The-Silurian-play-in-the-Tazhong-Uplift-Tarim-Basin-northwestern-China_2007

Marine and Petroleum Geology 24(2007)189–198

The Silurian play in the Tazhong Uplift,Tarim Basin,

northwestern China

Xiu Xiang Lu a,b,?,Qiuhai Fan a,b ,Fengyun Zhao c ,Qinghua Wang c ,Qilai Xie d ,Haijur Yang c

a

Basin and Oil Reservoir Research Center,China Petroleum University (Beijing),Changping,Beijing 102249,China

b

Oil &Gas Migration &Accumulation Mechanism Laboratory,China Petroleum University (Beijing),Changping,Beijing 102249,China

c

Tarim Oil Field Company,PetroChina,Korle,Xinjiang 841000,China

d

Guangzhou Geochemical Research Institute,Chinese Academy of Sciences,Guangzhou,Guangdong 510640,China

Accepted 26December 2006

Abstract

Silurian sandstone reservoirs in the Tazhong Uplift in the Tarim Basin,northwestern China are rich in bitumen and movable oil.Analysis of the biological markers indicates that the bitumen and movable oil came from two source rocks in the Cambrian and Middle/Upper Ordovician system,both of which are belong to marine facies.The Silurian system experienced three stages of oil ?lling,namely Late Caledonian period (400Ma),Late Hercynian period (240Ma)and Yanshan-Himalayan (40Ma)period.Most of the oil emplaced in the Late Caledonian was subsequently oxidized to bitumen.The movable oil found at the present time was mainly generated from the Middle/Upper Ordovician source rocks,with only a minor contribution from the Cambrian source rocks generating in the two later periods.Bitumen and heavy crude were largely trapped below a red mudstone unit while movable oil was concentrated below an underlying gray mudstone unit (the middle sub-section of bitumen sandstone section).Lateral migration into the Silurian reservoirs took place from northwest to southeast in each stage and was controlled by an inherited palaeohigh.While individual relatively limited in extent,the tidal-?at sandstones of the main reservoir have stacking patterns that generate extensive sand sheets.Faulting can both enhance connectivity and create traps.r 2007Elsevier Ltd.All rights reserved.

Keywords:Tazhong uplift;The Silurian play;Oil accumulation

0.Introduction

The Tarim Basin,northwest China,is the largest inland basin in China.Frequent regional tectonic movements in the basin developed multiple unconformities and multiple uplifting and denudation,resulting in the present pattern of secondary structural units (Jia,1997).

Based on the integrated research on the tectonics,classi?cation and the depositional system of Tarim Basin,the structural evolution falls into seven stages (Jia et al.,1995):formation stage of the basin base (Presinian),craton margin aulacogen stage (Sinian–Ordovician),downwarp in the craton stage (Silurian–Carboniferous),rift basin in

craton stage (Permian),foreland basin stage (Triassic),composite foreland basin stage.The Tazhong Uplift originated following the inversion phase in the craton margin aulacogen stage,and basically took shape in the middle of the Devonian (Zhang et al.,2004).During the formation of the Tazhong Uplift,intense tectonic move-ment occurred in the Silurian,resulting in uplift,faulting and great denudation (Zhang et al.,2004).

The secondary structural units from north to south are the Kuqa Depression,Tabei Uplift,Northern Depres-sion,Central Uplift,Southwestern Depression,Southern Uplift and Southeastern Depression (Fig.1).The Tazhong Uplift is located in the central part of the Central Uplift and is oriented NW–SE,bordering the Northern Depres-sion in the north and Southwestern Depression in the south (Fig.1)covering an area about 6?104km 2.The Manjiaer Sag,which abuts the Tazhong Uplift in the south,is a sub-structural unit in Northern Depression and is an important

https://www.360docs.net/doc/377417961.html,/locate/marpetgeo

0264-8172/$-see front matter r 2007Elsevier Ltd.All rights reserved.doi:10.1016/j.marpetgeo.2007.01.001

?Corresponding author.Basin and Oil Reservoir Research Center,

China Petroleum University (Beijing),Changping,Beijing 102249,China.Tel.:+8601089733357;fax:+8601089733423.

E-mail address:luxx7752@https://www.360docs.net/doc/377417961.html, (X.X.Lu).

oil source of Tazhong Uplift.Silurian formations onlap the slope between the Manjiaer Sag and the Tazhong Uplift.Abundant bitumen and movable oil are found in the Silurian sandstone on the Tazhong Uplift.

This paper summarizes the analysis of a large amount of data,including 2?2km seismic pro?le interpretation of the Tazhong Uplift,structural evolution pro?le of oil reservoirs by using drilling data and the statistics of the oil and gas shows from 55wells penetrating the Silurian reservoirs.Of those wells,seven wells,i.e.Tazhong-11,Tazhong-111,Tazhong-111H,Tazhong-117,Tazhong-47,Tazhong-471and Tazhong-169,obtained commercial oil ?ows,while seven other wells,i.e.Tazhong-20,Tazhong-12,Tazhong-30,Tazhong-31,Tazhong-35,Tazhong-50and Tazhong-58,yielded low oil ?ows.Most of the wells found Silurian bitumen and heavy crude.Some old wells with good oil and gas show or low-yield oil ?ow were brought under re-appraisal.Two wells (Tazhong-12and Tazhong-50)ob-tained commercial ?ow after sand fracturing.All the wells with oil ?ow or oil and gas shows are located on the northern slope of the uplift (Fig.1).Currently,four oil reservoirs have been found in the Tazhong-11,Tazhong-47,Tazhong-12and Tazhong-169wells.

Division and comparison of sand layers were made on 16wells by using log data (self-potential curve and gamma ray curve)and cuttings log data.Fluid inclusion temperature data were acquired from the Silurian oil legs in wells Tazhong-11and Tazhong-47.The GC/MS analysis para-meters of the extracted saturated hydrocarbon components were obtained from the samples of 13wells (Tazhong-4,Tazhong-6,Tazhong-11,Tazhong-12,Tazhong-16,Taz-hong-20,Tazhong-30,Tazhong-31,Tazhong-32,Tazhong-

35,Tazhong-47,Tazhong-117and Tazhong-167).The burial evolution history was simulated for three wells of Tacan-1,Tazhong-12and Tazhong-47.The results from the analysis of nitrogen compounds in 22bitumen-bearing core samples from nine wells (Tazhong-11,Tazhong-15,Tazhong-30,Tazhong-31,Tazhong-32,Tazhong-44,Taz-hong-47,Tazhong-67and Badong-2)and 16crude samples from nine wells (Tazhong-11,Tazhong-16,Tazhong-30,Tazhong-35,Tazhong-47,Tazhong-50,Tazhong-111,Taz-hong-117and Tazhong-161)were used in the study of the direction of oil migration and the formation of bitumen and movable oil.

The key control factors of oil accumulation are addressed in this paper.Operators for petroleum explora-tion have always had high expectations for the Silurian oil play which is distributed over a large area of the Tazhong Uplift since it appears to have all the factors required for commercial accumulation.However,predicting the dis-tribution of oil in the region is complicated by the occurrence of patchy development of high quality reser-voir,multiple sets of hydrocarbon source rocks,multiple stages of reservoiring,as well as destruction and deforma-tion in the geological history.Bitumen,heavy crude and conventional crude sourced from different stages have been found in the same wells or in the same sandbodies.It is quite common for exploration wells to have oil and gas shows but no oil and gas ?ow.1.Previous work

Since the Silurian system in the Tazhong Uplift became a key target for exploration in 1994,research has been

Fig.1.Schematic map of the top of the Silurian over the Tazhong Uplift showing the distribution of hydrocarbon.

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focused on the processes of oil accumulation in four key areas:

(1)The origin of bitumen in the sandstones.Attempts have

been made to search for the large-scale undestroyed primary oil reservoirs by means of tracing the distribu-tion of bitumen and heavy crude.

Two hypotheses have been proposed to account for the origin of the bitumen.One is that the large-scale oil reservoirs were destroyed and then the crude was exposed and oxidized to bitumen(Liu et al.,1999, 2000).The other is that the oil was oxidized to bitumen in the migration process(Lu et al.,1997).When the oil migrated in the Silurian sandstones,the thickness of the upper beds was only about1000m and had not completely solidi?ed,thus the meteorological fresh water could get in touch with the oil through percolating downward,which led to the oxidation action of oil.Good sealing of cap rock was required in the early period of reservoiring(Shi and Shen,1997).

(2)Oil sources.Oil-to-source correlation indicates that the

Silurian system contains oil from both Cambrian and Middle/Upper Ordovician source rocks(Zhang et al., 2001;Wang et al.,2001).

(3)Based on the measurement of?uid inclusion tempera-

tures and the analysis of burial history,the movable oil currently found is considered to have experienced two stages of emplacement after the formation of bitumen(Xiao et al.,1997;Liu et al.,2001;Li et al., 2002).

(4)A large amount of sedimentological research has been

done to evaluate the sedimentary facies of the highest quality reservoir and to predict reservoir distribution (Liu et al.,1996;Hou et al.,1997;Zhu et al.,2002).The tidal-?at environment is identi?ed as the main facies of Silurian sedimentation.

It has proved rather dif?cult to predict the distribution of the highest quality reservoirs from seismic owing to the small thickness of sandstone in the sand shale interbeds and the deep burial(deeper than4000m)of the target formations under the surface conditions of wide desert.

2.Geological background

The Tazhong Uplift experienced multiple stages of uplift and denudation(Jia et al.,1992;Lu et al.,1997). Sedimentary wave motion analysis method was used to study the structure evolution.Wave motion analysis is a new method for the study of sedimentary basin based on the wave theory of physics.Crustal movement was considered in the way of wave process.Wave motion describes the regular diffusion of stress point move-ment and its condition to time and space.To use this method,one has to get the statistical data of the strata, resume the thickness of the strata,calculate the sedimen-tary velocity and get the wave equation(Jin et al.,1996).The analysis of the wave motion?nds that the Tazhong Uplift undergoes seven major uplifts,which occurred, respectively,in the end of Middle Ordovician,Silurian, Early Devonian,Late Devonian,Early Permain,Jurassic and Early Tertiary.

The Cambrian and Lower Ordovician systems are dominated by carbonate sedimentation(Gu,1994).The Tazhong Uplift took its preliminary shape at the end of the Early Ordovician.The formation underlying the Silurian in wells Tazhong-10,Tazhong-11,Tazhong-12,Tazhong-14, Tazhong-30,Tazhong-31,Tazhong-32and Tazhong-33is Middle and Upper Ordovician mudstone.The formation underlying the Silurian of Tazhong-16,Tazhong-17, Tazhong-18and Tazhong-37is Lower Ordovician lime-stone.

The Silurian system of the Tazhong Uplift is character-ized by onlap against the palaeo-Tazhong Uplift at the bottom and denudation on the top(Fig.2).Uplift occurred at the end of the Silurian Period,eroding part of the Silurian and Middle/Upper Ordovician.The Devonian overlies the Silurian in wells Tazhong-10,Tazhong-20, Tazhong-201,Tazhong-37and Tazhong-18.The Carboni-ferous system lies unconformably above the Silurian in wells Tazhong-4,Tazhong-6,Tazhong-11,Tazhong-12, Tazhong-16and Tazhong-17.

Because of the uplift and the great denudation,some of the Silurian is absent in part of the central section of the Tazhong Uplift,although the strata sequence in the Tazhong northern slope is relatively complete.The Silurian system of the Tazhong northern slope is a tidal-?at sedimentary system(Wang et al.,2004).The pro?le is divided from bottom to top into the Kepingtag Formation, Tataiertag Formation and Yimugantawu Formation (Fig.3).The Tataiertag Formation includes an upper sandstone section and a lower red-colored mudstone section.The red-colored mudstone,about70m in thick-ness,is widespread.The Silurian bitumen and oil and gas shows are largely concentrated in the sandstone reservoirs under the red-colored mudstone section.The upper section of the Kepingtag Formation(also called the bitumen sandstone section)can be further divided into an upper sub-section(upper bitumen sandstone section),a middle sub-section(gray-colored mudstone section)and a lower sub-section(lower bitumen sandstone section).The upper and lower sub-sections have bitumen layers and oil legs.The lithology of the reservoirs is mainly siltstone and ?ne sandstone.

3.Characteristics of oil accumulation

There is a variety of petroleum types in the Silurian reservoirs,including the commercial oil?ow of different crude properties(light crude,conventional crude and heavy crude),bitumen sand(dry and soft),and oil sand. The geochemical analysis shows that the oil accumulation has the characteristics of multiple sources and multiple stages.

X.X.Lu et al./Marine and Petroleum Geology24(2007)189–198191

Fig.3.Generalized stratigraphic column of the Silurian in the Tazhong

Uplift.

Fig.2.Stratigraphic isopach map and schematic geological section of Silurian in the Tazhong Uplift.C,Carboniferous;D,Devonian;S 1y ,Yimugantawu;S 1t ,Tataiertag;S 1k ,Kepingtag;O 2+3,Mid-Ordovician;TZ,Tazhong;N,North.

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3.1.Multiple sources of oil

Based on the study of the biological markers in source rock and crude oil,Liang et al.(1998)proposed that there are two high-quality marine source rocks of Cambrian and Middle/Upper Ordovician in the Tarim Basin.Based on a large amount of GC/MS and GC/MS/MS analyses,Li et al.(1999)found that the Cambrian source rock and associated crude oil in the Tarim Basin contain high contents of dinosterane,4-methyl sterane,C 26–24-norcholesterol,C 28-sterane,gamma-cerane,and low contents of diasterane,while the Middle and Upper Ordovician hydrocarbon source rock and associated crude oil typically have the proportions reversed.

In this paper,the GC/MS and GC/MS/MS analyses were made on ?ve crude oil samples from Silurian reservoirs in wells Tazhong 11(4301–4307,4417–4435m),Tazhong-16(4125–4129m),Tazhong-30(4244.5–4260m)and Tazhong-50(4378–4385m).Using the biomarker characteristics established by Li et al.(1999),the crude oil in the 4301–4307m section in Tazhong-11is interpreted as having a Cambrian source rock.The biomarker parameters in the Silurian-reservoired crude oil from wells Tazhong-16(4125–4129m),Tazhong-30(4244.5–4260m)and Tazhong-50(4378–4385m)vary greatly and appear to indicate that the crude oil is a mixture from the Cambrian source rocks and the Middle/Upper Ordovician source rocks.The crude from well Tazhong-11(4417–4435m)correlates with the Middle/Upper Ordovician source rock.3.2.Multiple stages of reservoiring

The inclusions in Silurian formation in well Tazhong-47are many and are mostly distributed along the suture line in beaded shape.The uniform temperature of the salt water inclusions is divided into three groups—100–120,120–140and 160–2101C (Fig.4),of which the group of 160–2101C indicates the in?uence from the hot ?uid in the deep caused by the intrusions.In addition,the homogenization temperature of organic inclusions is divided into two groups:100–110and 120–1401C.As this is accordant with the homogenization temperature of the salt water inclu-sions,the conclusion that large-scale oil migration occurred twice when the formation temperature was 100–120and 120–1401C,respectively,can be drawn.

Thirteen oil-bearing core samples were extracted con-tinuously.The components of three different states sepa-rated (the free state,adsorbed state and inclusion),and GC and GC/MS analyses were made on these components.The accumulation periods could be judged by correlation of geochemical parameters of the saturated hydrocarbons (such as normal alkanes,terpane and sterane)(Pan et al.,2002).As a result,the movable oil in most samples can be interpreted as a mixture of two emplacement stages.The inclusions of the ?rst stage are characterized by normal crude,integrated n -alkane distribution and the main peak carbon of n C_(18)and of n C_(19).While,the inclusions of the second stage are characterized by higher content of low carbon n -alkane,rise of light component,and the main peak carbon of n C_(15).

Based on modeling the source rock thermal evolution history,the analysis of ?uid inclusions and the analysis of geochemical parameters of components from continuous extraction of core samples,it is considered that the Silurian system in the Tazhong Uplift experienced three stages of oil emplacement.The ?rst stage occurred in the Late Caledonian (at the end of Silurian period—400Ma).Cambrian hydrocarbon source rocks became mature and expulsion began,leading to extensive oil accumulation in the Tazhong Uplift.Major uplift between the ?rst and the second oil emplacement stage resulted in breaching of many traps and the degradation of the oil to bitumen.The second stage occurred in the Late Hercynian (Permian period—240Ma).The analysis of inclusions and burial history modeling indicates that this period of emplacement started from 240Ma.The third stage occurred in the Yanshan-Himalayan (mainly Tertiary period).The analy-sis of inclusions and burial history modeling indicates that this period of emplacement,largely sourced from the Middle/Upper Ordovician,started at 40Ma.3.3.Multiple types of oil trap

There are three types of oil traps currently found in the Tazhong Uplift (Fig.5).Tazhong-11and Tazhong-12were drilled on anticline structures.The structure at Tazhong-47is also an anticline,but it is penetrated by a volcanic and intrusive complex,with lateral sealing against the igneous body.Tazhong-169penetrates pinchout sand body.

Structural traps:a series of near east–west faulted structural belts in the Tazhong Uplift were produced by compressional tectonic activity following Silurian–Devo-nian sedimentation (Jia et al.,1992;Jia,1997;Kang,1992;Tang,1996).All the Silurian anticlines currently found in the Tazhong Uplift have this palaeo-structural setting.The palaeo-structure has persisted till the present time,creating the conditions for capturing multistage oil.

Volcanic rock barrier traps:major volcanic activity took place in the Tarim Basin in the later part of the Early Permian.The volcanic activity was mainly in the west of the Tazhong Uplift (Chen et al.,1997).Magma invaded the palaeo-surface (top of the Lower Permian)along major

20151050

<100

100-120

120-140140-160

160-210

Temperature (°

C)

F r e q u e n c y (%)

Fig. 4.Distribution of homogenization temperatures of salt water inclusions in Silurian sandstones in well Tazhong-47.

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basement faults.Magmatic doming separated the pre-existing anticline into three small traps,one of which was drilled by Tazhong-47.

Lithological pinchout traps:the northern slope of the Tazhong Uplift favors pinchout traps because of the Silurian onlap from the Manjiaer Sag to the Tazhong Uplift.The drilling results of wells Tazhong-9,Tazhong-11and Tazhong-68con?rm the existence of pinchout traps (Fig.2).The poor connectivity of the thin singles and bodies in this trap type is caused mainly by the tidal-?at facies architecture.4.Pattern of oil accumulation

4.1.Preservation conditions held the key to oil accumulation and distribution

There are a number of possible macroscopic and microscopic explanations for the genesis of the bitumen

in the Silurian sandstones,such as microbiological sulfate reduction,biological degradation,gas washing and meteo-ric fresh water leaching (Cai,1997;Liu et al.,2000).Taking into account the time relations between oil accumulation and destruction,the origin of the bitumen can be divided into two categories—bitumen formation after reservoiring and bitumen formation during reservoiring (Lu and Hu,1998).No matter what the origin,the bitumen is always below the red-colored mudstone at the base of the Tataiertag Formation over a large area (Fig.6).

The movable oil is concentrated mainly below the gray-colored mudstone of the middle sub-section of the Kepingtag Formation.The thickness of gray-colored mudstone in the lower sandstone section is usually 20–30m,and,especially,there is no much change in the thickness in the northern slope.The mudstone is a high-quality cap rock for the movable oil currently found in the Tazhong Uplift.The oil reservoirs of Tazhong-11,Taz-hong-47,Tazhong-12and Tazhong-169,discovered at the present time,are all accumulations ?lled in the second (Late Hercynian)and third (Yanshan-Himalayan)periods of emplacement.Bitumen in Silurian reservoirs is mostly concentrated below the red-colored mudstone section of the Tataiertag Formation,indicating that the gray-colored mudstone of the middle sub-section was easily penetrated by oil during the ?rst stage of accumulation.Accordingly the current good sealing capacity of the gray-colored mudstone is probably the result of later diagenetic changes.4.2.Inherited structural background guided direction of oil migration

Since the Ordovician period,the Tazhong Uplift has acted as an inherited growth palaeohigh (Jia,1997).With the Manjiaer Sag to the north is taken as the kitchen for hydrocarbon generation,the Tazhong Uplift appears to have guided the direction of oil migration through its history (Xiao et al.,2000).The analysis of the nitrogen compounds in the crude con?rms that the main migration in the Tazhong region was from northwest to southeast up the palaeoslope of the early Tazhong Uplift (Liu and Kang,1999).This study used 22bitumen core samples from nine wells and 16oil samples from nine wells in the Tazhong Uplift for the analysis of nitrogen compounds.Nitrogen compounds have strong polarity (Bakel and Philp,1990;Wiesner et al.,1990).Nitrogen compounds in oil change regularly with increasing distance of migration (Yamamoto,1992),with which the direction of oil migration can be determined.The trend of variation in the target com-pounds,such as trimethyl carbazole (A)/trimethyl barbazole (C),re?ects the northwest–southeast hydrocarbon migration direction in ancient oil reservoirs and current oil reservoirs.4.3.Connectivity of thin sand body controlled horizontal connection of oil areas

The drilling results con?rm that the Silurian system is a set of thinly interbedded sand-shale sequences of tidal-?at

-3300-3400-3500

-3200-3300

-3200-3100-3000-2900-2800-3300

-3200-3100-3000-2900-2800E l e v a t i o n (m )

E l e v a t i o n (m )

E l e v a t i o n (m )

-3850-3900-3950-4000-4050-4100

V olcanic rock

Fault TZ47 reservoir

Fig.5.Cross sections illustrating the three main types of trap in Tazhong oil ?eld.E,East.

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facies.The thickness of a single sand body is usually3–5m. The lateral distribution of reservoir quality changes rapidly.

The thickness of the poorly connected sand bodies changes greatly from one place to another(Fig.7).Core observations indicate that bitumen and heavy crude are distributed in thin layers.The sand bodies with relatively higher permeability and porosity functioned not only as the migration pathway but also as the reservoir space for oil during the migration process.Sand bodies with low permeability and porosity show no sign of oil.This indicates that lateral oil migration in the sandstone bodies was selective—oil migrating along the highest poroperm pathways(Lu,2000).The connectivity and distribution of sand bodies controlled the lateral connection and distribu-tion of oil.The lack of lateral connectivity in the tidal-?at facies architecture of Silurian sandstone in the Tazhong Uplift made it dif?cult for single large pools to form. However,the thin sand layers are often superimposed vertically.In space,a large amount of individual layers interlink together,and in this way,single sand bodies are connected to form a large-scale reservoir.

4.4.Conducting system of oil migration controlled the

oil-bearing properties of sand body

Potentially,faults,unconformity surfaces,volcanic pier-cing bodies and high-quality sand channels might provide secondary migration pathways from the Cambrian and Middle/Upper Ordovician source rocks to the Silurian reservoirs.Seismic pro?les indicate development of high-angle faults.Silurian oil in the Tazhong Uplift is mainly distributed close to the faults,suggesting that faults were the most important types of vertical conducting system.High-quality sand bodies could be charged with oil only after being penetrated by the faults.Oil?rst passed through the faults and then moved into the sand body even in the case of lithological pinchout oil reservoirs.Then,oil passed through the sand bodies and migrated laterally for accumulation(Fig.8).In addition,the oil in the Tazhong Uplift had another conducting system—an intrusive body. The intrusion pierced the source rock and sand bodies so that oil migrated vertically up along the walls of the intrusion to accumulate in those sand bodies with relatively high permeability(such as Tazhong-47).

5.Conclusions and discussion

(1)Multiple stages of reservoiring and multiple sources of oil:Hydrocarbons in the Silurian play in the Tazhong Uplift originate from both Cambrian and Ordovician source rocks.The petroleum now represented by bitumen was generated from the Cambrian source rock while movable oil mainly came from the Ordovician source rock. Analysis of both organic inclusions and geochemical parameters of different components from continuous extraction of core samples indicate that bitumen and oil in Silurian reservoirs are the result of three stages(the end of Silurian period,Permian period and mainly Tertiary period)of?lling.Analysis of nitrogen-bearing organic indicates that migration was always southeastwards from the Manjiaer Sag to the northwest up the northwestern palaeoslope of the palaeo-Tazhong Uplift.

(2)Effective cap rock:The red-colored mudstone at the base of the Tataiertag Formation forms the top boundary of bitumen-bearing sandstone and appears to have been an effective caprock for palaeo-pools during the?rst(Late Caledonian)emplacement phase.There is no evidence that

Fig.6.East–west cross section showing the distribution of bitumen and oil shows in the Silurian of the Tazhong Uplift.TZ,Tazhong;E,East.

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the underlying gray-colored mudstone within the Keping-tag Formation was effective at this time.However,it forms the top boundary of movable oil now and thus appears to have been effective during the second (Late Hercynian)and third (Yanshan-Himalayan)emplacement phases.Although detailed analysis is not available,it is thought that diagenesis between the ?rst and second emplacement phases are the most likely cause of permeability reduction that increased its effectiveness.

(3)Characteristics of oil migration and accumulation :Bitumen and oil accumulation are controlled by the physical properties and spatial organization of the reser-voirs.The oil legs were relatively thin in all the forenamed three types of traps (structural traps,volcanic rock barrier

traps and lithological pinchout traps),but generally occurred in multiple stacked sets.For single sand bodies in a tidal-?at environment,as here,high quality reservoir is thin and of limited lateral extent,and although some lateral continuity is produced by superimposition of sand bodies,the net result is that oil abundance is not high.Never-theless,when those high-quality sand bodies were linked to the source rocks by the faults or volcanic channels,oil could migrate and accumulate over a large area.

Therefore,the basic characteristics for Silurian oil distribu-tion in the Tazhong Uplift can be summarized as follows:(A)Silurian sandstones are preserved over a large part of

the northwest slope of the palaeo-expression of the

Fig.7.Sand interval correlation chart for the upper member of the Silurian Kepingtag Formation in the Tazhong 14and 15

wells.

Fig.8.Integrated model of hydrocarbon accumulation in the Silurian upper interval of Kepingtage on the palaeohigh background.SE,southeast.

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Tazhong Uplift,which they onlap to the southeast.

Reservoir is composed of tidal sandstone bodies that, individually,have limited lateral and vertical continu-ity.Stacking patterns,faulting and,locally,volcanic channels,to some extent enhance connectivity but small pools are typical.

(B)The reservoir was charged three times in its history

(Late Caledonian,Late Hercynian and Yanshan-Himalayan),from two source rocks(Cambrian and Ordovician)which matured in the Manjiaer Sag to the north.Migration was always from northwest to southeast within the Silurian section.Petroleum types are variable across the region.

(C)The red-colored mudstone at the base of the Tataier-

tag Formation and the underlying gray-colored mudstone within the Kepingtag Formation were the principal effective caprocks but were effective at different times:the red-colored mudstone during the ?rst(Late Caledonian)charge and the gray-colored mudstone during the second and third phases(Late Hercynian and Yanshan-Himalayan).The?rst migra-tion-emplacement phase was followed by a period of degradation of the oil pools to bitumen.Accordingly, movable oil is currently found largely below the gray-colored mudstone.

(D)Traps are largely formed by faulted anticlines but,

locally,lithological pinchout and abutment against volcanic piercing bodies are effective. Acknowledgments

This project is funded by the State development plan for key basic research(Serial number:G1999043308).Tarim Oil Field Company,PetroChina provided the basic materials such as core and crude samples.The State Key Organic Geochemistry Laboratory of Guangzhou Geo-chemistry Research Institute,Chinese Academy of Sciences conducted GC/MS analysis of crude oil.The State key Heavy Crude laboratory of China Petroleum University analyzed nitrogen-containing compounds.Thanks are extended to them for their help in this research project. The authors further acknowledge Norman Oxtoby and David G.Roberts for the constructive comments. References

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