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Spatial-distribution-and-geochemistry-of-the-nearshore-gas-seepages-and-their-implications-to-natura

Spatial distribution and geochemistry of the nearshore gas seepages and their implications to natural gas migration in the Yinggehai Basin,offshore South China Sea

Baojia Huang a ,b ,*,Xianming Xiao a ,*,Xushen Li b ,Dongshen Cai b

a State Key Laboratory of Organic Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China b

CNOOC Ltd,Zhanjiang 524057,China

a r t i c l e i n f o

Article history:

Received 21August 2007

Received in revised form 8April 2008Accepted 16April 2008

Available online 10May 2008Keywords:

South China Sea Yinggehai Basin Gas seepage Gas source

Gas migration model

a b s t r a c t

About 120gas seepage vents were documented along the west and southwest coast of the Hainan Island,South China Sea,in water depths usually less than 50m.The principal seepage areas include the Lingtou Promontory,the Yinggehai Rivulet Mouth,Yazhou Bay,the Nanshan Promontory and the Tianya Prom-ontory.They occur along three major zones,re?ecting the control by faults and lateral conduits within the basement.It is estimated that the total gas emission from these seepage vents is 294–956m 3/year.The seepage gases are characterized by a high CH 4content (76%),heavy d 13C 1values (à38to à33&)and high C 1/C 1–5ratios (0.95–1.0),resembling the thermogenic gases from the diapiric gas ?elds of the Yinggehai Basin.Hydrocarbon–source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried Miocene source rocks and sandstone reservoirs in the central depression.The 2D basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas-bearing ?uids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up-dip direction along the sandstone-rich strata of Huangliu Formation to arrive to seabed and form the nearshore gas seepages.It is suggested that the seepage gases are sourced from the Miocene source rocks in the central depression of the Yinggehai Basin.This migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation.

1.Introduction

Natural hydrocarbon seepages from sea bottom have been reported in quite a few offshore petroleum-bearing basins,such as North Sea of Netherlands (Hovland and Judd,1988;Schroot and Schuttenhelm,2003),Gulf of Mexico (Behrens,1988),offshore California (Scott et al.,1999),Great Britain (Selley,1992),the Torry Bay of Scotland (Judd et al.,2002),and southeast Asia (Macgregor,1993).Because hydrocarbon seepages are proxies to migration pathways and/or deeper hydrocarbon reservoirs (Abrams,2005),they are very important clues for petroleum exploration.The hydrocarbon seepages along the eastern margin of the Yinggehai Basin,South China Sea,have been known and recorded for more than 100years (Zhang,1965).However,little information has been reported on their geochemistry and origin.With increasing

petroleum exploration activities in the Yinggehai Basin,it has been realized that the gas seepages may provide new information to trace the hydrocarbon sources and thus reduce the exploration risk in the basin.In this paper,we present the spatial distribution,gas ?ux rates and geochemical characteristics of the hydrocarbon seepages,and discuss their origins and migration models in the basin.

2.Geological setting

The Yinggehai Basin is one of the most gas-rich Cenozoic rift basins in China (Gong,1997;Huang et al.,2003).It trends northwest–southeast,and is separated from the Hainan Island to the east by the No.1fault (Fig.1).The tectonic evolution of the basin can be divided into two stages:a Paleogene extensional rifting event and a Neogene post-rift thermal subsidence (Gong,1997).The basin was ?lled with thick deposits of Cenozoic clastics on Paleozoic and Mesozoic basement rocks (Fig.2).It was estimated that the maximum thickness of the Cenozoic deposits reaches 16km at the basin center (Huang and Xiao,2004).

*Corresponding authors.State Key Laboratory of Organic Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China.Tel.:t862085290176.

E-mail addresses:huangbj@https://www.sodocs.net/doc/2c17122557.html, (B.Huang),xmxiao@https://www.sodocs.net/doc/2c17122557.html, (X.

Xiao).

Contents lists available at ScienceDirect

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Marine and Petroleum Geology 26(2009)928–935

The basin is characterized by a rapid subsidence rate,a high geothermal gradient and overpressure(Zhang et al.,1996;Huang et al.,2003;Huang and Xiao,2004).It is evaluated that the maxi-mum sedimentation rate was up to1.2mm/year.The geothermal gradient was as high as4.25–4.56 C/100m during the Tertiary period(Huang et al.,2002,2003).The maximum pressure co-ef?cient reaches2.0–2.3(Zhang et al.,1996;Huang et al.,2003) with the major part of the basin being an overpressure system.The combination of overpressure and high palaeo-geothermal gradient had an important in?uence on the generation,migration and ac-cumulation of natural gases in the basin.Available geological and geochemical data indicate that the main hydrocarbon–source rocks in the basin reside in the Meishan and Sanya Formations(Huang et al.,2003).These source rocks contain gas prone higher plant-derived organic matter(Huang et al.,2003).

Another important geological character of the basin is the widely developed diapiric structures in the basin center area. The diapiric faults act as preferential pathways for upward mi-gration of natural gases from the deep sources into the Pliocene sandstone bodies and Quaternary strata,forming the major exploration targets in this basin.Several gas?elds related to the diapiric structures have been discovered in the Yinngehai Basin in recent years(Dong and Huang,1999;Huang et al.,2002, 2003).

The discovered natural gas pools principally occur in the Plio-cene–Quaternary marine sandstones from the central diapir zone, with a burial depth of390–2000m.Most of them are of diapiric origin.The gases are believed to be derived from a set of Type II2–III sources rocks in the Meishan and Sanya Formations occurring in the central depression of the Yinggehai Basin(Huang et al.,2003; Huang and Xiao,2004).The gas seepages occur in the west and southwest coastal area of the Hainan Island,and cover the south-east part of the hanging wall of the No.1fault in the Yinggehai Basin (Fig.1),with water depths usually ranging from10to50m.Most of the sea?oor topography appears to be relict and related to the erosion of older sedimentary strata.A thin layer of surfacial sedi-ments consisting of silts,clays,and sands is deposited in this area. The basement rocks comprise inter-bedded deltaic sandstones, siltstones and shales of Pliocene and late Miocene age.Some of the gases derived from the deep buried source rock in the central basin migrate to the seabed through tortuous pathways and exit from the sea?oor surfacial sediments,leading to the formation of the near-shore gas seepages in the Yinggehai Basin,which will be discussed in following sections.

N HAINAN

ISLAND

P E A

R L

R I V

E R

M O

U T H

B A S

I N

DONGSHA

XISHA

ZHONGSHA

SOUTH CHINA SEA

VIETNAM Q I O

N G

D O

N G

N A

B A

S I N

GUANGZHOU

108

109

Lingtou

Yinggehai

Yacheng

Nanshan

Haninan Island

YC13-1

DF1-1

LD8-1

LD22-1

LD15-1

Gas seep

Gas field

Northwest

Section

Middle

Section

Southeast

Section

E a

s t

S l o

p e

No.1 Fault D

Z o

n e

A’

B’

C’

(1)

(2)

(3)

(4) BEIBUWAN BASIN

Fig.1.(a)General map showing the location of the Yinggehai Basin and(b)the dis-tribution of the Yinggehai nearshore gas seeps.Fig.2.Schematic stratigraphic column of the Yinggehai Basin.

B.Huang et al./Marine and Petroleum Geology26(2009)928–935929

3.Methods and experiments

The data contained in this paper are mainly from a joint CNOON–BP research project and a CNOOC survey project on the seepage investigation in the Yinggehai Sea,carried out in 1991and 2001,respectively (Huang and Zhang,1992;Xie et al.,2001).

The surveys were navigated by Global Positioning System (GPS).The cruising speeds for the surveys were about 3–5.5knots.The gas seepage investigation was made using two side-scan sonars equipped on the surveying ships with frequencies of 3.5kHz and 12kHz,respectively.The analog signals were recorded by a paper chart recorder.The acoustic intensity returned from gas bubbles in water is proportional to their cross-sectional area.

Most of the seepage gas samples were collected from the water about 5–25m below sea level using a funnel.Some gases in the Yazhou Bay were sampled directly from the sea?oor using a re-motely operated vehicle (ROV).Sea?oor sediment samples were collected by a few methods,for example,the shallow core samples were collected using a lead bomb tube with a maximum sampling depth of 30cm,and mudsnapper samples were also collected at or near the surface of the sea?oor.

The sediments were extracted in dichloromethane and were analyzed by GC and GC/MS.The seepage gases were analyzed for their composition using a Hewlett Packard 5890II gas chromato-graph,equipped with a thermal conductivity detector.Methane and ethane gaseous hydrocarbons and CO 2were separated for d 13C measurements following procedures similar to those described by Schoell (1980)using a Finnigan-MAT251mass spectrometer.The d 13C values are reported in reference to the PDB standard,with an analytical precision of ?0.02&.Stable deuterium–hydrogen iso-tope ratios were determined for some of the samples.The d D C1values are reported in reference to SMOW,with an analytical pre-cision of ?3&.

4.Results and discussion

4.1.Gas seepage distribution

The two investigations have found that there are more than 120gas seepage vents along the west and southwest coast of the Hainan Island,in water depths usually less than 50m (Fig.1).They are distributed mainly in the Lingtou Promontory,the Yinggehai

Rivulet Mouth,the Yazhou Bay,the Nanshan Promontory and the Tianya Promontory to form three major seepage zones (Fig.4).Gas bubbles cover these areas to form nearshore bubble zones.For in-stance,within an area measuring about 100m by 50m in the Yinggehai Rivulet Mouth,there are numerous (>40)individual vents,with each producing visible bubbles (1–2cm in diameter for a single bubble)continuously,displaying spectacular seeps (Fig.3).The gases escape from the sea?oor sediments through fractures in

15.5m

Sea level Depth (m)

Sea floor

18.4m

Sea level

Sea floor

Depth (m)

Fig.3.Sonar records of 3.5kHz through the Yinggehai Rivulet Mouth seep site.Seep bubbles in water column are shown as maroon curtains or bubbling

plumes.

Sea floor

Pockmark

12.5ms

100

100m

Time (ms)

Sea floor

Pockmark

100

Time (ms)

100m b

B’

A’

Fig.4.Sparker pro?ler records showing pockmarks at the Yinggehai gas seepage site.The locations of cross sections are shown in Fig.1.

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930

the Pliocene Yinggehai Formation strata or the sea?oor outcrop of the pre-Tertiary basement rocks.

Since the leaking hydrocarbons have affected shallow sediments (Chen et al.,2005)and sea?oor (Hovland and Judd,1988;Taylor,1992;Schroot and Schuttenhelm,2003),the seabed morphology features also provide evidence of near-surface ?uid expulsions.The sparker seismic pro?les (Fig.4)show seabed pockmarks in the Yinggehai seep site.These pockmarks have sizes of 0.5–2m wide and 0.5–1m high,and are believed to be related to gas emissions.A similar scenario was reported in the Norwegian Trench where pockmarks occur in weak zones in the soft sediments within the seepage areas (Hovland and Judd,1988).

Based on the gas seepage occurrences,it is believed that the gases are originated from deeper burial rocks,and not from sea?oor soft sediments.4.2.Gas ?ux rates

Gas seepage ?ux rates were estimated using the displacement of water by an inverted funnel or ROV.The methods have been shown to be capable of accurate measuring ?ux over short time periods (Scott et al.,1999;Judd et al.,2002).Twelve samples were collected at the gas seepage areas of the Yinggehai Basin using an inverted 25cm diameter plastic funnel,and transferred to the measuring cylinder through a short plastic tube.The gas was stored in a 200-cm 3glass cylinder.Two samples were taken by ROV at the Yazhou Bay and the gas bubbles were collected in a 200-cm 3glass cylinder which was sealed in water with a silicone septa and aluminum cap.

The results are summarized in Table 1.The ?ux rates vary widely in the different gas seepage areas.The highest ?ux rate of individual seep vent occurs at the Yazhou Bay,and the lowest at Lingtou Promontory,with ?ux rates of 280ml/h and 910ml/h,respectively.Assuming that the ?ux rate measurements un-dertaken are representative,a crude approximation of the total gas ?ux from the Yinggehai Basin can be estimated.By applying the total 120seepage vents,the total ?ux rates range from 294to 956m 3/year.

It should be pointed out that the ?ux rate is possibly under-rated,because some continuous seepage may be occurring in water depths up to 50m,but no measurements have been made.The ?ux rate and volume of hydrocarbon seepage to the surface provide direct and critical information to the petroleum system analyst regarding source and gas potentials (Hunt,1996;Abrams,2005).The Yinggehai Basin is typical of a basin containing rich gas re-sources which will be further elaborated in the following sections.4.3.Geochemistry and origin of gas seepages

The geochemistry of samples from the gas seeps shows that the composition is dominated by methane,with minor N 2,CO 2and O 2(Table 2).The gas seepage samples collected by ROV contain high methane,nitrogen and carbon dioxide.The difference between samples collected from the sea?oor by ROV and from the water surface by a funnel (SWF)suggests that the SWF samples were contaminated by a small amount of air during sampling.The seepage gases have a high dry index (0.95–1.00)with relatively heavy d 13C 1values (à33.91to à38.24&)and light deuterium–hy-drogen isotope ratios (à131to à162&),indicating a thermogenic origin without biogenic gas presence (Fig.5a).

In order to trace the seepage gas sources,a comparison was made between the seepage gases and the subsurface trapped gases in the central diapiric zone of the Yinggehai Basin with regard to their compositions and compound-speci?c isotopic ratios.Huang et al.(2003)reported the geochemical characteristics of the res-ervoired gases from the Yinggehai Basin.The

thermogenically

Table 1

Table 2

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931

derived gases have dry indexes ranging from 0.93to 0.96,and d 13C 1values from à47to à30&,and a maturity corresponding to a source maturity of 1.0–2.2%Ro.As shown in Fig.5b,the geo-chemical data from the seepages fall within the reservoired gas boundaries,indicating that they may have derived from the same source.

A source rock rich in humic organic matter may have generated mainly gases with a small amount of oil (Hunt,1996)and required much greater temperature to expel the remaining hydrocarbons as gas.Dark mudstones with a signi?cant thickness in the Meishan and Sanya Formations occur mainly in the central area of the Yinggehai Basin.They have a TOC of 0.4–2.97%,with a typical Type II 2–III kerogens (Huang et al.,2003),and are believed to be the main source rocks for the reservoired gases in the Yinggehai Basin (Huang et al.,2002,2003).The burial depth in Meishan and Sanya Formation source rocks varies greatly,from 4000to 7500m in the basin,which led to the great discrepancy in the maturity levels of the source rocks from VRo of 1.0to 2.5%(Huang et al.,2003).Therefore,it is reasonable to consider the deeply buried source

rocks in the Meishan and Sanya Formations in the central area of the Yinggehai Basin as the seepage gas source.

The total soluble extract yields for all of the surfacial sediment samples from the Yinggehai seep sites were low.Only two extracts (91-SR6and 91-SR7)yielded suf?cient quantity to perform con-ventional GC and GC/MS https://www.sodocs.net/doc/2c17122557.html,pared with the extracts from the reservoir sandstone samples from DF1-1gas ?eld,the seepage sample extracts seem to contain less lower-molecular weight n -alkanes due to slight biodegradation,but their GC spectra are still quite similar (Fig.6).The Pr/Ph (pristane/phytane)ratios are moderate (1.5–1.8),together with an observed C 29aaa sterane predominance (Fig.7),which may imply a terrestrial source input in a marine depositional environment.The hopane and sterane molecular maturity parameters are fully isomerized,indicating that they are derived from matured source rocks.Based on the hopane and sterane bimarkers,the characteristics of extracts from the surfacial sediments in the seepage areas are similar to those of the source rocks from the Meishan and Sanya Formations and oils from the gas-reservoir in the basin.This suggests that the extracted hydrocarbons from the surfacial sediments in the Yinggehai seep-age sites are mainly originated from mature source rocks in the Yinggehai Basin.

4.4.Migration of gas seepages

Based on the above geochemical data and discussion,the seepage gases were likely sourced from the matured source rocks in

-20-30

-40

-50

-60

-70

-80

D C1

C 1

C 1/C 1-5

Fig.5.(a)The Yinggehai seep gas genetic classi?cation diagram by d 13C 1versus d D C1(Scheoll diagram,Scheoll,1980;Whiticar et al.,1986).(b)Cross plot of d 13C 1values versus C 1/C 1–5for the seep gas and the reservoired gas samples collected from the Yinggehai Basin,showing that both seep gas and reservoired gas are thermogenic in origin,and may have same sources.The boundary lines were taken from Tissot and Welte (1984).

Fig.6.Gas chromatogram of saturated hydrocarbons for two sand extracts from the Yinggehai seepage site and a gas-reservoir sandstone extract from DF1-1gas ?eld,showing the light biodegradation of the seepage hydrocarbons and their correlation with the reservoired hydrocarbon.

B.Huang et al./Marine and Petroleum Geology 26(2009)928–935

932

the Meishan and Sanya Formations in the Yinggehai Basin.How-ever,the Yinggehai nearshore gas seepages are primarily in the areas with no gas ?elds or discoveries,and there are no Meishan and Sanya Formations or any other possible source rocks around the seepage areas.Therefore,the widely developed matured source rocks in the Meishan and Sanya Formations in the central area of the Yinggehai Basin is the most likely source for the seepage gases.For a further explanation,the migration pathway analysis should be made using both geological–seismic data and ?uid ?ow modeling (Selley,1992).This is also critical in understanding the surface seepage in terms of petroleum system dynamics (Abrams,2005).

As pointed out in the above section,the Yinggehai seepages are not randomly distributed,but con?ned in three zones (Yinggehai Rivulet Mouth,Yazhou Bay,Nanshan Promontory,see Fig.1b),im-plying the possible control by faults and lateral conduits in the basement.In fact,all the nearshore gas seepages investigated are directly or indirectly related to the shale diapirism,or to the lateral gas migration induced by pressure difference.Seismic records show that widely developed high angled faults related to the diapirs extend from a few meters to 2km,and cut through seal rocks,even reaching to sea?oor in the Yinggehai Basin (Fig.8,Xie et al.,2001;Huang et al.,2005),leading to deeply sourced gases to be expelled from overpressured compartments into the overlying strata to form gas-reservoirs or migrate laterally into higher permeability units.The DST data from drilled wells in the Yinggehai Basin show that the burial depth of the overpressured zone is from 1500m to 2480m in the diapiric structure zone of the basin center,and be-comes deeper toward the east slope (to about 4300m).The over-pressure is believed to be the main driving force for the formation

of the diapir structures and the migration of overpressured ?uids (Huang et al.,2002).

Under above geological and dynamic framework,a 30km long section from the source rock center to the gas seepage sites is se-lected to model the ?uid ?ow using the IES’PetroFlow software.In this model,the sedimentary sequences and lithologies were based on seismic interpretations and well data,the paleogeothermal gradients and heat ?ow data were taken from Gong (1997),relative permeability curves were determined experimentally on reservoir rocks from the basin,and the modeled maturities and overpressure were calibrated using measured vitrinite re?ectances and

DST

LD3011well Source rock 3432m Miocene

Migrated hydrocarbon from the seafloor sand of the seepage site

C27

C28

C29

C30

C27

C28

C29

Ts Tm

C29

m/z217

(a)

C30m/z191Oil from

the Df1-1 gas field Huangliu Formation.

Fig.7.Representative m /z 191and 217mass fragmentograms showing the correlation among migrated hydrocarbons from the sand of the Yinggehai seepage site,the oil–sandstone in DF gas ?eld and potential source rocks in the central basin.

1.0

2.0

3.0

4.0

T w o -w a y t i m e (s )

No.1 Fault

D i a p i r

Upper Yinggehai FM

Lower Yinggehai FM

Huangliu FM Meishan FM

C C’

Fig.8.A seismic pro?le across L8-1diapir and the eastern part of the Yinggehai Basin,

showing the diapir piercing zone and associated faults.The location of seismic section is shown in Fig.1.

B.Huang et al./Marine and Petroleum Geology 26(2009)928–935933

pressure data,respectively.It was assumed that source rocks in the Meishan and Sanya Formations distributed in the central area of the Yinggehai Basin,with an average TOC of 1.05%and Type II 2–III kerogens.The modeling result (Fig.9)provides an overall in-dication of the major gas ?ow directions in the section.The ?uids migrate predominantly along faults,fractures and stratigraphic surfaces.The mud diapiric fault system would act as an effective pathway for ?uids releasing from the overpressured source rocks of the Meishan and Sanya Formations in the basin center.The ?uids migrate upward and were trapped over the diapir structures or migrate laterally through the east slope along the pathways below the sealing surface in the sandstone-rich strata of the Huangliu Formation.The gases may seep in the outcrops of the Neogene strata or accumulate to form gas pools in suitable structures along the pathway.

Fig.10shows the lateral–vertical–lateral migration model of gases along a west to east cross-section across the eastern part of the Yinggehai Basin.The migration directions are based on the integration of the modeling results with the regional stratigraphic data.Gases generated from the overpressured source rocks in the basin center may migrate laterally through the framework sand-bodies of the Huangliu Formation to the major basin boundary (No.1fault)by the drive of abnormal differential pressure.The gases would then migrate upward into the overlying sand-rich strata mainly along the No.1fault,and then migrate laterally into higher permeability up-dipping units.Consequently,the gases migrate laterally to the margin of the basin through coarse-grained sedi-ments,which unconformably overlie the impermeable basement,and eventually escape from the sea?oor basement outcrop un-conformity and/or the sea?oor sediments through fractures.

In this model,the No.1fault acts as an important pathway for ?uid migrating upward.This fault extends 250km to the northwest and appears to have connected to the Red River fault system (Fig.1).The fault can be divided into three sections:the northwest section,the middle section and the southeast section based on its activity which increases gradually from northwest to southeast.New data collected in recent years suggest that the middle and the southeast sections of the fault were active from pre-Tertiary to Pliocene or Quaternary,with relatively small fault displacement from approx-imately 10.5to 2.7Ma B.P,during the deposition of the deltaic sand and mudstone sequence.The results of lithology contact analysis of the fault’s two sides indicate that sand/sand juxtapositions in the upper Miocene or Pliocene have occurred in different parts of the southeast and middle sections (Fig.9),which would clearly favor gas migration cross the fault and along the permeable up-dip layers of the hanging wall (Knipe,1997).This may account for the

concentrated distribution of gas seeps in the shallow water area along the southeast coast of the Hainan Island.4.5.Implications for favorable sites of gas accumulation

The constructed gas migration model for the Yinggehai nearshore seeps has a signi?cant implication for hydrocarbon exploration in the region.Hunt (1996)reported that the largest oil-producing area in the world was discovered directly by visible oil-seepages.In south-east Asia,seep-rich basins are usually rich in subsurface reserves,and eight of nine basins with more than 35documented seepages contain at least one billion barrels oil equivalent (BBOE)proven reserves (Macgregor,1993).The Ying-gehai Sea seepage gases originating mainly from deeply buried source rock (Miocene)must have migrated through complex pathways to the sea?oor surface along the eastern margin of the Yinggehai Basin.Consequently,in addition to the diapiric structures in the central depression of the Yinggehai Basin,the pathways of migrating gases on the east slope zone should be an ideal location for searching for important gas accumulation in the Yinggehai Basin.

5.Conclusions

The Yinggehai gas seeps cover a relatively large nearshore area of the Yinggehai Basin,extending about 100km along the west to southeast coasts of the Hainan Island,in water depths usually less than 50m and have been documented for nearly 100years.The major sources for the seepage gases are the Miocene source rocks in the central depression of the Yinggehai Basin.Our investigation indicates that these gas seeps,which have mainly originated from deeply buried source rocks (Miocene)in the central depression,migrate laterally and vertically to the hanging wall of the No.1fault through the Huangliu Formation framework sand-bodies on the east slope,and escape from the sea?oor sediments through frac-tures or subcrops of the basement.This suggests that the Yinggehai Basin contains huge gas potential and future exploration should be focused on the east slope zone between the active gas kitchen and the Yinggehai seepages.Acknowledgements

We would like to thank Dr.R.W.T.Wilkins for improving the English,CNOOC Ltd,Zhanjiang for making available the data,and Professor Roberts and a reviewer for their comments and suggestions on the original manuscript.The work was ?nancially supported by Chinese Academy of Sciences (Project Grant No.KZCX2-YW-114),a CNOOC key project,and the Natural Science Fund for Distinguished Young Scholars (Grant No.

40625011).

Quaternary

Upper Pliocene

Lower Pliocene Upper Miocene

Middle Miocene

Lower Miocene

(1)

(3)(2)

(4)(5)

(1)(2)(3)(4)(5)(7)

sea water

No.1 Fault

3km D e p t h (m )

C C’

8000

700060005000400030002000

10000

(3) Lower Yinggehai Formation (4) Huanliu Formation

(5) Meishan Formation (source rock)Fig.9.2D gas migration modeling results at present day along a transect across the eastern part of the Yinggehai Basin,showing possible sources and migration directions of seeping petroleum.

No.1 Fault

D e p t h (m )

S S

Fig.10.Migration pathway and formation model of the Yinggehai seeps along the west-southwest coast of Hainan Island.The location of section is shown in Fig.1(C–C 0).

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