Table of Content

25 December 2005, Volume 26 Issue 6

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Analysis of highly efficient gas reservoiring process in Kuqa depression

Zhao Wenzhi, Wang Hongjun, Shan Jiazeng, Wang Zhaoyun, Zhao Changyi, Wang Zecheng

2005, 26(6):  703-710.  doi:10.11743/ogg20050601

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Kuqa depression in Tarim basin has the basic geologic conditions to form medium and large gas pools.The Triassic-Jurassic source rocks not only have very large cumulative gas generating intensity,but also have experienced a very rapid gas generating and discharging process since Neogene;thus they have become highly efficient gas kitchens and played a key role in the formation of Kela 2 type high efficient gas pools.The piedmont thrust zone in Kuqa depression has been strongly deformed due to the compression since Neogene.Vertical superimposition of Cretaceous-Jurassic strata below the halite resulted in increment of rock volume.Being sealed by good caprock,the rock volume enlargement would lead to structural swabbing,an important process for the formation of Kela 2 type high efficient gas pools.Simulation experiments of hydrocarbon-generating kinetics and tectophysics have proved these new understandings and further revealed the effective gas generation of Jurassic source rocks and effective gas migration and accumulation process in Kuqa depression.

Time-space structure of reservoiring elements and hydrocarbon enrichment—concurrently discussing near-source reservoiring

Zhou Xingxi

2005, 26(6):  711-716.  doi:10.11743/ogg20050602

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Time-space structures of reservoiring elements and process influence the level of hydrocarbon enrichment.In plays that are adjacent to the source rocks and regional cap rocks,when the reservoiring occurred close to the hydrocarbon-generating period,the reservoirs would have relatively high hydrocarbon enrichment.Most of the major reservoirs in a basin or petroleum system belong to this type of reservoirs.Vertically,if there is no large and prominent tectonic events or depositional breaks between the source and reservoir rocks,we call it near-source combination,and its specific time interval is preliminarily determined to be no longer than an "Age"(in Mesozoic and Cenozoic)or an "Epoch"(in Paleozoic);otherwise we call it far-source combination. Laterally,near-source and far-source areas can be defined according to their location to be within or outside the effective source rock area—hydrocarbon kitchen.The available exploration results indicate that most of the discovered large and medium oil and gas fields in Tarim basin,such as Hadson oilfield,Lungu-Tahe oil/gas field,Kela 2 gas field and Yaha oil/gas field,are(indigenous-or)near-source combinations.Over 95% of the proved oil reserves in Songliao basin is distributed in Saertu,Putaohua and Gaotaizi oil reservoirs between the major source and cap rocks.Therefore,"near-source exploration" is an important and practical principle for discovering prolific oil and gas enrichment areas.Exploration of near-source oil and gas fields is commonly very fruitful.However,most of the near-source oil and gas fields are distributed in structural zones with high exploration difficulties,in complex plays or in deep and ultra-deep horizons of the exploration area,thus they are commonly discovered behind the "far-source" oil and gas reservoirs.

A preliminary discussion on geochemical dynamic tracing of oil/gas reservoiring process—taking Dawanqi structure in Kuqa depression as an example

Liu Wenhui, Zhang Dianwei, Zheng Jianjing, Chen Mengjin, Wang Xiaofeng, Gao Bo

2005, 26(6):  717-723,729.  doi:10.11743/ogg20050603

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In Dawanqi oil/gas pool in Kuqa depression,Tarim basin,the enrichment of ³⁸Ar was related to the migration of anoxic fluids and volatile constituents along large faults.Tuzimazha fault is the major pathway,through which ³⁸Ar generated from fluid activities would have migrated to the oil and gas reservoirs.The low concentration of ³⁸Ar in Dabei reservoir reveals that gas would have been reservoired near the source rocks.Dawanqi oil/gas field is not an oil/gas pool as a result of later gas invasion,but is a primary oil/gas pool.The light components of gas partially dispersed along the fault due to the poor sealing ability of the trap at later reservoiring stage,leading to the formation of the current condensate gas reservoir with relatively low saturation.Hydrocarbons were generated from the Triassic and Jurassic major source rocks below,and were accumulated in the reservoir rocks above,i.e.,hydrocarbons have been generated from the organic matters in the Mesozoic fresh lake facies sediments during mature to high-mature evolutional phases.During the reseroviring process of Dawanqi oil/gas pool,hydrocarbons first migrated upward along Tuzimazha fault,and then entered Dawanqi structure from north to south.Dawanqi oil/gas pool has experienced early and late charging processes.The high-mature to over-mature natural gas charged in later stage would have mainly accumulated in the upper part,due to their relatively low density;while the relatively high density natural gas generated in early stage would have accumulated in the lower part.

Study of carrier system of typical gas reservoirs in China—taking Sulige gas field in Ordos basin as an example

Zhu Xiaomin, Liu Chenglin, Zeng Qingmeng, Men Xiangyong

2005, 26(6):  724-729.  doi:10.11743/ogg20050604

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For gas reservoirs in the fold-thrust belts of foreland basins and in rift basins,faults constitute the main carrier systems,and hydrocarbons migrate mainly in vertical direction with short migration distance and high delivery efficiency.While for lithologic gas reservoirs in craton basins and gas reservoirs in slope zones of foreland basins,sandbodies act as the main carrier system,and hydrocarbons migrate both laterally and vertically with relatively long migration distance and low delivery efficiency.Sulige gas field is of lithologic gas reservoirs in craton basin,its main carrier system is of sandbody type,with a few fault type and sandbody-fault type carrier systems.The sandbody carrier system can be subdivided into thick belt sandbody,thin belt sandbody and lenticular sandbody.The delivery capacity of carrier system varied with time.It peaked in 200-150 Ma during the strongest dissolution and faulting.At the critical reservoiring period(J₃-K₁),the framework of carrier system was high in the west and north and low in the east and south,and the major carrier system consisted of sandbodies,faults and fractures.Hydrocarbons migrated northwestward with high delivery efficiency.While in the post-reservoiring adjustment period(K₂-Q),the framework of carrier system was basically high in the east and north and low in the west and south,and the major carrier system consisted of sandbody and fractures.Hydrocarbon migration efficiency was low and not evident.

Elastic properties of gas-bearing strata and seismic response

Wang Shangxu

2005, 26(6):  730-735.  doi:10.11743/ogg20050605

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Based on analysis of variation of elastic properties of natural gas and formation water under reservoir conditions,calculation of compressional velocity and shear velocity through gas-bearing strata,and physical si-mulation and experimental analysis,it is believed that gas-bearing strata are characterized by the following seismic responses: compresional velocity of gas with larger specific gravity is higher than that of gas with smaller specific gravity;compresional velocity in rocks containing saline water increases with increasing water salinity;and fluid properties have relatively large influence on compressional velocity,but have little influence on shear velocity.There would be a certain different seismic responses in strata bearing different fluids.Reflected energy in gas-bearing strata is relatively strong,while that in water-bearing strata is relatively weak.Seismic response in he-terogeneous gas-bearing strata would be characterized by relatively well developed diffracted wave.

Distribution and occurrence of kerogen fatty acids in low mature marl and calcareous mudstone

Meng Qianxiang, Zheng Jianjing, Fang Xuan, Sun Minzhuo

2005, 26(6):  736-741.  doi:10.11743/ogg20050607

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Three acid series,including straight chain monoacid,straight chain binary acid and aromatic acid,have been detected in the kerogen of low mature Tertiary source rocks with low organic carbon content in Qaidam basin.Relative abundance of straight chain monoacid is the highest among these three acid series,and C_16:0 acid occurs as the main peak(saturated fatty acid),with obvious even carbon number predominance.The volume of kerogen fatty acid is over three times that of enhanced kerogen fatty acid,and the former has much larger relative abundance of straight chain binary acid than the latter.In addition,the volume of fatty acid that enrich in carbonate crystals of source rocks in the form of calcium(magnesium)fatty acid salt and fatty acid is far greater than that occur in kerogen network.When these carbonate crystals were decomposed at high temperature(and at complex geologic conditions),the fatty acid salt and fatty acid might be cracked into small molecular hydrocarbon,i.e.,gaseous hydrocarbon.The sulfur in deposited organic matters in the study area occur as pentabasic,hexabasic and octatomic ring stable states,and would make contribution to the enrichment and existence of fatty acid(salt)due to its strong electron negativity.

Comprehensive evaluation of sealing ability of cap rock in China's large and medium gas fields and their contribution to gas accumulation

L? Yanfang, Fu Guang, Yu Dan

2005, 26(6):  742-745,753.  doi:10.11743/ogg20050608

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Sealing ability of cap rock in large and medium gas fields mainly depends on the thickness of cap rock,displacement pressure,interior energy of gas reservoir(pressure coefficient)and gas property(flowing viscosity).Cap rock sealing index(CSI)of gas reservoir is directly proportional to gas accumulation efficiency.The accumulation efficiency of China's large and medium gas fields can be classified into three categories:(1)High efficiency gas reservoirs have accumulation efficiency of over 100?10⁶ m³/(km²·Ma).They are mainly distributed in Tarim,Qaidam,Yinggehai-Qiongzhou,Bohai Bay and Songliao basins,of which Tarim,Qaidam and Yinggehai-Qiongzhou basins are in the majority.(2)Moderate efficiency gas reservoirs have accumulation efficiency in the range of 10?10⁶-100?10⁶ m³/(km²·Ma).They are mainly distributed in Sichuan,East China Sea,Songliao,Bohai Bay,Tarim and Turpan-Hami basins,and most of them are in Sichuan basin.(3)Low efficiency gas reservoirs have accumulation efficiency of lower than 10?10⁶ m³/(km²·Ma).They are mainly distributed in Ordos and Sichuan basins.For the formation of large and medium gas fields in China,the CSI value of cap rocks should be over 10⁹m/s.For gas fields with relatively higher accumulation efficiency,however,the CSI value of cap rocks should be raised even higher.

Characteristics of diagenesis and pore evolution of gas reservoirs formed in different tectonic settings

Liu Chenglin, Zhu Xiaomin, Zhu Yuxin, Zeng Qingmeng

2005, 26(6):  746-753.  doi:10.11743/ogg20050609

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Gas reservoirs in craton basins have experienced multicyclic tectonic movement during the long geologic history,resulting in varied diagenetic environments,complex diagenesis and long-term superimposition.Their pores have experienced several cycles of evolution and the present porosities of reservoirs are low.According to the structural location,trap type and reservoir nature,the gas reservoirs can be further subdivided into three types,including palaeohigh structural gas reservoirs,anticlinal gas reservoirs,and lithologic gas reservoirs.Their diagenetic sequences and pore evolutions are somewhat different.Gas reservoirs in the thrust belt and slope zone of foreland basin are characterized by their relatively short geologic histories,relatively less tectonic cycles,relatively rapid burial and relatively simple diagenetic sequence and pore evolution,so the present porosities are relatively small.While gas reservoirs in rift basin are characterized by their short geologic histories,less tectonic(cycles),rapid burial and simple diagenetic sequence and pore evolution,so the reservoirs have relatively large(porosities.)

Gas accumulation efficiency and its main controlling factors in China's large and medium gas fields

Fu Guang, Lu Yanfang, Yu Dan

2005, 26(6):  754-759.  doi:10.11743/ogg20050610

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Statistics of gas in place,gas-bearing area and reservoiring period of over 60 large and medium gas fields in China show that their accumulation efficiencies vary greatly.China's large and medium gas fields can be classified into 3 types,including high,moderate and low efficiency gas fields,according to their accumulation efficiencies.Gas accumulation efficiency of high efficiency gas fields is over 100?10⁶ m³ /(Ma·km²),that of moderate efficiency gas fields is in the range of 10?10⁶-100?10⁶ m³ /(Ma·km²),and that of low efficiency gas fields is below 10?10⁶ m³ /(Ma·km²).The gas accumulation efficiencies of China's large and medium gas fields are mainly controlled by gas generating intensity of source rocks,gas delivery rate,sealing(ability)and timing of gas reservoiring period.The larger gas generating intensity,higher gas delivery speed,larger comprehensive sealing index of cap rocks(CSI),and later gas reservoiring period would lead to higher gas accumulation efficiency,and vice versa.

Influence of lithology,porosity and variation of pore fluid saturation on rock elastic property

Li Shengjie

2005, 26(6):  760-764.  doi:10.11743/ogg20050611

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A relational expression between fluid modulus and bulk modulus of fluid-bearing rocks is derived based on the Gassmann's equation.It reflects the influences of such factors as fluid modulus and lithology on the elastic properties of fluid-bearing rocks in the process of fluid displacement.Theoretical calculation and analysis show that fluid variation has relatively small influences on rocks with relatively large rigidity(hard),and has relatively large influences on rocks with relatively small rigidity(soft).The lower the rock porosity is,the more sensitive the bulk modulus of fluid-bearing rocks is to fluid variation,i.e.,the compressional velocity of seismic wave decreases with increasing rock porosity.When rock porosity and fluid modulus are relatively small,compressional velocity is relatively more sensitive to variation of fluid saturation.The results of fluid displacement experiments in real rock samples agree with the theoretical analysis.

Reservoiring process and accumulation efficiency of Feixianguan Formation gas pool in Luojiazhai,northeastern Sichuan basin

Xie Zengye, Li Jian, Shan Xiuqin, Zhang Lin, Gao Jiayu

2005, 26(6):  765-769.  doi:10.11743/ogg20050612

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Luojiazhai gas field in Lower Triassic Feixianguan Fm is the third largest gas field in Sichuan basin with gas reserves of over 50 BCM.The natural gas is overmature high H₂S dry gas,with methane content gene-rally in the range of 75.29%-84.95%,ethane content in the range of 0.07%-0.11%,and hydrogen sulfide content in the range of 7.13%-10.49%.Natural gas in Luojiazhai gas field are mainly sourced from the Upper Permian and enriched in the structural traps overlain by the(Lower-Middle)Triassic regional gypsum-halite cap rock,and are controlled by faults.This gas field is of vertical migration and accumulation system with near-normal pressure and allogenetic gas source,i.e.,hydrocarbons have been generated from the source rocks below,accumulated in the reservoirs above and sealed by caprock on the top.It is superior in reservoiring conditions.Bitumens that commonly occur in the reservoirs are the intermediate products resulting from pyrolysis of crude oil into gas in fossil oil reservoirs,and the content of bitumen is positively correlated with porosity and gas deliverability.The gas pool has experienced several processes,such as hydrocarbon accumulating in oolitic rocks and liquid hydrocarbon being deeply buried and pyrolyzed during Late(Indosinian-Early)and Middle Yan-shanian,as well as gas accumulation controlled by structure in the course of structure formation and local adjustment of fluids during Himalayan.(Middle-Late)Jurassic is the main hydrocarbon charging period. The high accumulation efficiency of Luojiazhai gas field has been ensured by the highly efficient carrier system,good reservoir space and favorable sealing conditions.

Evolution of Mesozoic source rock's organic maturation in Kuqa foreland basin and its influence factors

Yang Shuchun, Lu Qingzhi, Song Chuanzhen, Yuan Yusong, Wang Hongjun, Hu Shengbiao

2005, 26(6):  770-777,785.  doi:10.11743/ogg20050613

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The main body of Jurassic source rock in Kuqa foreland basin is in its peak of oil generation,with R_o value in the range of 0.55%-1.88% and average value of 0.93%.While that of Triassic source rock is in its late period of oil generation,with R_o value in the range of 0.60%-2.59% and average value of 1.1%.The(available)borehole temperature measurements indicate that the temperatures at 2 000,4 000 and 6 000 m in Kuqa foreland basin vary in the range of 40-60℃,60-90℃,and 100-150℃,respectively.Thermal history restoration shows that Kuqa foreland basin has always been in relatively low heat flow conditions since Triassic. The surface heat flow was relatively high in Mesozoic(50-55 mW/m²)and has been continuously and slowly lowering since Cenozoic and finally to the contemporary 40-50 mW/m².The Triassic and Jurassic source rocks entered into low maturity stage in 160-170 Ma and 100 Ma respectively,and entered into gas generation stage in 145 Ma and 75 Ma respectively.The earliest area that entered into low maturity stage is Kelasu,followed by Qiulitage structural zone,and Baicheng sag was the last one.The maturity of organic matter began to increase remarkably since 23 Ma,and both the Triassic and Jurassic source rocks are in their gas generation stage now.The maturity of source rock is mainly controlled by sedimentary and burial history and thermal history of the basin.Overthrusting and gypsiferous rock have also certain influences on maturity of source rock,the intensity of which depends on the displacement of overthrust fault and the thickness and lateral variation of gypsiferous rock.

Relationship between distribution of fatty acids and gas-generating potential in Quaternary in Qaidam basin

Kang Yan, Wang Wanchun, Zhang Daowei, Ren Junhu

2005, 26(6):  778-785.  doi:10.11743/ogg20050614

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Fatty acids(methyl ester)of dissoluble organic matter in Quaternary mudstone samples obtained from 540-1 490 m interval in Sebei area of Qaidam basin are analyzed through GC-MS.Even-odd predominance index of normal fatty acids varies greatly in 1 200-1 381 m interval.It is caused by various factors,of which the input of organic materials and bacterial action are dominant.In intervals of 796-810.8 m,1 188.24-1 210 m and 1 291.54-1 300.74 m,several biomarker indexes of fatty acids,such as content of fatty acids,even-odd predominance index and component dispersancy index,are at their valleys,as the major organic matters input in these intervals are of aquatic organism type,which are easy to be degraded by bacteria.All these indicate that source rocks dominated by indigenously sourced organic matter type have relatively high gas generating potential.Abundance of unsaturated acid and isomeric acid are discovered in 1 188.24-1 210 m interval,indicating that the geochemical environment and type of gas source rock in this interval are suitable for living and growth of bacteria.This specific fatty acid distribution might indicate high quality gas source rocks.

Genesis of authigenic kaolinite and gas accumulation in Bashijiqike Fm sandstone in Kuqa thrust belt

Zou Huayao, Hao Fang, Liu Guangdi, Sui Cheng

2005, 26(6):  786-791,799.  doi:10.11743/ogg20050616

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Kela 2 structure is the largest gas pool in Kuqa thrust belt and authigenic kaolinites are best developed in Cretaceous reservoir,with an average relative content of over 20%,and occur as book and vermicular kaoli-nites between the skeletal grains.While authigenic kaolinites are underdeveloped in Kela 3,Kela 1 and Tubei 1 traps where exploration failed.In Kela 2 structure,authigenic kaolinites have been precipitated from the dissolution of feldspar in the stratal acidic media,which have been associated with the formation of secondary pores.The diagenetic environment was in an open or half-open flow system,which allowed both the charge of acidic fluid from the underlying source rocks and the discharge of dissolved feldspar and other materials.The formation of authigenic kaolinite and secondary pores and charging of natural gas are two associated processes with closely related mechanisms.Through-going and linking-up faults pierced downward through source rock and broke upward through gypsum and halite cap rocks and constituted the major migration pathway of overpressured fluid.Areas with through-going faults had not only been provided with conditions for formation of authigenic kaolinite and secondary pores,but were also favorable for gas accumulation.Comparison of fault carrier types shows that Kelasu structural zone is more favorable for gas accumulation than Qiulitake structural zone.

Distribution characteristics and formation process of Upper Paleozoic abnormally low pressure zones in Ordos basin

Yuan Jihua, Liu Guangdi

2005, 26(6):  792-799.  doi:10.11743/ogg20050617

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In Ordos basin,abnormally low and low pressures are common,while high pressure is rare in Upper Paleozoic.Laterally,the Upper Paleozoic in Suligemiao area displays abnormally low and low pressures,while that in Mizhi area dispalys normal and high pressures,and the other areas mainly display normal pressure and low pressure.Vertically,pressure of acquifers basically distribute along the trend line of hydrostatic pressure;while that of gas-bearing acquifers and gas-bearing formations deviate from the trend line of hydrostatic pressure,the latter has larger extent of deviation than the former,and the pressure coefficient decreases with increasing depth.Both the test data of inclusions and the burial-denudation data of the basin indicate that the Upper Paeozoic gas pools with abnormally low pressure in Ordos basin would have been formed in two stages: gas charging and pressurizing from Late Jurassic to Early Cretaceous,and gas-dispersion,temperature lowering and depressurizing stage from the end of Early Cretaceous up to present time.The depressurization of gas pool would have mainly resulted from basin uplift,temperature lowering,gas dispersion and density contrast of gas and water.

Characteristics of fluid inclusions in reservoirs and its application in study of natural gas reservoiring in southern West Sichuan foreland basin

Zhang Li, Liu Guangdi, Xie Zengye, Gao Jiayu, Shan Xiuqin

2005, 26(6):  800-807.  doi:10.11743/ogg20050618

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In the Upper Tertiary primary gas reservoirs and Jurassic secondary gas reservoirs in Qiongxi,Pingluo-ba and Baimamiao gas fields in southern West Sichuan foreland basin,organic inclusions are mainly in gas phase and gas-fluid biphase.They are usually irregular in shape,with dominant size in the range of 3-6 μm, and are mainly distributed in the fissures of quartz grains and in the calcite veins.The homogenization temperature of the fluid inclusions in the 2nd member of Xujiahe Fm is in the range of 60-170℃,with the main peak to be 90-130℃,while that in Jurassic is in the range of 70-130℃,with the main peak to be 80-100℃.Based on the results of homogenization temperature and in combination with the analysis of evolutional history of Triassic source rocks,it is believed that the charging of gas in reservoirs in the 2nd member of Xujiahe Fm is earlier than that in Jurassic gas reservoirs.The gas reservoirs in the 2nd member of Xujiahe Fm has experienced a complete hydrocarbon charging process from small amount of charging in late Indosinian,to large amount of charging in Yanshanian and to adjustment and minor charging in Himalayan,of which Yanshanian(Jurassic-Cretaceous)is the major charging period.The Jurassic gas reservoirs are secondary gas accumulations,with Himalayan as the main charging period.Based on reconstruction of palaeo-fluid potentials of inclusions formed in different geologic periods,it is believed that the fluid potentials of gas reservoirs in the 2nd member of Xujiahe Fm in southern West Sichuan foreland basin are in random distribution.Gas migration is dominated by vertical migration between source rocks and reservoir rocks with unevident lateral migration.

Sedimentary facies and sequence stratigraphy of Donghe Sandstone in Akekule Uplift

Guo Jianhua, Zhu Meiheng, Liu Chensheng, Shi Yuanyuan

2005, 26(6):  808-815.  doi:10.11743/ogg20050621

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Donghe sandstone in Akekule,composed of mud-pebbly sandstone,well sorted(medium-to)fine-grained sandstones,siltstone and mudstone,is a normal sequence with a complete transgressive-regressive cycle,the top and bottom of which are defined by unconformities,i.e.,type Ⅰsequence boundaries.Donghe sandstone can evidently be divided,from bottom to top,into lowstand system tract,transgressive system tract and highstand system tract.As the lowstand system tract is subaerial fluvial deposit,it is underdeveloped in the basin with limited distribution.The transgressive system tract is mainly composed of shore deposit controlled by tide.A series of shallowing and fining upward parasequences constitute the retrograding parasequence group.The parasequences gradually change upward from subtidal sandbar in the lower part to intertidal sand flat,mixed sand and mud flat,and mud flat.The highstand system tract is mainly distributed in the upper Donghe sandstone.Duing the continuous deposition of Donghe sandstone, the inshore sea water circulated smoothly and(unimpededly)after the largest marine flooding surface was reached,leading to the formation of wave-controlled shore deposition system with wave action as the main geologic agent,which is mainly characterized by grayish white(medium-to)fine-grained sandstone. They constitute the characteristic "Donghe sandstone".

Depth imaging technique in complex conditions

Fang Wubao, Sun Jianguo

2005, 26(6):  816-822.  doi:10.11743/ogg20050622

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Wave equation-based prestack depth migration technique can accurately describe the wave field in complex media conditions and perform high resolution imaging.Its key techniques include datum correction through upward and downward continuations of wave field,velocity model modification of imaging points based on common imaging gather in depth domain,and migration anti-alias filter through interpolation of migration data before and after application of imaging condition equation.Practical application shows that prestack depth migration technique based on wave equation is obviously better than conventional techniques in respects of resolution of wave-groups,detail of fault,imaging accuracy of deep target and preservation of amplitude,etc.

Estimates of shallow biogas resources in Hangzhou Bay area and its geological implications

Lin Chunming, Zhuo Hongchun, Li Guangyue, Chen Haiyun

2005, 26(6):  823-830,839.  doi:10.11743/ogg20050623

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Late Quaternary gas source rocks in Hangzhou Bay are mainly of flood-plain and shallow marine fa-cies.The total amount of gas generated from the source rocks in flood-plain facies is estimated to be 103.252 BCM,with the total adsorbed gas 27.466 BCM,total dissolved gas 69.31 BCM,total diffused gas 429 MCM,and total free gas 6.047 BCM.While those from the source rocks in shallow marine facies are 141.275BCM,55.668 BCM,74.983 BCM,1.028 BCMand 9.596 BCM,respectively.These estimations show that the biogas generated in shallow layers in the study area would have first been dissolved in formation water and adsorbed by clays,and most of the gas dissolved in water in clay layers would migrate in the form of dissolved gas.Free methane can appear only when it has been saturated in the formation water.Free gas mainly occurs in the inter-granular spaces of sediments.The amount of free gas migration in shallow layers is much smaller than that of dissolved gas.The amount of free gas in flood-plain facies is less than that in shallow marine facies.The total amount of adsorbed gas in shallow marine facies sediments is over twice as large as that in flood-plain facies sediments,while the amount of adsorbed gas in mudstone is over 18 times of that in sandstone.All these indicate that preservation of methane is related to the grain sizes of sediments in shallow layers,and small grain size and large specific surface area are favorable for adsorption of methane. These estimates also indicate that Hangzhou Bay has large potentials for exploration and development of shallow biogas.

Types and characteristics of Jurassic prototype basins in southwestern Tarim basin

Qin Du

2005, 26(6):  831-839.  doi:10.11743/ogg20050624

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Southwestern Tarim basin is located in the superposition area of Pal-Asia structural domain and Thetys structural domain.Laterally, Jurassic is in zonal distribution on the southern edge of southwestern Tarim,where a faulted basin system occurred in Jurassic period and might connected with the faulted basin on the southern edge of Karakum landmass to the west.During Jurassic period,deep water sedimentary area was located in a place where the marginal major normal fault had the largest displacement,while fluvial and swamp depositional environments were located on the superposition area of adjacent faults or in a place where the major fault had relatively small displacement.The largest fault displacement usually occurred in the middle part of the fault,and gradually decreased toward both ends of the fault,thus the subsidence occurring on the hanging wall would also decrease,and correspondingly,there would be the sedimentary environments of fluvial or alluvial fan fa-(cies).In addition to the piedmont zone,the Jurassic strata in southwestern Tarim were also developed in a series of fault-controlled lake basins.The development of oil source rocks had been confined to the semi-deep lacustrine sediments.

Characteristics of petroleum geology and formation conditions of large oil and gas fields below halite in southern Pre-Caspian basin

Li Yonghong, Burlin Y K

2005, 26(6):  840-846.  doi:10.11743/ogg20050625

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The series of strata below the halite have favourable petroleum geological conditions in southern Pre-Caspian basin.The extensively developed mudstones and marls have large hydrocarbon generating potentials,the high quality carbonate reservoirs,especially the bioherms,are widely distributed,and the thick halite cap-rocks in Kungurian Fm are well developed in the whole basin except some local areas on the basin margin.Therefore,the source-reservoir-cap rock combination is very good in this basin.The distribution of oil(gas)pools and fields are controlled by the development and distribution of traps and high quality carbonate reservoirs of shallow water platform facies(such as Astrakhan condensate gas field)and biohermal facies(such as Tengiz and Kashagan oil fields)that developed on the long-term stable palaeohighs and palaeoslopes below the thick halite layers.The western South-Enba depression(the northern part of Caspian sea)is the most favourable area for exploration of oil and gas pools below the halite in southern Pre-Caspian basin,followed by the eastern South-Enba depression,the northern Caspian uplift zone and Biikzhal uplift zone,while the Astrakhan quaquaversal dome with well developed deep carbonates,the southern Astrakhan structural zone and the South-Enba uplift are relatively favourable exploration areas.