic dolograinstone and oomoldic porosity can be observed(Type V;Table1,Fig.5G).In case of the Ska?pe1well,the section is dominated by the previously described algal-lump dolopackstone-bindstone facies(Type IX;Fig.5B e O)with scarce micritic ooidal dolograinstone-packstone(Type VI;Table1).

On the other hand,in a number of wells located so far on the peritidal inner platform?at,sediments representing the low-angle slope facies were found(the Struga1,Radnica1,Chro s nica1,and Papro c29wells;Fig.1C).There,one can observe sequences of microdolograinstones,calcisiltites and calciturbidites and grain slumps similar to those noted in the Babimost,Kargowa,Mysze?cin, Gry_zyna and O1obok areas(Fig.1C)included in the platform slope. They document the existence of signi?cant height differences in the area.They evidence the occurrence of vast depression zones separated by distinct slopes from areas of highs on which mainly ooidal and microbial-algal-lump facies were deposited.The exis-tence of such distinct slopes is also indicated by interpretation of the2D seismic data on which these paleogeographic features are similar to those interpreted on the platform margin(Figs.3,4). 5.Geochemistry of Zechstein Main Dolomite e results and discussion

5.1.Quantity of organic matter

Data from the Rock-Eval pyrolysis were analysed to acquire in-formation on quantity and quality of organic matter.The study of its geochemical characteristics was performed on core samples taking into account the paleogeographical position of the wells.General paleogeography of the Zechstein Main Dolomite Basin was adopted after Kotarba and Wagner(2007),despite the fact that the achieved results of microfacies analysis showed some departure from it.

The following geochemical characterization of the Main Dolo-mite rocks was carried out within the new paleogeographic pattern proposed in this article and the arrangement of facies given in Table1.With regard to considerable changes in the paleogeo-graphic pattern,the basinal?at zone has not been characterized due to the absence of the core material.

Within the carbonate platform slope and toe-of-slope,299core samples were collected,among which calcisiltites and dolo-mudstones(facies Type II;Table1)are dominant,representing approximately65%of the analysed population(Table2).Dolo-packstone,dolograinstone and dolobindstone are subordinate. Measured organic carbon content in the dolomudstones reaches 3.4wt.%,nevertheless lean samples(under0.2wt.%)are dominant (Fig.7).The average is only0.2wt.%(Table2).Such a low median value results,?rst of all,from a very low organic carbon content in the Mysze?cin1and Wilcze7wells(Fig.6),which constitutes as much as45%of the analysed population(Table2).Nevertheless,in these wells also the highest values were measured,which raises an argument for existence of some horizons with very good source properties within the generally very lean rock(Fig.6).

In remaining facies units also low content,usually under 0.2wt.%,is prevailing(Fig.7).Only in the Wilcze4well,in the mixed facies(dolopackstones,dolograinstones and dolo?oat-stones;facies type III/II/IX in Table1)representing slump-breccia facies,the TOC average amounts to3.9wt.%,with the median be-ing as low as in facies II(Table2).Similarly,the hydrocarbon po-tential(the Rock-Eval S1tS2)in the analysed population from the carbonate platform slope is very low(Fig.7).Measurable content was only observed in about30%of the analysed population.Low

TOC (%)

(

)

(

)

TOC (%)

Figure7.Petroleum source quality diagram for organic matter from Zechstein Main Dolomite in western part of the Wielkopolska Platform.Classi?cation after Hunt(1996) and Peters and Cassa(1994).

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values are prevailing,which evidences lean source rock at most (Table 2,Fig.7).However,in some cases the observed values are characteristic for moderate or even good source rocks,according to the criteria given by Peters and Cassa (1994).This is especially clear in carbonates of facies II and XI,occurring also on the platform slope,where relatively low TOC values are accompanied by high hydrocarbon content (Table 2,Fig.7).It is obviously an indicator of good reservoir properties of the dolomite there.

Among the measured hydrocarbons,slight predominance of epigenetic hydrocarbons is indicated by the productivity index (PI)values that range from 0.22to 0.89,with the average in particular facies usually over 0.5(Table 2).

For the quantitative evaluation of the source rock,also the hydrogen index (HI)was used.Like for TOC and hydrocarbons,the HI values are very low,usually under 100mg HC/g TOC (Table 2,Fig.8).There is no direct correlation between the TOC content,hydrocar-bon content or facies type and the value of the hydrogen index.

Of course,the measured low values of TOC,hydrocarbons and HI,suggesting the low source potential of the Zechstein Main Dolomite on the slope and toe-of-slope,are an effect of a relatively high degree of organic matter alteration (Fig.8)and advancement of organic matter transformation into hydrocarbons.On the other hand,the great realized petroleum potential of the Zechstein Main Dolomite is evidenced by the bitumen content that reaches 5380ppm,with the average usually in excess of 1430ppm (Table 2,Fig.9).

According to the classi ?cation of Peters and Cassa (1994),these values indicate occurrence of poor to good oil-prone source rock on the slope of carbonate platform (Figs.6,9;Pletsch et al.,2010).

The marginal platform barrier zone is poorly represented,which results from the fact that this zone is not well developed in this part of the Wielkopolska Platform.Only the results of 8core samples from Radoszyn 3well were used for geochemical charac-terization (Table 3,Fig.1).The research material represents prin-cipally the facies V e ooid dolograinstones and packstone facies,with subordinate proportion of dolorudstone and dolograinstone (mixed facies XII/V,Table 1).The averages of TOC content and hy-drocarbon content are higher than in the adjacent zone of the carbonate platform slope (Table 3),which results from higher proportion of the epigenetic fraction in the hydrocarbons:PI ranges widely from 0.68to 0.86(Table 3)but the average for the whole population is as high as 0.83(Table 3).The TOC content for the prevailing facies type V reaches 1.92wt.%(Table 3,Fig.7).As high TOC content can be observed in the mixed facies type XII/V where the average is as high as 0.75wt.%,with maximum content reaching 1.13wt.%.Similarly,the hydrocarbon content in the facies type IV reaches 9.9mg HC/g rock,and in the mixed facies XII/V it reaches 11.8mg HC/g rock,with the average of 6.6mg HC/g rock (Table 3,

Fig.7).Also HI values are higher than in the adjacent zone and reach 164mg HC/g TOC in the facies type V and 207mg HC/g TOC in the mixed facies (with the average of 156mg HC/g TOC)(Table 3,Fig.8).The higher petroleum potential of the barriers is also con ?rmed by the bitumen content that reaches up to 12800ppm at equally high average of 9560ppm (Table 3,Fig.9).Such high bitumen content results from a high proportion of epigenetic hydrocarbons,which is evidenced also by the bitumen fractions,with dominant saturated and aromatic hydrocarbons (Table 3).

The carbonate platform zone was sampled just as densely as the slope zone.A total of 159core samples were collected,principally from the facies type IX and mixed with facies V,VI,XII and XI (85samples)(Table 4).Subordinately,core samples representing facies type II (30samples)and X (19samples)were collected.Contribution of the analytical material from the remaining facies is small (Table 4).In the dominant microbial-algal dolopackstone and bindstone facies IX and bioclastic-peloidal dolowackestone-pack-stone facies X,organic carbon content is very variable and ranges from values near zero to 1.3wt.%.However,averages are as low as 0.23wt.%for both the facies.The highest TOC content was measured in the dolomudstones from facies II (maximum 2.95wt.%,average 0.35wt.%;Table 4).In spite of the fact that rocks of this facies are characteristic for the platform slope,in consequence of sharp morphologic differentiation of the platform they also occur in the discussed paleogeographic zone.This zone and these facies are naturally considered to be sources for hydrocarbons occurring in the Zechstein Main Dolomite rocks,and this is why the maximum values are so high.In the remaining facies,TOC values are much lower,both the maximum and mean values.In the micrite-microsparites (facies XIV),the TOC content reached 0.66and 0.84wt.%,with averages of 0.37and 0.27wt.%,respectively (Table 4).Microbial dolobindstones,microbial-algal dolobindstones,dolomudstones and dolopackstones of facies XI,mostly predis-posed to be source rocks,reveal the lowest values of the organic carbon content.In the carbonates of facies XI the measured TOC content reached up to 0.71wt.%(average 0.16wt.%;Table 4,Fig.7).Also the hydrocarbon content (S 1tS 2)in the facies type II,IX and X is higher than in the other facies (Fig.7).Despite the visible facies-dependent differences of the content,more than half of the analysed population have hydrocarbon content below the threshold of the Rock-Eval apparatus sensitivity.In the microbial-algal-lumps dolopackstones and bindstones representing facies IX,hydrocarbons were found in w 35%of the analysed population,and in bioclastic-peloidal dolowackestones nad packstones (facies

(C)temperature H y d r o g e n i n d e x (m g H C /g T O C )

400450465T

(C)

temperature 430

500

Figure 8.Hydrogen index versus Rock-Eval T max temperature for Zechstein Main Dolomite.Maturity paths of individual kerogen types after Espitaliéet al.(1985).

C o n t e n t o f C h y d r o c a r b o n s (p p m )

101

TOC (wt%)

0.1

110

10Figure 9.Petroleum source quality diagram for organic matter of Zechstein Main Dolomite in the analysed part of the Wielkopolska Platform.Classi ?cation after Hunt (1979)and Leenheer (1984).

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type X)only in 25%of the population (Table 4).Like in the other paleogeographic zones,the depletion in hydrocarbons is also an effect of the considerable degree of organic matter transformation.The bitumen content in carbonate rocks of the carbonate platform is higher than in analogous facies of the carbonate platform slope and in the predominant dolomudstone,dolopackstone and bind-stone facies it reaches 15760ppm,with the average of 4930ppm (Table 4,Fig.9).However,the group composition is very similar and there is a slight dominance of aromatic and saturated hydrocarbons (average 26and 40%,respectively)over resins and asphaltenes (average 14and 20%,respectively)(Table 4).Correlation between TOC and S 1tS 2and C 15thydrocarbon content indicates that in both the paleogeographic zones the dolomudstone,dolopackstone and bindstone from facies type II and IX represent a very good source for oil (Figs.7,9).

In the general quantitative evaluation of the source character of the Zechstein Main Dolomite,deposits of the facies distinguished on the carbonate platform demonstrated the highest organic car-bon and hydrocarbon content (Table 4,Fig.7;Pletsch et al.,2010).However,in the geochemical pro ?le of the carbonate formation,wide variations occured.High TOC and HC content is not seen along the whole pro ?le but only in narrow depth intervals (Fig.7;Kotarba and Wagner,2007).Similar variability can be observed in the mudstone facies on the carbonate platform slope:high content in narrow depth intervals separated by barren intervals (Fig.6).Grain-supported carbonates,i.e.packstone and grainstone,contain less organic matter.On the carbonate platform as well as on the

platform slope,the organic matter content usually does not exceed 0.5wt.%and hydrocarbon content is usually below 1mg/g rock.Only the barrier zone facies reveal reservoir properties and the TOC and hydrocarbon content grows signi ?cantly.Table 5

The maceral composition and vitrinite re ?ectance of the Zechstein Main Dolomite strata in the study area.

Well

Depth (m)

Facies

Pyrite %

Macerals (%)

OM %

R o (%)

Range of measur.

No.of meas.

I Chro s nica 12243.3

II 0.5 2.00.1

2.10.970.78e 1.2071Chro s nica 12251.2IV 0.211.011.00.970.76e 1.20114Chro s nica 1

2255.2IX 0.4 3.4 3.4 1.070.85e 1.23125Gry _zyna 42498.7II 3.00.90.30.3 1.50.760.60e 0.8025Gry _zyna 42499.85II 18.0 1.5 2.3

0.5 4.30.820.75e 1.2084Jablonna-22256.7XI/X <0.10.1tr.0.10.690.58e 0.7655Jastrze ?bsko-12325.4II tr.tr.tr.tr.n.m.Jastrze ?bsko-12359.5II 0.20.80.10.90.720.60e 0.8260Mozów 11981.0XIV 0.60.10.15

0.050.30.60.46e 0.7821Mozów 11992.4XI/XIV 0.30.3tr.0.30.550.46e 0.7044Mozów 11996.3XI/XIV 0.50.4tr.0.40.580.49e 0.7371Mozów 12004.7XI/XIV 0.00.70.5tr. 1.20.740.52e 0.8354Mozów 12011.5XI 1.6 2.80.10.1 3.00.630.49e 0.80103Mysze ?cin-12618.6II 1.3 6.50.2tr. 6.7 1.000.84e 1.20110Mysze ?cin-12730.5II 0.2 2.6tr.

tr. 2.6 1.050.88e 1.2366O 1obok 12475.9II 0.3 1.0 1.00.700.59e 0.8624O 1obok 12481.85II 0.20.8tr.0.80.890.75e 1.1056O 1obok 12487.35II 0.30.40.4 1.060.85e 1.2632O 1obok 12516.0II 0.00.4tr.0.40.820.68e 1.0560Paproc 212308.7II tr.tr.n.m.Paproc 212315.3II tr.0.3tr.

0.30.790.63e 1.0038Paproc 212326.45II 0.20.80.80.800.60e 0.9350Paproc 212328.7II 0.10.10.10.840.75e 0.9920Paproc 212333.5II 3.00.1tr.

0.1n.m.0.92e 0.978Radoszyn 32384.1IV 0.118

180.880.60e 0.9096Radoszyn 32387.75V/XIII 0.0ab.Radoszyn 3

2393.3V/XIII tr.tr.0.840.80e 0.935Stare Kramsko-12164.4II/IV tr. 1.00.1 1.10.800.69e 1.0593Stare Kramsko-12176.1II/IV 4.70.50.2tr.0.70.890.75e 1.2790Templewo-12688.4IX/V 0.1 1.6tr.tr. 1.60.970.72e 1.1071Templewo-12716.2IX 0.20.90.1

0.1

1.10.820.72e 0.9961Wilcze-72163.2II 0.1<0.1<0.10.980.97e 1.108Wilcze-72189.1II tr.0.10.1 1.020.95e 1.1131Wilcze-7

2284.0II/IV 0.5 4.7tr.tr.

4.7 1.100.90e 1.27114Zba ?szy n -52208.4IX/XIII tr.tr.tr.n.m.0.74e 0.887Zba ?szy n

-52228.1

<0.1

0.4

tr.

0.4

0.90

0.69e 1.05

Vitrinite-macerals.with optical characteristics similar to vitrinite.It is mostly solid bitumen;OM e organic matter;n.m.e not measured.V e vitrinite macerals (kolotelinitu i witrodetrynitu i zwitrynityzowanych szcza ?tków organicznych);SB e solid bitumen;L e liptinite macerals;I e inertinite macerals;AOM e organo e mineral association.

2000

2500

3000

Depth (m)Figure 10.Distribution of vitrinite re ?ectance R o versus burial depth in the western part of the Wielkopolska Platform.

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5.2.Type of organic matter and environment of deposition

The location of the Polish part of the Zechstein Basin in the subtropical climatic zone with extremely dry climate determined the predominance of genetic precursors of marine origin and subordinate proportion of the component representing higher plants from occasional effects of terrestrial vegetation(Kosakowski,2004;Kotarba and Wagner,2007;S1owakiewicz and Miko1ajewski, 2011).Therefore,in all the paleogeographic zones of the Zechstein Main Dolomite basin signi?cant predominance of sapropelic kerogen can be observed,with subordinate and variable pro-portions of humic kerogen.

The genetic type of the organic matter in the Zechstein Main Dolomite was determined,in the?rst place,on the basis of

the Figure11.Vitrinite and inertinite macerals with fragments of pyrite in carbonates of carbonate slope.(A)Gry_zyna4,2499.85m;(B)Myszecin1,2730.0m;(C)Myszecin1,2730.0m;

(D)Stare Kramsko1,2176.1m;(E)Stare Kramsko1,2618.6m;(F)Wilcze7,2284.0;(G)Wilcze7,2189.1m;(H)Zbaszy n5,2228.1m.

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results of maceral composition analysis and results of geochemical studies e the Rock-Eval pyrolysis,elemental and biomarker analyses.

Microscopic examination of the maceral composition was car-ried out on36polished specimens from the depth from the depth interval of1981.0e2688.4m in13wells(Table5,Fig.10).The analysed Zechstein Main Dolomite rocks are characterized by highly variable contents of organic matter.Its quantity,determined by point counting,ranges from traces(the Jastrze?bsko1,Papro c21, Radoszyn3and Zba?szy n5wells)to over10%(11%in the Chro s nica 1well and8%in the Radoszyn3well)(Table5).Also carbonate rocks in the Mysze?cin1and Gry_zyna4wells and in basal parts of the Zechstein Main Dolomite in the Wilcze7well are distinctly enriched in organic matter(4.3e6.7%).The organic matter contains mainly primary components represented by macerals of the vitri-nite group(colotelinite),composed of gelled plant tissues,

which Figure12.Solid bitumen with fragments of pyrite in carbonates of carbonate platform.(A)Chro s nica1,2251.2m;(B)Jastrzebsko1,2359.5m;(C)Mozów1,2011.5m;(D)Jab1onna 2,2256.7m;(E)Mozów1,1996.1m;(F)Papro c21,2328.7m;(G)Templewo1,2688.4m;(H)Templewo1,2716.2m.

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occur most frequently in clay intercalations within carbonates (Figs.11and12).It is commonly accompanied by solid bitumen, which is secondary organic material resulting from alteration of the primary components.It usually?lls pore space of the rock and sometimes shows brown?uorescence in the UV light(Fig.13). Macerals of the inertinite group(mainly inertodetrynite)occur most often as traces or w0.1%(Table5).Only in the samples from the Gry_zyna4well the quantity reaches0.3e0.5%(Fig.11A).Pri-mary liptinite macerals occur in a few wells only.They are most often composed of liptodetrinite originating probably from algal material(alginite),spores and plant pollen(Fig.13A e F).A few samples from the Mozów1and Gry_zyna4wells are enriched in lipid components(0.3e0.5%;Table5)that show yellow and yellow-orange?uorescence.In the Gry_zyna4well at the depth of 2499.85m the liptinite content reaches2.3%(Table5,Fig.13B).In the examined deposits,also numerous bituminous impregnations occur(Fig.13G).That are also characterized by highly variable py-rite content,ranging from<0.1to18%(Gry_zyna4well;Table5). High pyrite concentrations(1.3e4.7%;Table5)were recorded in a few samples from the Mozów1,Mysze?cin1,Papro c21and

Stare Figure13.Macerals in UV light.(A)Mozów1,1996.3m;(B)Gry_zyna4,2499.85m;(C)Myszecin1,2618.6m;(D)Stare Kramsko1,2176.1m;(E)Stare Kramsko1,2164.4.6m;(F) Templewo1,2716.2m;(G)Zbaszy n5,2228.1m.

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Kramsko 1wells.All the results generally indicate low proportion of organic matter,with predominance of maceral material that evidences oil-prone type II kerogen.

Geochemical studies,particularly the results of the Rock-Eval analysis and correlations between basic quantities from this anal-ysis,i.e.between the temperature T max and the hydrogen index HI,did not reveal any predominance of the oil-prone type II kerogen;in case of the mud-supported facies even apparent predominance of the gas-prone type III kerogen is observed (Fig.8).The suggested considerable proportion of the terrestrial component in the ana-lysed samples,especially evident in the facies II and IV on slope zones (Fig.8A),and facies XI and XIV on platform (Fig.8B),is most probably a result of low organic carbon content and hydrocarbon content,thus lower reliability of T max determinations and HI and OI calculations.

The presence of the type III kerogen has not been con ?rmed by examination of n -alkanes and isoprenoids,carried out for the samples most rich in TOC,where short-chain n -alkanes are clearly dominant (Table 6).Correlation between Pr/n -C 17and Ph/n -C 18even suggests presence of the type I kerogen (Fig.14).

On account of high thermal maturity,the elemental composition analyses (Table 7)did not univocally indicate the kerogen genetic type.Contribution of the type III kerogen is only shown by a single sample from the Ska ?pe 1well located in the outer-barrier zone (Figs.1B,15).

In general,it should be stated that despite a low organic carbon content in the Zechstein Main Dolomite,the oil-prone type II kerogen occurs in these rocks,with local and quantitatively unde-termined admixtures of the type III kerogen and type I kerogen.

The organic matter in the Zechstein Main Dolomite was deposited in the anoxic conditions.The Carbon Preference Index (CPI)calculated for the C 17e C 31n -alkanes range (Kotarba et al.,1994)exhibits values generally close to 1(Table 6;Didyk et al.,1978).These values can be attributed to carbonate or evaporitic environments and they agree with the low Pr/Ph ratios,varied from 0.12to 0.72(Table 6).This Pr/Ph ratio is considered characteristic of the redox potential in the hypersaline depositional environment (Didyk et al.,1978;ten Haven et al.,1985,1987,1988;Peters et al.,2005).The calculated isoprenoids to n -alkanes ratios Pr/C 17and Ph/C 18,exhibit greater variability,compared to the Pr/Ph,possibly due to different thermal maturity.

The homohopane index (HoI),calculated as the ratio (C 35ab -hopane [S tR]/C 31ab -hopane [S tR])(Peters and Moldowan,1993),is considered to be an indicator of the redox potential of marine sediments during diagenesis and it is affected by the ther-mal maturity (Peters et al.,2005).In the analysed zone,the average value of HoI amounts to 0.31and only in one sample from the Templewo-1well its value is below 0.1and equals to 0.07(Table 8).In the remaining samples,the values are distinctly higher and reach 0.71.Higher C 35homohopane abundance than that of the C 34ho-mologues is associated with conditions of deposition of marine carbonates and evaporites (ten Haven et al.,1988;Bishop and Farrimond,1995).

The presence of gammacerane and high values of the gamma-cerane index (GI >ca.0.1;Table 8)indicate the hypersaline sedi-mentation environment (Sinninghe Damste et al.,1995).The composition of the regular steranes as well as the ratios calculated based on steranes,diasteranes and hopanes distribution (Table 9,Fig.16)also indicate the anoxic,salinar environment of deposition (Peters et al.,2005).

5.3.Maturity of organic matter

The thermal maturity of the Zechstein Main Dolomite samples was assessed by Rock-Eval T max ,vitrinite re ?ectance R o values and biomarker maturity ratios.Due to the low TOC and hydrocarbon

Table 6

Indices calculated from distribution of the n-alkanes and isoprenoids in bitumen extracted from the Zechstein Main Dolomite.Well

Depth (m)

Facies

Indices CPI (Total)

CPI (17e 23)CPI (25e 31)Pr/Ph Pr/n-C 17Ph/n-C 18Jastrze ?bsko 12351.0II 1.130.910.980.340.57 1.15Kije 92125.7II 0.980.96 1.080.310.140.31Papro c -292347.6II 0.920.900.940.220.320.89Mysze ?cin 12618.6II 1.020.78 1.070.510.470.6Mysze ?cin 12622.6II 0.950.98 1.050.230.230.36Wilcze 72189.1II 1.020.840.870.410.310.51Wilcze 72275.1II 1.040.880.830.560.250.4q agów 12951,0II/IV 0.940.940.940.460.260.47q agów 1

2956,0II/IV 0.97 1.000.910.630.470.70Stare Kramsko 12176.1II/IV 1.060.880.930.470.370.65Stare Kramsko 12164.3II/IV 1.010.99 1.100.630.270.48Wilcze 4

2190.4III/II/IX 0.98 1.020.930.570.140.31Stare Kramsko 12174.5IV 1.010.99 1.100.600.290.48Mozów 12011.5XI 0.880.870.870.870.210.58Jab 1onna 22256.7XI/X 1.010.830.90.460.430.71Zawisze 1K 2602.5XI 0.920.920.900.160.150.50Ska ?pe-1

2265.8IX 0.890.850.940.150.230.54Templewo 1

2698.2

1.11

0.91

0.95

0.72

0.25

0.23

CPI (Total)?(C 17tC 19t.tC 27tC 29)t(C 19tC 21t.tC 29tC 31)/2(C 18tC 20t.tC 28tC 30);CPI (17e 23)?(C 17tC 19tC 21)t(C 19tC 21tC 23)/2(C 18tC 20tC 22);CPI (25e 31)?(C 25tC 27tC 29)t(C 27tC 29tC 31)/2(C 26tC 28tC 30)(indices after Kotarba et al.,1994);Pr/Ph e Pristane/Phytane.

Phytane/n -C 18

P r i s t a n e /n -C 17

Figure 14.Genetic characterization of bitumens from Zechstein Main Dolomite,in terms of pristane/n -C 17and phytane/n -C 18.Categories after Obermajer et al.(1999).

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content,the T max values were determined only for 82samples,that is about 20%of the analysed sample population.The measured T max values,which range from 423to 436 C (Tables 2e 4),point out that the organic matter occurs in the whole thermal maturity interval of the “oil window ”(Fig.8).Only a few T max values indicate the immature to early-mature phase (below 430 C).In the analysed sample population,signi ?cant proportion of epigenetic hydrocar-bons can be observed,which is con ?rmed by values of the

productivity index PI reaching 0.94and the amount of the bitumen extracts reaching 15760ppm (Table 4,Fig.9).Such large proportion of the epigenetic fraction may result in erroneous T max values.Considering that,re ?ectance measurements conducted on solid bitumen seem to be more reliable.The obtained re ?ectance values,converted to the vitrinite re ?ectance scale,range from 0.55to 1.10%R o ,with the average of 0.85%R o (Table 5),which means that they also fall within the whole interval of the “oil window ”(Fig.17).Higher transformed organic matter is observed in the area of the Chro s nica 1,Mysze ?cin 1and Wilcze 7wells (Fig.1),that is in the western part of the study area.Despite such signi ?cant proportion of epigenetic hydrocarbons,the obtained values of the temperature T max and vitrinite re ?ectance demonstrate correlation and fall within the interval of the “oil widow ”.The re ?ectance values converted to maximum paleotemperatures indicate that these ranged from 65to 100 C and in the zone with the highest maturity of organic matter even the range of 100e 120 C was reached (Gaupp and Batten,1985).

Additionally,the re ?ectance and temperature T max calculated on the basis of the sterane,methyldibenzothiophene and methyl-phenanthrene distribution were used for the evaluation of the organic matter maturity.In general,these indicators also record the organic matter maturity in the interval of the “oil window ”(Table 10).Only in case of the q agów 1and Mysze ?cin 1wells,a considerable discrepancy between the calculated and measured values of T max and R o can be observed (Tables 2and 10).The values calculated from the indicators R cal(DBT)and T max(DBT)as well as R cal(MPI)and T max(MPR)in general suggest higher maturity (Table 10).Some discrepancies between the measured and calculated values may be related with an analytical error caused by generally low concentrations of methyldibenzothiophenes.

Table 7

Elemental composition of kerogen from the Zechstein Main Dolomite.Well

Facies

S Total Fe Total

Composition (wt %,daf)Atomic ratio (%wag.)

C H O N S H/C O/C N/C S/C Mozów-1IX/XI 11.6 4.880.2 5.7 5.9 1.27.00.860.060.0130.033Papro c -29II 10.9 3.680.7 4.6 6.60.67.50.690.060.0060.035Ska ?pe-1IX

9.7 5.378.5 5.010.3 1.5 4.80.760.100.0160.023Wilcze-4III/II/IX 9.0 1.176.8 4.9 5.50.911.90.770.050.1000.058Zawisze-1K

9.2

3.580.3

5.1

7.4

1.2

6.0

0.76

0.07

0.013

0.028

daf e dry,ash-free basis.

0.5

1.0

(H /C )a t

(O/C)at

Figure 15.Genetic characterization of organic matter from Zechstein Main Dolomite in the analysed part of the Wielkopolska Platform.Fields representing natural maturity paths for individual kerogens after Hunt (1996).

Table 8

Selected characteristics of hopanes from the Zechstein Main Dolomite.Well

Micro-facies

Hopanes Ts/(Tm tTs)

C 29Ts/(C 29H tC 29Ts)31S/(S tR)30M/H HoI GI Jab 1onna 2IX/VIII 0.830.570.530.180.31n.c.Jastrze ?bsko 1II 0.710.810.570.850.35 1.38Kije 9II 0.54n.a.0.56n.a.n.a.n.a.Mozów 1XI 0.610.380.580.100.320.23Mysze ?cin 1II 0.780.650.420.520.17n.c.Mysze ?cin 1II n.a.n.a.n.a.n.a.n.a.n.a.Papro c 29II 0.790.490.670.120.710.37Ska ?pe 1

IX 0.510.290.630.150.340.32Stare Kramsko 1II/IV 0.810.060.490.110.260.34Templewo 1IX

0.690.460.580.190.070.15Wilcze 4III/II/IX 0.68n.a.0.59n.a.n.a.n.a.Wilcze 7II 0.600.340.520.10.150.17Wilcze 7II n.a.n.a.n.a.n.a.n.a.n.a.Zawisze 1K

0.45

0.25

0.59

0.09

0.41

0.27

HoI e homohopan index (C 35ab-hopane [S tR]/C 31ab-hopane [S tR])(Peters and Moldowan,1993);GI e gammaceran index (gammaceran/C 30ab-hopan)(Peters and Moldowan,1993);31S/(S tR)e homohopanes ratio C 3122R/(22R t22S);30M/H e moretane ration C 3017b(H)21a(H)/norhopanu C 3017a(H)21b(H)(Seifert and Moldowan.1980);Ts/Tm e 18a(H)-22.29.30-trisnorneohopan/17a(H)-22.29.30-trisnorhopan;C 29Ts/C 29H e C 29Ts-18a(H)-30-norneohopan/C 29-17a(H)-norhopan;n.a.e not analysed.

P.Kosakowski,M.Krajewski /Marine and Petroleum Geology 49(2014)99e 120

116

In general,after analysis of all the obtained maturity parameters it should be maintained that the organic matter from the Zechstein Main Dolomite is in the maturity stage corresponding with the whole interval of the “oil window ”(Figs.8,17)and could have been the hydrocarbon source for the accumulations located in the study area.

6.Conclusions

Our studies revealed a number of discrepancies between the so far accepted paleogeographic and facies model accepted so far and the conclusions presented in this work.The previous model,which assumed the existence of a shallow-water rimmed platform bounding narrow basinal zones does not fully con ?rm with the results obtained most recently as regards paleogeography and facies.This implies the need of veri ?cation of the previous views.Evaluation of deposition of the Zechstein Main Dolomite was controlled by such factors as the sea-level oscillations,subsidence,morphology,block-faulted structure of the pre-Zechstein base-ment,and synsedimentary tectonics.At the present stage it is dif ?cult to demonstrate a detailed paleogeographic image as further detailed studies are required.A more satisfactory paleo-geographic and facies image more satisfactory than the previous may be characterized as follows.The part of the platform which comprises the Fore-Sudetic Monocline represents a system of rather narrow highs that strike NW-SE and refering to the complex multidirectional systems of faults bounding horsts (paleohighs)and grabens detected in the basement.In case of the Zechstein Main Dolomite it seems that the extent of the platform and microplatforms corresponds with paleohighs of the basement.On these highs (horsts of the basement),oolitic shoals and microbial-algal-lump facies developed.As in the Wilcze and Gry _zyna areas,

the platform slopes were probably tectonically sutured,referring to the main tectonic directions.In the study area,the slopes are in most cases gentle with the mud-dominated facies.The NW continuation of the interpreted highs may be represented by

Table 9

Maturity indices calculated based on biomarkers distribution.Well

Micro-facies

Steranes Dia./Reg.

20S/(20s t20R)C 29sterane

C 29abb /(aaa tabb )

C 27(%)

C 28(%)C 29(%)C 29/C 27Jastrze ?bsko 1II 25.756.218.10.700.550.340.64Papro c 29II 25.224.050.8 2.020.140.530.56Mysze ?cin 1

II 35.730.334.00.950.740.410.55Stare Kramsko 1II/IV 40.624.534.90.860.370.460.6Wilcze 7II

42.325.132.60.770.340.410.68Wilcze 4III/II/IX n.a.n.a.n.a.n.a.n.a.0.520.57Ska ?pe 1

IX 38.420.641.0 1.070.210.550.52Templewo 1IX 38.133.928.00.730.60.510.64Mozów 1XI 42.325.532.20.760.200.540.52Jab 1onna 2XI/X 35.320.644.2 1.250.370.430.55Zawisze 1K

33.2

21.3

45.5

1.37

0.20

0.55

Dia./Reg.e C27diasterane/(C27sterane tC27diasterane);n.a.e not calculated due to lack of biomarkers.

C 29

C 27

Figure 16.A ternary plot of C 27vs.C 28vs.C 29steranes (as normalised percentages;after Huang and Meinschein,1979;classi ?cation after Peters et al.,2005;modi ?

ed).

Figure 17.Plots of sterane ratio:C 29(20S t20R)versus C 29(bb taa )and C2920S/(20S t20R)versus C 29bb /(bb taa ),for organic matter from Zechstein Main Dolomite.Maturity ?elds after Peters and Moldowan (1993).

P.Kosakowski,M.Krajewski /Marine and Petroleum Geology 49(2014)99e 120

117

numerous microplatforms,the Gorzów Platform and Grotów Peninsula.The NW continuation of the depressions with the slope facies found in the Fore-Sudetic Monocline may be represented by the bays observed on the Gorzów and Wielkopolska platforms,e.g.the Rzepin,Witnica or Note c bays.Like in adjacent areas,some of the elevations represent the depositional model of microplatforms,often reminiscent to atolls,frequently observed to the north of the Fore-Sudetic Monocline.

The paleogeographic pattern changed in this manner un-doubtedly in ?uenced the assessment of values of particular zones for hydrocarbon exploration.Unquestionably the slope of carbon-ate platform gained considerable importance as a zone with high source potential and also a zone in which hydrocarbon ?elds are located.The characterization of the source potential of the Main Dolomite carbonate rocks,presented in a scheme not applied pre-viously in the Polish part of the Ca2basin,proved that good oil-prone rocks occur locally and are generally related to the calti-siltites of facies II,and dolomudstones and partly to the bindstones of facies IV.Carbonates rocks of these facies occur also in other paleogeographic zones where they may constitute sources of hy-drocarbons present in the petroleum system.The microbial-algal facies IX and XI seem to signi ?cantly supplement the hydrocar-bon potential in this zone of the Wielkopolska Platform.In spite of the fact that the measured organic carbon content is low,with respect to thermal maturation degree they reveal favourable source characteristic.The hydrocarbon accumulation potential was con ?rmed by the grain-supported facies III,V and VI in which low organic carbon content is associated by relatively high hydrocarbon content.These facies may also supplement the hydrocarbon po-tential of Ca2rocks.

The growth of the importance of the slope of platform zone has resulted also from the considerable increase in the extent of this paleogeographic zone at the cost of the bay of the basin zone.It increases the general hydrocarbon generation potential of this paleogeographic zone.

The carbonate platform,extent of which was also changed,demonstrated geochemical characteristics similar to slopes of the Ca2.In this platform,microbial-algal dolopackstones and bind-stones deposited in the moderately energetic shoals and skeletal mounds (reefs)are dominant.They show somewhat better source characteristics than those of the slope of carbonate platform,but

considering the degree of thermal maturity we can even speak of good source potential.Similarly,the microbial-algal facies X and XI,and dolomudstones and caltisiltesof facies II,despite their not very high TOC values,originally presented good hydrocarbon potential.The effective source potential of carbonate rocks in the carbonate platform zone is evidenced by numerous oil ?elds and even gas ?elds discovered in this area.

To sum up,the considerable revisions in the paleogeographic pattern of the Zechstein Main Dolomite veri ?ed the geochemical characteristics of the carbonate rocks,which enabled recapitulation of the assessment of values of particular paleogeographic zones for hydrocarbon exploration and created the basis for search for ana-logues in other zones of the Zechstein Main Dolomite basin.Acknowledgements

Financial support of this work arrived from the statutory fund of the AGH University of Science and Technology No.11.11.140.175,and grant of the Polish Ministry of Science and Higher Education Grant No.N N307460638.Dr.A.Kowalski and Mr.H.Zych helped with the analytical work,Special thanks go to Prof.Dr O.Weidlich and anonymous Reviewer for constructive comments and critical sug-gestions,and Prof.M.Wendorff for language review of the manuscript.References

Bathurst,R.G.C.,1982.Genesis of stromatactis cavities between submarine crusts in

Palaeozoic carbonate mud buildups.J.Geol.Soc.Lond.139,165e 181.

Bishop,A.N.,Farrimond,P.,1995.A new method of comparing extended hopane

https://www.sodocs.net/doc/eb7055240.html,.Geochem.23,987e 990.

Clark,D.N.,Tallbacka,L.,1980.The Zechstein deposits of southern Denmark.Con-trib.Sedimentol.9,205e 231.

Czeka n

ski,E.,Kwolek,K.,Miko 1ajewski,Z.,2010.Hydrocarbon ?elds in the Zech-stein main dolomite (Ca2)on Gorzów block (NW Poland).Prz.Geol.58,695e 703(in Polish with English summary).

Didyk,B.H.,Simoneit,B.R.T.,Brassell,S.C.,Eglinton,G.,https://www.sodocs.net/doc/eb7055240.html,anic geochemical

indicators of paleoenvironmental conditions of sedimentation.Nature 272,216e 221.

Dunham,R.J.,1962.Classi ?cation of carbonate rocks according to depositional

texture.In:Ham,W.E.(Ed.),Classi ?cation of Carbonate Rocks,1.American As-sociation of Petroleum Geologists Memoir,pp.108e 121.

Embry,A.F.,Klovan,J.E.,1972.Absolute water depth limits of late Devonian Paleo-ecological zones.Geol.Runds.61,672e 685.

Table 10

Maturity indices calculated based on distribution of phenantrene and dibenzotiophene and their methyl derivatives in bitumens of the Zechstein main Dolomite strata.Well

Depth (m)

Micro-facies

Indices R cal(MDR)R cal(DBT)

T max(DBT)( C)

R cal(MPI)R cal(MPR)

DBT/Ph

MDR MPI 1MPR (%)(%)Jab 1onna 22256.7

IX/VIII 1.23

3.93 1.000.79

0.95Kije-92103.7P/G 2.700.290.00.71437

0.55q agów 12951.0I/III 9.72 2.000.0 1.22 1.57q agów 12956.0I/III 10.5 2.190.0 1.27 1.68Mozów-12011.5IX 1.780.510.8

0.64

4320.710.51

Mysze ?cin 12618.6R 3.72

6.600.97 1.45

0.96Mysze ?cin 12622.6I 24.9n.m.n.m 2.75580n.c.Papro c 292347.6I 3.14 1.27 2.30.74439 1.16 1.03Ska ?pe 1

2265.8VI 2.130.66 1.1

0.66

434

0.770.67

Stare Kramsko 12176.1I/III 3.82 2.720.690.74

0.78Stare Kramsko 12174.5III 3.710.560.0

0.780.71Templewo 12698.2VI 2.58 5.14 1.110.95 1.04Wilcze 72189.1I 2.79 3.090.760.750.83Wilcze 72275.1I

2.29

3.72 1.590.78

1.32Wilcze 4219

2.4II/I/VI 5.590.820.00.92452n.c.Zawisze 1K 2602.5IX/X 2.230.33

0.1

0.67

434

0.60

0.35

Zba ?szyn 5

2228.1

0.77

3.55

0.77

Ph e Phenantrene;DBT e Dibenzotiophene;MDR e Metyldibenzotiophene Ratio (4-MDBT/1-MDBT (Radke et al.,1986);MPI 1(Metylphenantrene Index 1)?1.5(2-MP t3-MP)/(P t1-MP t9-MP);MPR (Metylphenantrene Ratio)?2-MP/1-MP,R cal(MDR)?0.40t0.30(MDR)-0.094(MDR)2t0.011(MDR)3(Radke,1988);R cal(DBT)?0.51t0.073MDR (Radke,1988);T max(DBT)?423t5.1MDR (Radke,1988);R cal(MPI)?0.60MPI 1t0.4dla MPR <2.65(Radke and Welte,1983);R cal(MPI)?à0.59MPI 1t2.59dla MPR >2.65(Radke,1988);R cal(MPR)?1.1logMPR t0.95(Radke,1988);n.m.e not measured;n.c.e not calculated.

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Hydrocarbon-potential-of-the-Zechstein-Main-Dolomite-in-the-western-part-of-the-Wielkopolska-platfor

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