abnormal high pressure in kuqa foreland thrust belt of tarim basin

PROGRESS INNATURAL SCIENCE　　　　　　　　　　　　　　　　　　　　　　　　Vol.16 , No.12 , December 2006

＊ Supported by the Nat ional Program on Key Basic Research Project s(Grant No.G19990433)and CNPC Pet roleum Science and Technology Foun-dation

＊＊ To w hom correspondence should be addressed.E-mail:[email protected]

Abnormal high pressure in Kuqa foreland thrust belt of Tarim basin:Origin and impacts on hydrocarbon accumulation

＊

ZENG Lianbo1＊＊ and LIU Benming1 , 2

(1.Key Laboratory of Hydrocarbon Accumulat ion in China Pet roleum University , Minist ry of Educat ion , Beijing 102249 , China;2.The Inst itute of Oil and Gas Exploration and Development , Tarim Oilf ield Company , Pet roChina Company Limited, Korla 841000 ,

China)

Received March 28 , 2006;revised April 24 , 2006

　　Abstract　　Follow ing analyses of the abnormal high pressu re distribution characteristics , based on the geological characteristi cs ,tectonic stress field and physical simulation , w e invest igated the format ion mechanisms of abnormal high pressure and it s im pacts on hydro-

carbon accumulation in the Kuqa foreland th rust belt.The abnorm al high pressure appears at the bottom of the Paleogene and obviously

exists in the Triassic and Jurassic.How ever , the pressure coeff icient in the Triassic and Ju rassic is low er than that in the Cretaceous and

at the bot tom of the Paleogene.Horizontally , the abnormal high pressure dist ribut ion is characterized by E-W orientation zoning.The

maximum pressure coeff icient lies in the Kelasu-Dongqiu-Dina tectonic zones in the center of the Kuqa foreland thrust belt and decreasesaway f rom the tectonic zones.The format ion of abnormal high pressure w as mainly related wi th the intense tectonic compression in the

Early Pleistocene time , and tectonic uplift ing , undercompact ion and hydrocarbon generation w ere secondary factors con tributing to abnor-mal high pressure.Under the rapid and intense tectonic compression in the Early Pleistocene , the rock f ramework f irstly undertook 1/ 4 of

the compression st ress and the other was borne by the pore fluids.Due to the presence of great seal of gypsum-salt or gypsum-mudstonebeds in the Paleogene , the p ressure of pore f luids increased rapidly and led to the abnormal high pressure in the Kuqa foreland thrust belt.

The abnormal high p ressure has important impacts on hydrocarbon accumulation.It is one of the necessary condit ions for format ion of

large oil and gas fields in the Kuqa foreland thrust belt.

　　Keywords:　abnormal high pressure , formation mechanism , oil and gas reservoir , Kuqa foreland thrust belt , Tarim basin.

　　The abnormal high pressure in sedimentary basinis referred to as the pressure of strata obviously higher

than hydrostatic pressure (the pressure coef ficient isover 1.2)[ 1 ,2] .There are more than 180 high pres-sure basins in the wo rld.Of them , about 160 are richin gas and oil[ 1 ,2] .In China , the oil and gas reser-voirs in over 30 areas of 10 main basins are related

w ith high pressure[ 3].Especially , in the foreland

thrust belts of the basins in west China , abnormalhigh pressure plays an important role in hydrocarbon

accumulation and dist ribution[ 1—5] .

In the early 1930s , abnormal high pressure w asmainly studied f rom the aspect of compaction.Asmore and more basins were found to be overpressed ,part icularly the breakthrough in gas and oil explo-ration in abnormal high pressure zones , the abnormalhigh pressure formation mechanism and i ts relation

w ith hydrocarbon accumulation have been studied by

numerous oil companies and scholars.Previous studiesshow that the format ion mechanisms fo r abnormal

high pressure are mainly composed of undercom-

paction , hydrothermal pressuring , hydrocarbon gen-eration , dehydration of clay minerals , tectonism ,temperature change and cementation of the po re

space.Of them , undercompaction and hydrocarbon

generation are well known[ 1—15] .

In recent years , with the discovery of the Kela2Gas Field and the signif icant breakthrough in oil and

gas exploration in the thrust belts of the Kuqa , thewestern Sichuan Basin , and the southern Jungar

Basin , research on abnormal high pressure in the

fo reland thrust zone of western basins becomes mo re

and more important.The unbalanced compact ion

caused by rapid subsidence , hydrocarbon generat ionand tectonism is the possible formation mechanism of

abnormal high pressure in these regions[ 16—24] .Onthe basis of geological analysis , and according to

physical simulation experiment and the relationship

betw een abnormal high pressure and tectonic st ress

field , it is concluded that the strong tectonic compres-sion in the Early P leistocene is the main reason fo r the

fo rmation of abno rmal high pressure in Kuqa foreland

thrust belt.

1　Geological setting

The Kuqa foreland thrust belt is located at the

northern Tarim basin and south of the Tianshan

M ountains , NW China.The belt is adjacent to theNorthern Tarim uplif t(Fig.1), 45 km long and 20 —60 km w ide.The st rata mainly consist of Mesozoicand Cenozoic sequences.The Mesozoic is generally2000 —3000 m thick w ith a maximum thickness of

over 4000 m.It consists of sandstones and mudstonesof lacust rine , sw amp and f luvial facies wi th coal mea-sures , carbonaceous shale and oil shale developed inthe middle and low er parts.The Cenozoic is generally3000 —5000 m thick and has a maximum thickness of

over 8000 m.It is composed of sandstones and mud-stones of lacust rine and fluvial facies with tw o sets of

g ypsum-mudstone and gypsum-salt beds developed in

the middle and lower parts.There are three sets ofreservoir-seal assemblages in the Jurassic , Paleogeneand Neogene.

The tectonic sty les in the studied area are domi-nated by fold-thrusts developed during Cenozoic

t imes.They have the characterist ics of zoning in

no rth-south o rientation and vertical layering .Hori-zontally , there are four detachment and thrust zonesand tw o sag s , which are the monocline tectonic zone ,Kelasu-Yiqikelike tectonic zones , Baicheng-Yangxiasag s and Qiulitag f ront tectonic zone f rom no rth to

south (Fig.1).Controlled by four detachments of

gypsum-mudstone beds in the Neogene Jidik forma-tion , Paleogene gypsum-salt beds , Jurassic and Trias-sic coal measures and cry stalline basement detach-ment , the vertical deformation of rock w as obviouslyof layering , and the st ructures beneath and above thegypsum-salt bed are clearly unsymmetrical.

Fig.1.　The locat ion of the Tarim basin and the studied area.

2 　Distribution characteristics of abnormalhigh pressure

According to mudstone acoustic time difference

logging and actual drilling data , there is obviously ab-normal high pressure in the Kuqa foreland thrust

belt.For example , in the Kela 2 Gas Field , the sand-stones at the bottom parts of the Paleogene Kumuge-limu Formation and in the Low er Cretaceous Bashiji-jik Formation belong to the same pressure system.Their pressure coefficients are generally 1.7—2.0.The maximum pressure coef ficients are over 2.2 and

the residual pressures are 30—38 MPa[ 19—24] .Theactual drilling pressure in the middle parts of gas-bearing interval of Dina 2 well is 101.1 MPa and thepressure coef ficient is 2.18.The maximum residual

pressure in Dongqiu 5 w ell is 47.6 MPa[ 23] .In planeview , the pressure coefficient is dist ributed by E-Worientation zoning

[ 22].The maximum coeff icient of

fo rmation pressure lies in the Tubei , Kelasu , Dongqiuand Dinan tectonic zones , where their pressure coeff i-cients are mo re than 2.0 and decrease southw ards andno rthw ards.The northw ard decreasing rate is muchg reater than the southward rate , which probably re-

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sulted f rom intensive faulting and signif icant strata u-plif ting at the f ront of the Tianshan Mountains.These tectonic activi ties led to the rapid unloading of

pressure.The pressure system is close to normal in

the Northern Tarim uplif t w here the pressure coef fi-cients are less than 1.3.

The abnormal high pressure dist ribution is close-ly related w ith gypsum-salt and gypsum-mudstonebeds in the Paleogene.If there were no thick gyp-sum-salt or gypsum-mudstone beds , no abnormal highpressure w ould exist.Generally , if the thickness ofg ypsum-salt or gypsum-mudstone beds is over 400 —500 m , there is possibly abnormal high pressure un-derneath this place , but no t v ice versa.Besides hugegypsum-salt or gypsum-mudstone beds , the abnormalhigh pressure dist ribution is also controlled by st ruc-tural posi tion.The good sealing property of gypsum-salt o r gypsum-mudstone beds in the Paleogene is thenecessary condition for the formation of abnormal

high pressure.

Vertically , the format ion pressure is normal

above the Paleogene gypsum-salt o r gypsum-mudstonebeds w here the pressure coef ficient is less than 1.2.The abno rmal high pressure begins to appear at the

bottom of the Paleogene.When the pressure coef fi-cient reaches it s maximum , i t will decrease g raduallyw ith depth.However , the pressure remains the same(Fig.2), which means the bo ttom parts of the Paleo-gene and the Cretaceous belong to the same pressure

sy stem[ 19] .Because of several non-permeable layers inthe Paleogene and Cretaceous , the pressure is uneven-ly changed and has division properties w ithin the ab-normal high pressure system of reservoi rs.The ab-normal high pressure also exists in the Jurassic and

Triassic w here the abnormal high pressure appears in

the Paleogene and Cretaceous.There are a few actual

drilling data for pressure in the Jurassic and Triassic.According to a few pieces of actually measured data of

pressure at several drilling w ells , such as Yinan 2 ,Yinan 4 and Yishen 4 in the south of Yiqikelik tec-tonic zone , the pressure coef ficients are usually 1.6 —1.8 , with the maximum over 1.9 , which is low erthan that at the bottom parts of the Paleogene and the

Cretaceous.

Fig.2.　The dist ribut ion of actual formation pressure and f racturepressures of st ratum in Kela 2 w ell.

3　Mechanism of abnormal high pressure

The possible mechanisms of abno rmal high pres-sure in the Kuqa foreland thrust belt are mainly un-dercompaction , hydrocarbon generation , and tectonic

uplif t and compression[ 16—24] .Among them , tectoniccompression is considered to be the principal factor by

numerous studies[ 18—22]

.Under the strong tectonic

compression in the Neogene and Early Pleistocene ,there w ere intense uplif ting and significant erosion a-long w ith the further thrusting and folding , whichresulted in erosion of over 2000 m in the Neogene

Kuqa formation.In theory , if the format ion pressurew as not changed follow ing uplif ting and erosion , thehydrostatic pressure in the stratum over 6000 m of

burial depths in the geological period could be turned

into present-day abnormal high pressure.If takingthe factors of temperature reduction , rock pore vol-ume change and fo rmation w ater change into consid-eration , which were accompanied w ith uplift ing , thepressure after uplif ting and erosion is no t high but

static or even tends to be lower[ 20—22] .Therefore ,the influence of tectonic uplif ting on abnormal high

pressure formation is considered to be less significant.　　

The undercompaction and hydrocarbon genera-tion are also the possible facto rs for abno rmal high

pressure formation[ 23 , 24]

.However , these mecha-nisms are not supported by the actual geological data.Firstly , there are no abundant oil and gas in all of thest rata or structures w ith abnormal high pressure.Sec-ondly , the Triassic coal bearing st rata and the Jurassiclacust rine mudstone are the main hydrocarbon source

rocks , but the known pressure coefficients in these

st rata are low er than those at the bot tom parts of the

1309Prog ress in Natural Science　Vol.16 No.12　2006　www .tandf.co.uk/ journals

Paleogene and the Cretaceous.Thus , there are manydoubts for the hydrocarbon generation mechanism.Inaddi tion , acco rding to the microscopic observation ofcements and diagenesis in the Cretaceous system ,there were obvious recrystallization and replacement

of cements at the late diagenetic stage.It means thatthe abnormal high pressure w as formed late and un-dercompaction did not cause the abno rmal high pres-sure[ 17] .

After ruling out the possible mechanisms of un-dercompaction , hydrocarbon generation and tectonic

uplifting , the horizontal tectonic compression is themain factor fo r formation of abnormal high pressure

in the Kuqa foreland thrust belt.The formation ofthe abnormal high pressure is related with intensi ty of

tectonic compression , such as strain and stress magni-tude.Through the statistical analysis of abnormal

high pressure , folding intensity and compression ratiofrom the Mesozoic sequences , the residual pressure islinearly related to the ex tent of closeness of traps and

the compression rat io.Wang concluded that the tec-tonic compression w as the main factor for the increase

of po re fluid pressure[ 23] .

According to studies of the relat ionship betw een

tectonic st ress field and abnormal high pressure , it isknow n that there is a good co rrelation betw een them.Firstly , in view of the formation time of abnormal

high pressure and evolvement of tectonic st ress f ield ,the abno rmal high pressure w as formed during the in-tense compression periods.Using the estimation f romrock memorial information measured by acoustic e-mission[ 25] , the mean maximum ef fective st ress val-ues w ere 28.8 MPa in the Jurassic (208—135 M a),41.1 MPa in the Cretaceous (135 —65 M a), 58.5MPa in the Paleogene(65—23.3 Ma), 63.3 MPa inthe Neogene (23.3 —2.6 Ma), and 76.4 MPa in theEarly Pleistocene(2.6 —0.78 M a).The mean maxi-mum effective st ress values of the Cretaceous reser-voirs in the Early Pleistocene were 80.9 MPa.Theyindicate that the compression intensity became

stronger and stronger since Jurassic time , and the in-tensi ty reached its peak in the Early Pleistocene.TheEarly Pleistocene w as the main period both fo r abnor-mal high pressure formation and for rock deforma-tion.Analy ses of 4 balanced cross sections indicate

that the sho rtening ratio w as 77%—80%during this

period.

From abnormal high pressure and regional tec-tonic position[ 25] , it can be seen that the abnormal

high pressure has good corresponding relations w ith

the severe rock defo rmation and stress concentrat ion

zones.Horizontally , the maximum pressure coeff i-cient is mainly dist ributed at the Tubei-Kelasu-Dongqiu-Dinan tectonic zones where the rocks w ere

severely deformed in strong tectonic st ress field.Thepressure system at the Northern Tarim uplif t is close

to normal due to the weak tectonic compression.Ver-tically , the bo ttom parts of the Paleogene and the

Cretaceous , where the largest pressure coef ficient andresidual pressure exist , are also the stress concentra-tive intervals(Fig.3).

Fig.3.　The isoline of the mean st ress in Kela2 st ructu re.

According to the actual measured data of forma-tion pressures and the corresponding tectonic stress in

several drilling wells , the formation pressures and

residual fo rmation pressure linearly increase w ith the

maximum tectonic principal st ress[ 22] .Although themeasured data at drilling w ells are limited , this distri-bution in each drilling well is very clear and complete-ly consistent.This show s the direct inf luence of hori-zontal tectonic compression on the abnormal high

pressure formation.

4　Physical simulation experimentation

4.1　Experimental condi tions

To model the mechanism of abnormal high pres-sure , phy sical simulation experimentation has been

carried out.The simulation was made on the im-proved PVT equipment of RUSKA-2730A type

which consists of a sample container w ith high pres-sure , an ax is pressure loading system , a confiningpressure loading sy stem and a measure sy stem.Therock samples are the Low er Cretaceous sandstone at

the depth of 3825.7 m in Kela 2 well , which have aporosi ty of 12% and a permeability of 8×10-3 μm2.


The samples were enclosed by rubber band to approx-imately simulate the good sealing ef fect underneath

the gypsum-salt or gypsum-mudstone beds of the Pa-leogene on reservoi r.The pressure loaded by the con-f ining pressure loading sy stem w as 62.7 MPa to ap-proximately simulate the confining pressure caused by

overlying st rata.The pressure loaded by the axis

pressure loading sy stem w as 10.3—67.7 MPa to ap-proximately simulate the influence of tectonic com-pression st ress on the change of pore fluid pressure.

4.2　Result analyses

According to the simulation experimentation da-ta , a curve of bulk pore volume w ith dif ferent axispressures w as obtained at fi rst(Fig.4).It show s thatthe bulk pore volume decreased with the increasing

axis pressure.The curve could be divided into twosect ions and the incline ratio of section I w as bigger

than that of section II.The decreasing rate of bulkpore volume w ith stress was faster at the beginning

than that in the latter state.Acco rding to the curve ofpore f luid pressure with dif ferent axis pressures(Fig.5), with the axis pressure being added , the pore fluidpressure also increased.The curve could also be divid-ed into tw o sections and the slope of the latter section

w as bigger than that of the former one.The increas-ing rate of pore f luid pressure w ith st ress w as faster in

sect ion II than that in section I , indicating that w henthe tectonic st ress is added , the st ress w as mainlyborne by rock g rains and turned into rock st ructure

stress.The st ress increased by 22 MPa and the rockpore f luid pressure by about 9 MPa.That is to say ,about 60% of st ress w as borne by the rock frame-work.When the tectonic stress was over 25 MPa ,the stress w as mainly borne by pore f luids and turned

into pore f luid pressure.The stress increased by 34M Pa and the pore fluid pressure by about 26 MPa.This means that about 76% of stress w as bo rne by

pore f luids.

Because of the limit of experimental conditions ,the axis pressure could no t ex ceed 70 MPa.Accord-ing to studies of tectonic st ress field

[ 25], the maxi-

mum principal st ress in the Neogene could be 100 —120 MPa.Under this st ress status , the rock fluid

pressure could reach 70—85 MPa by the section II

trend of Fig.5.It is approximately equivalent to theactual formation pressure in the studied area.Al-though the simulation condi tions w ere not completely

consistent with the actual geological condit ions , espe-cially micro-creep deformation mechanism of rock

could not be satisfied in the experimentation fully ,this simulation experimentation verif ied the influence

of tectonic compression on pore fluid pressure.Thest rong horizontal tectonic compression could form ab-normal high pressure in the Kuqa fo reland thrust

belt.

Fig.4.　The change curve of rock pore volume and st ress obtainedby physical simulation experiment data (Samples are sandstones

f rom the Low er C retaceous of Kela 2 w ell at 3825.7 m).

Fig.5.　The change curve of rock pore fluid pressure and st ressobtained by physical simulat ion experiment data(Samples are sand-

stones f rom the Lower Cretaceous of Kela 2 w ell at 3825.7 m).

5　Mechanism of tectonism

According to the geological analy ses and phy sical

simulation experiment results , the st rong horizontal

tectonic compression in the Early Pleistocene is the

main mechanism fo r abnormal high pressure forma-tion in the Kuqa foreland thrust belt.

Based on the st ress analy sis of rock , the st resscan be divided into tw o parts.One is the mean st ress(σ)which causes the bulk deformation of rock(σ=(σ1+σ2+σ3)/3), the other is the shear st ress (τ)


which makes the shear st rain of rock (τmax =(σ1 -σ2)/2).The fo rmer is the main st ress resulting in

abnormal high pressure[ 26 , 27] .Under the closed con-di tions that there is no f luid discharge , when rock isaffected by tectonic stress field , some part of the tec-tonic st ress is borne by rock framework and turns into

effective st ress , the other part is borne by f luid inpores and turns into po re fluid pressure.The changeof rock bulk under the st ress is

ΔV =V(σ-Pp)

K,

where ΔV is the change of bulk rock volume , V is

the bulk rock volume , σis the mean st ress , PP is thepore fluid pressure , and K is the elastic module of

rock.

Under the st ress , the change of bulk fluid vol-ume in pores of rock is

ΔV V =nPp V

K V,

where ΔVV is the change of bulk po re f luid volume ,n is the porosity , and K V is the compression coef fi-cient of pore bulk.

After diagenesis , the rock framewo rk volume iscompressed lit tle.Thus , the change of rock bulk isapproximately equal to that of pore bulk , namely ΔV≈ΔVV .Then , the relationship between pressure ofpore fluid and mean st ress can be expressed as P P≈Bσ, where B =K V/(nK +K V), and B is the in-vert coef ficient depending on seal degree and satura-tion.To the completely sealing and saturated terrane ,the compressibility of w ater is much lower than that

of rock f ramew ork , so B≈1 , that is to say , all of themean stress can turn into pore fluid pressure.To thedry rock or the fully open sy stem , the compressibilityof rock pore is much higher , so B ≈0 , that is to say ,the mean stress can no t turn into pore f luid pressure.To unsaturated w et rock or incompletely sealing sys-tem , B =0 ～ 1 , that is to say , part of mean stresscan turn into po re f luid pressure.The higher satura-tion or sealing degree of terrane is , the bigger B is.According to the results of physical simulation experi-mentation and i ts analog y to the actual geological con-di tions , the invert coef ficient B can be 0.76.

The impacts of tectonic compression on abnormal

high pressure have become more and more recognized

by numerous studies[ 28—34] .Under the tectonic com-pression , the st ress t ransmission and bearing are lim-ited by the actual geological conditions such as seal of

cap rock , saturation , tectonic intensity , and so on.The good seal of huge gypsum-salt or g ypsum-mud-stone beds in the Paleogene is the essential condit ion

fo r abnormal high pressure formation in the Kuqa

fo reland trust belt.Under the intense and rapid tec-tonic compression during the Early Pleistocene time ,the pore volume of rocks w as reduced.Because of theadequate sealing by gypsum-salt or gypsum-mudstonebeds in the Paleogene , the fluids could not be

squeezed out , and the majority of the tectonic com-pression st ress w as bo rne by po re fluids.It caused therapid rise of pore f luid pressure and formed the abnor-mal high pressure in the Kuqa foreland thrust belt.According to the results of tectonic st ress field analy-sis and its numerical simulation data , the mean stressof the Kela 2 w ell w as over 90 MPa and could cause

more than 68 M Pa fluid pressures in sandstone reser-voi rs at the bot tom parts of the Paleogene and the

Cretaceous.By this w ay , we can deduce that mo rethan 80% of abnormal high pressure in Kela 2 w ell

w as caused by tectonic compression.The contribut ionof undercompaction and hydrocarbon generation was

less than 20%.Therefo re , the fo rmation process ofabnormal high pressure in Kela 2 w ell could be divid-ed into tw o stages.The mechanism of abnormal high

pressure in the first stage was mainly by undercom-paction , but the maximum pressure coefficients by

this w ay w ere less than 1.3.The mechanism of ab-normal high pressure in the second stage w as mainly

by tectonism , and the maximum pressure coefficients

by this w ay were more than 2.2(Fig .6).

Fig.6.　The sketch map showing the formation process of abnor-

mal high pressure in Kela 2 Gas Field.

6　Influence on oil and gas accumulation

The abno rmal high pressure has quite important


influences on oil and gas accumulation.Being advan-tageous or not , it depends on the actual geological

conditions.It can not only expedite the conversion ofo rganic materials to hydrocarbons , but also seal thediffusive phase natural g as in concentration , increasethe sealing capabili ty of cap rocks and faults , and pre-vent g roundwater , oxygen and bacteria from destroy-ing the entrapped oil and gas.However , when abnor-mal high pressure in reservoi rs w as fo rmed before oil

and gas charging , or was higher than that in hydro-carbon source rocks , it w as very dif ficult to let oil andgas mig rate into reservoirs and accumulate.Further-more , abnormal high pressure can also make pressureof gas reservoirs be released off and natural gas leak

out.These are the disadvantages fo r oil and gas accu-mulation[ 17—19] .

In view of distribution of the known oil and gas

reservoi rs , the large oil and gas reservoi rs , speciallythe large gas reservoi rs , are closely related w ith ab-normal high pressure in the Kuqa foreland thrust

belt.The format ion pressure coef ficients at oil and

gas reservoirs are significantly larger.Due to the re-quirement of quite good preserved conditions for ab-normal high pressure preservation , the preserved con-di tion w as generally excellent where abnormal high

pressure existed.Because abundant oil and gas

sources exist in the studied area , large oil and gas

fields , specially large gas fields , could be formed inthe tectonic traps w here good preservation conditions

are present.For example , the st ructures of Kela 2 ,Kela 1 and Kela 3 belong to the same tectonic zone ,but the formation pressure is low er than the f racture

pressure of the cap rock in Kela 2 structure(Fig.2),i.e.the Kela 2 structure had bet ter preservation con-di tions.This is the key factor for formation of the

large gas field of Kela 2.Kela 1 and Kela 3 st ructureshad unfavorable preservation conditions , so i t was dif-f icult for gas to accumulate and to fo rm a large gas

field in these two st ructures.

The main hydrocarbon source rocks are the coal

bearing st rata and mudstones in the Triassic and

Jurassic in the Kaqu fo reland trust zones , and the

main reservoir rocks are sandstones in the Low er Cre-taceous and at the bot tom parts of the Paleogene.These sandstones together wi th the gypsum-salt o rg ypsum-mudstone beds in the Paleogene consti tute

one set of reservoir-seal assemblage.The tectonic

traps were principally formed at the end of Neogene ,and they provided the good trapping condi tions fo r

gas and oil accumulation[ 35] .The peak oil and gas

charging occurred mainly at the end of Neogene and

Early Pleistocene.Structural t raps and abno rmal highpressure were finalized in the Early Pleistocene.Theabnormal high pressure w as mainly caused by unbal-anced mechanical compact ion prior to the Neogene

and w as only dist ributed in the Middle and Lower

Jurassic hydrocarbon resource rocks and compart-mented by mudstone beds in the Upper Jurassic and

Low er Cretaceous.Under the strong horizontal tec-tonic compression at the end of Neogene , it formedno t only st ructural traps , but also faults connectinghydrocarbon source rocks w ith reservoi rs.Under thedriving forces of strong tectonic st ress and abnormal

high pressure of hydrocarbon resource rocks , the oiland gas generated by the Middle and Low er Jurassic

hydrocarbon source rocks migrated vertically and

episodically along faults and accumulated to form oil

and gas pools in reservoi r rocks sealed by the Paleo-gene thick gypsum-salt or gypsum-mudstone beds.Simultaneously , intense tectonic compression led to

uplif ting and erosion and dest royed the balance of f lu-id pressure in the upper st rata , which is the other fac-to r for oil and gas migration.Furthermore , whenfurther oil and gas charging occurred again in the Ear-ly Pleistocene , the abno rmal high pressure of reser-voi rs at the bottom parts of the Neogene and Creta-ceous sy stems was formed.The abnormal high pres-sure in reservoirs st rengthened sealing capabili ty of

the cap rocks and prevented g roundw ater , oxygenand bacteria f rom destroying the entrapped oil and

gas.Thus , oil and gas could be preserved to form the

large fields.When the primary oil and gas reservoirswere destroyed by subsequent faulting , the re-mig rat-ing oil and gas could form secondary accumulations in

the favorable posi tions above gypsum-salt or gypsum-mudstone beds in the Neogene.

In sum , the matching of space and time amongthe abnormal high pressure occurrence , traps forma-tion and gas and oil charging is the key factor to de-termine the validity of format ion of oil and gas reser-voi rs in abnormal high pressure zones[ 36] .The struc-tural st raps in the studied area developed early .Theybegan to appear since the Paleogene , and were formedon large-scale at the end of the Neogene and finalizedin the Early Pleistocene.Gas and oil charging and ab-normal high pressure format ion occurred almost at the

same time , with the charging slightly earlier as indi-cated by the sequence of events.The good matchingof the above-mentioned three factors w as the neces-


sary geological condition for forming the large-sizedoil and gas fields in the Kuqa fo reland thrust belt.

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abnormal high pressure in kuqa foreland thrust belt of tarim basin

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