Table of Content

25 December 2004, Volume 25 Issue 6

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Evolution of Meso-Cenozoic Qaidam basin and its control on oil and gas

Jin Zhijun, Zhang Mingli, Tang Liangjie, Li Jingchang

2004, 25(6):  603-608.  doi:10.11743/ogg20040601

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The Meso Cenozoic Qaidam basin has experienced four evolution stages including the Early to Middle Jurassic chasmic stage, the Late Jurassic to Cretaceous compression stage, the Early Tertiary (Lulehe age) to Late Pliocene (Shangyoushashan age) regional compressive downwarping and local strike slipping and squeezing out stage, as well as the Late Pliocene (Shizigou age) to Quaternary compressive napping stage. The Middle and Lower Jurassic source rocks were deposited in the northern edge of Qaidam basin during the chasmic stage. The Tertiary source rocks were deposited while the massive deppression developed in western basin during the compressive downwarping stage. The Quaternary source rocks were deposited in the Quaternary depression in eastern basin along with the uplifting of western basin and the strong subsidence of eastern basin during the compressive napping stage. Fault and fold structures are well developed in Qaidam basin. Structural reservoirs are the main reservoir types, of which anticlinal and fault nose reservoirs are mainly distributed in Mangya sag in western basin, while the fault-related fault nose and fault block reservoirs are mainly distributed in the downfaulted area in the northern edge of Qaidam basin. The discovery of bubble inclusions in the Tertiary indicates that pulse-migration of hydrocarbons under faulting is an important mode of reservoiring in Qaidam basin.

Petrologic characteristics of igneous rock reservoirs and their research orientation

Zhao Hailing, Liu Zhenwen, Li Jian, Di Yongjun, Luo Xia, Liu Qinghua, Zhao Guoquan

2004, 25(6):  609-613.  doi:10.11743/ogg20040602

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Igneous rock reservoirs are characterized by their extensive distribution, long geologic time and no exclusiveness of rock types. The primary pores, such as intercrystal pores and shrinkage pores, formed while igneous rocks cool down are not influenced by compaction. These characteristics, being different with that of sedimentary reservoirs, are very favorable for seeking deep oil and gas accumulations. In view of the problems concerning the characteristics, formation and evolution mechanism, time space distribution and prediction of igneous rock reservoirs, it is proposed that igneous petrology, volcanic geology, thermodynamics, physical properties of magma and hydrodynamics, as well as geophysics should be strengthened in study of igneous rock reservoirs.

Main factors controlling hydrocarbon accumulation in Qaidam basin

Dang Yuqi, Xiong Jihui, Liu Zhen, Ma Dade, Wang Liqun, Li Weilian

2004, 25(6):  614-619.  doi:10.11743/ogg20040603

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Systematic study of the geological conditions and processes of hydrocarbon accumulations in Qaidam basin shows that there were six main factors controlling hydrocarbon accumulations. (1) Hydrocarbon generating sag controlled the distribution pattern of oil and gas. (2) Reservoir quality influenced the formation of oil and gas reservoirs. (3) Abnormal hydrodynamic field controlled the migration and accumulation of hydrocarbons. (4) Faults in source rocks controlled the formation of oil and gas reservoirs. (5) Middle and late Himalayan movements determined the formation and development of structural traps. (6) Occurrence of trap belts determined the specific distribution of pools.

Nature of Mesozoic basins in the northern edge of Qaidam basin

Liu Zhihong, Yang Jianguo, Wan Chuanbiao, Liu Zhenwen, Zhang Liguo

2004, 25(6):  620-624.  doi:10.11743/ogg20040604

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A series of half graben like faulted basins were developed in Jurassic in the northern edge of Qaidam basin, which was mainly controlled by extensional tectonic framework. The sedimentation of the Middle and Lower Jurassic in the fault depression was remarkably controlled by faults, thus the Early Middle Jurassic basin was faulted basin. While faults basically had no impacts on the sedimentation of the Upper Jurassic, thus the Late Jurassic basin was a downwarped basin. In Cretaceous, the northern edge of Qaidam basin was mainly controlled by compressional tectonic framework, with fault propagation folds to be formed in the process of compression in the foreland of Qilianshan. The Cretaceous sedimentation was controlled by the fault propagation folds formed during the process of compression, thus the Cretaceous basin was a compressional basin.

Discrimination of mixed gases in western depression of Qaidam basin

Ma Liyuan, Zhang Xiaobao, Hu Yong, Duan Yi, Li Xiufen, Song Chengpeng

2004, 25(6):  625-628.  doi:10.11743/ogg20040605

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Chemical compositions and carbon isotopes of 34 natural gas samples, collected from 13 oil and gas fields in western depression of Qaidam basin, are measured and analyzed. Based on the carbon isotopes of methane, ethane and propane in the gas samples, the natural gas in western depression of Qaidam basin can be divided into 4 different types, including sapropel type oil associated gas, mixed type oil associated gas, coal related (humic) gas and hybrid gas. Most of the gases in western depression are cogenetic and synchronous sapropel type oil associated gas, mixed type oil associated gas and coal related gas. Hybrid gas, consisted of biogas, immature mixed type oil associated gas and low mature mixed type oil associated gas, distribute only in the shallow layers (Neogene) in Gas Hure oilfield in Gas faulted depression, the Yuejin No. 2 west intersection point, Youshashan oilfield and Wunan oilfield in Kunbei fault bench zone.

Preliminary study of relation between geochemical field of surface elements and energy minerals in Ordos basin

Pan Aifang, He Ying, Ma Runyong

2004, 25(6):  629-633,685.  doi:10.11743/ogg20040606

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Hydrocarbon alteration and adsorption can influence geochemical field of elements through enhancing dissolubility of fluids, changing redox conditions and pH values of media and impacting wettability of soil particles. The geochemical field of elements is closely related with the energy minerals in Ordos basin. The superposition zones of low background of iron group and chalcophile elements are the major lodging points of coal and natural gas. The transitional zones between low backgrounds of W Mo group elements and iron group and chalcophile elements are the favorable enriched zones of uranium deposits. The low background areas of W Mo group elements are the prospective enriched zones of hydrocarbons and coals. Large scale and high ranked low or high backgrounds of elements are generally correlated with large and rich deposits. The geochemical field of surface elements in Ordos basin and the surrounding areas can be divided into 4 zones, of which zoneⅠis enriched with oil, natural gas and coal, zoneⅡis the enriched zone of natural gas, coal and uranium, zone Ⅲ is the coexisting zone of oil and coal, and zone Ⅳ is the enriched zone of polymetralic and noble metallic ore and uranium deposits.

Structural features and evolution of contemporaneous reversed faults in Gas faulted depression in western Qaidam basin

Ding Wenlong, Wang Xiepei, Li Yanda, Zheng Qiugen

2004, 25(6):  634-638.  doi:10.11743/ogg20040607

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Reversed faults and folds are well developed in Gas faulted depression. The faults, closely accompanying the folds, are located on one side or both sides of anticlines and the highs of each structural layer superimpose vertically. Most faults have obvious control over sedimentation. The regional NWW contemporaneous reversed faults in Gas faulted depression would have controlled the distribution and scope of local sedimentary sags (oil generating sags), the depocenters of which are located near the downthrow blocks; while the relatively small contemporaneous reversed faults are mainly developed within sags or swells and would have controlled the formation and development of local structural traps. The faults and anticlines were not developed at the same time. The faults were formed earlier on the basement as synsedimentary faults, which thrusted while depositing. The anticlines were formed later as a result of compressional or compresso torsional structural stress in Late Tertiary. Anticlinal structures did not show synsedimentary characteristics of thin and coarse on the crest, and thick and fine on the limb until the early Miocene.

Genesis and distribution pattern of fractures in western Qaidam basin

Tong Hengmao, Cao Daiyong

2004, 25(6):  639-643,649.  doi:10.11743/ogg20040608

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Fractures in different structural zones in western Qaidam basin can be divided into 3 types, namely "Nanyishan type", "Xianshuiquan type" and "Shizigou type". The combination patterns and genesis of the fractures are correlated with the mode and strength of deformation and boundary conditions. Quantitative analysis of the relationship between the genetic types of fractures and the deformational strength (strain) of structural zones shows that there are 3 distribution patterns of fractures in compressed and folded region, including transverse tensile fractures+longitudinal tensile fractures+bedding fractures (type Ⅰ), transverse tensile fractures+longitudinal tensile fractures+planar conjugate shear fractures+bedding fractures (type Ⅱ), and transverse tensile fractures+longitudinal tensile fractures+bedding fractures+planar conjugate shear fractures+profile (conjugate) shear fractures (type Ⅲ). The larger the deformation strength is, the more complex the combination patterns of fractures would be. As a whole, the structural fractures in western Qaidam basin are mainly transverse and longitudinal tensile fractures.

Analysis of characteristics of thermal evolution in Hefei basin

Chen Gang, Zhao Zhongyuan, Li Pilong, Ren Zhanli, Li Xiangping, Li Zongliang

2004, 25(6):  644-649.  doi:10.11743/ogg20040609

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Based on statistical analysis of vitrinite reflectance (Ro) data of 23 mudstone samples from five wells in Hefei basin, the thermal evolution characteristics of the basin during Mesozoic and Cenozoic are comprehensively studied by sieve correlating the slope of vitrinite reflectance vs. depth profile (Ro H ) and the corresponding geothermal gradients. TheRo values of the Permian, Lower Jurassic and Lower Cretaceous in different blocks of the basin vary significantly and range from 0.82% to 2.70%, while the slopes of the correspondingRo H profiles are remarkably larger than that of the Upper Cretaceous and Lower Tertiary, indicating an obvious hiatus between the Lower and Upper Cretaceous. The geothermal gradient in the Lower Cretaceous ranges between 70.80℃/km and 130.90℃/km, and that in relatively stable area is near 70.80℃/km, which indicate that there have been a relatively large regional tectonic thermal event during the Early Cretaceous. TheRo of the Upper Cretaceous and Lower Tertiary distribute narrowly between 0.40% and 0.74% and the geothermal gradient is 38.30℃/km. The basin has been uplifted, as a whole, and cooled down since Late Tertiary, lowering the geothermal gradient to the current 27.50℃/km.

Characteristics of time-space combination of lowstand system tracts and their exploration significance in eastern Qiongdongnan basin

Wei Kuisheng, Chu Meijuan, Cui Yingkai, Shen Hua, Liang Jianshe, Yang Guozhong, Liu Tieshu

2004, 25(6):  650-655.  doi:10.11743/ogg20040610

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Qiongdongnan basin has experienced chasmic and depression stages, which can be further divided into 5 substages. Correspondingly, there can be 2 mega sequences (TA, TB), 5 super sequences and 19 sequences. Lowstand system tracts have been developed in every evolutionary stage and are characterized by their multistage and multiplicity. The sedimentary systems of lowstand system tracts include basin floor fan, slope fan, progradational wedge, delta, fan delta, alluvial fan, submarine valley and incised valley. These sedimentary systems are characterized by their time space combination, and can be categorized into isolated type, fault controlled suspension type and coexisting type. Sedimentary systems during chasmic stage are mainly of isolated type, fault controlled suspension type; while those during depression stage are mainly of coexisting type. These lowstand sediments are favorable for the development of source reservoir cap rock combinations, hence there can be various composite traps, such as lithologic, stratigraphic lithologic and structural lithologic traps, which would have large exploration and development potentials.

Study on palaeo-productivity of the lake during the deposition of the 1^st member of Shahejie Formation in Dongying sag

Li Shoujun, Wang Jicun, Zheng Deshun, Jiao Yehong, Wang Jinxiang, Li Xueyan

2004, 25(6):  656-658.  doi:10.11743/ogg20040611

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Palaeo productivity values of the 1^st member of Shahejie Formation encountered in hundreds of wells in Dongying sag have been calculated with the formula of lake's productivity based on organic carbon method. The conture map shows the distribution of paleo productivity of the lake during the deposition of the 1^st member of Shahejie Formation. These values are then compared with those of the 3^rd member of Shahejie Formation, as well as with those of modern lakes having different nutrition types. The study indicates that Dongying sag was a super eutrophic lake during the deposition of the middle and low intervals of the third member and the first member of Shahejie Formation, which would be the most favorable for oil generation. The sag was a eutrophic lake during the deposition of the upper interval of the third member of Shahejie Formation, which also had certain capacity of oil generation. The quality of source rocks degrades in a sequence from the low interval of the 3rd member to the 1st member, the middle interval and the upper interval of the 3rd member of Shahejie Formation.

Sealing capacity of faults and its control on hydrocarbon migration and accumulation in Gas fault depression in Qaidam basin

Zhang Chunlin, Gao Xianzhi, Li Weilian, Ma Dade

2004, 25(6):  659-665.  doi:10.11743/ogg20040612

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The key factors that influence sealing capacity of fault include fault activity, shale smearing factor, lithologic juxtaposition on both sides of fault, and relation between pressure on fault plane and shale stratum ratio. Taking these four key influential factors as appraisal parameters and integrating with tectonic evolution of the basin, the vertical and lateral sealing capacities and sealing history of the faults in Gas fault depression in Qaidam basin are studied. It is shown that the sealing capacities of faults vary largely in time and space in western Qaidam basin and have played different roles in hydrocarbon migration and accumulation. When acting as migration pathways of hydrocarbon, the faults would interconnect source rocks and traps and controlled the horizons and times of vertical migration and accumulation of hydrocarbons, such as the Ⅶ and Ⅺ faults. When acting as barrier beds, they would control the formation of traps for hydrocarbon accumulation, such as Wunan fault. Reactivation of faults would destruct oil and gas reservoirs, leading to the redistribution of oil and gas, such as the faults in Youshashan and Nanyishan areas.

Geochemical origin of oil in Lower Oligocene reservoirs in Gas Hure,western Qaidam basin

Li Sumei, Liu Luofu, Wang Tieguan

2004, 25(6):  666-670,691.  doi:10.11743/ogg20040613

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Organic geochemical analysis shows that oils in the lower Oligocene reservoirs in Gas Hure,western Qaidam basin,are of typical salt lake origin and have the characteristics of both low mature oil and mature oil. Source correlation shows that oils in the lower Oligocene reservoirs in Gas Hure were mainly sourced from the lower Tertiary near mature or mature source rocks buried over 3500m in the adjacent area, which were deposited in salt lakes with high concentration of calcium and salt in strong reducing environment. Salt and high carbonate content in the source rocks significantly inhibited isomerization of steroid and terpenoid, wich would delay the maturation of lower Tertiary source rocks. Oils currently discovered in the lower Oligocene reservoirs in Gas Hure are mainly low mature oil, with rare immature and mature oils. Analysis shows that mature source rocks are mainly distributed in deep layers with a burial depth of over 4000m, thus deep layers would be the potential targets for discovering mature oils.

Geological characteristics of Fula oilfield in Muglad basin, Sudan

Nie Changmou, Chen Fajing, Bai Yang, Wang Wangquan, Ma Pengshan

2004, 25(6):  671-676.  doi:10.11743/ogg20040614

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Fula oilield in Muglad basin, Sudan, is mainly consisted of shallow ordinary heavy oil reservoirs in Cretaceous Aradeiba and Bentiu Formations and light oil and gas reservoirs in Cretaceous Abu Gabra Formation. The reservoir in Bentiu Formation in the major Fula North Block is a faulted anticlinal massive reservoir in a horst block with a dipping oil water contact. In the Aradeiba Formation there are shallow lake facies thin sandstone reservoirs with high porosity and permeability. While braided river facies thick bedded massive sandstone reservoirs with high porosity and permeability are developed in the Bentiu Formation. The Aradeiba and Bentiu reservoirs are characterized by shallow burial depth, loose consolidation, good poroperm characteristics, high sand production and low formation pressure, and the crude oils are characterized by high density, high viscosity, high acid value and heavy oil. The layered reservoirs in Abu Gabra Formation are of delta front facies thin sandstone reservoirs with medium porosity and medium permeability, being characterized by small fault block, rapid facies change and low hydrocarbon column. The oil and gas accumulations in Abu Gabra Formation are mainly controlled by lateral sealing conditions and poroperm characteristics of the reservoirs. Effective source-reservoir-cap rock combination, and lateral blocking of mudstones are the main control factors of oil and gas accumulations in Darfur Group and Bentiu Formation.

Hydrocarbon generation history of major source rocks in the northern edge and western part of Qaidam basin

Lin Lamei, Jin Qiang

2004, 25(6):  677-681.  doi:10.11743/ogg20040615

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Source rocks in the Xiaomeigou Formation in Kunteyi sag,northern edge of Qaidam basin,experienced two hydrocarbon generation processes.At the end of Jurassic,some part of the deep sag reached the hydrocarbon generation threshold,but oil generation ceased subsequently due to uplifting.The second widespread hydrocarbon generation began in Eocene.Thus Lenghu structure experienced two stages of hydrocarbon accumulations.The Dameigou Formation,the main source rock in Saishiteng sag,generated hydrocarbons only once.The Jurassic source rocks in Yuka sag entered the oil generation threshold in early Oligocene with only one hydrocarbon accumulation stage.Due to the influences of Kunlun mountain tectonic movement,the major Tertiary source rocks in western Qaidam basin,which were depositedin salt lake and saline water environments,did not entered the oil generation threshold until the early and middle Pliocene,and the maturities were generally low with only some areas could reach peak of oil generation,and the time and depth that the source rocks entered the oil generation threshold varied to some extent in different tectonic positions.

Characteristics of subtle oil/gas reservoirs and exploration strategy in central Junggar basin

Wu Jincai, Meng Xianlong, Wang Lichi, Cao Bo, Zhang Jianzhong, Zhu Yunhui

2004, 25(6):  682-685.  doi:10.11743/ogg20040616

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Central Junggar basin is rich in petroleum resources,much of which are accumulated in subtle oil/gas reservoirs. The subtle oil/gas reservoirs are characterized by control of multiple sedimentary systems, structural lithologic combination, multiple oil-bearing layers and multi stage reservioring. Both new ideas and relatively mature methods should be used in exploration of subtle oil/gas reservoirs. Firstly, determining favorable horizons for development of subtle traps through high resolution sequence stratigraphic division and correlation; then looking for slope break zones and favorable facies zones through palaeotopographic and sedimentary environment studies; and finally locating subtle traps and searching for subtle reservoirs through seismic processing.

Evaluation of geological effects of acid-fracturing in deep carbonate reservoirs:taking the fractured-vuggy carbonate reservoirs in Tahe oilfield as an example

Chen Zhihai, Zhang Shicheng

2004, 25(6):  686-691.  doi:10.11743/ogg20040617

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Acid fracturing treatment of deep carbonate reservoirs is to establish communication between the fractures generated and natural fractures and vugs. Based on the variation modes of operating curves of acid fracturing treatment, and the geological effects of the treatment, 4 appraisal systems have been suggested each of which corresponds to one of the acid fracturing modes, including no fracture have been created (type Ⅰ), fractures have just been created and then stop (type Ⅱ), fracture have been created but have no communication with natural fractures and vugs (type Ⅲ), and fractures have been created and have communication with natural fractures and vugs (type Ⅳ). Acid fracturing treatments in 27 wells in Tahe oilfield have been categorized and evaluated with this method. Of these wells, 56% have reached the type Ⅳ acid fracturing mode and these wells can have high and stable productivities in a relatively long period; 22% are of type Ⅱ and Ⅲ; while the rest 22% are of type Ⅰ, and their acid fracturing effects need to be further improved. The evaluation results are in good agreement with the actual performance of the wells.

Main factors controlling hydrocarbon accumulation in western part of northern edge of Qaidam basin

Jiang Zhenxue, Pang Xiongqi, Luo Qun, Zhang Hong, Tian Fenghua

2004, 25(6):  692-695.  doi:10.11743/ogg20040618

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Studies on the geological conditions and process of hydrocarbon accumulation in western part of northern edge of Qaidam basin show that the main factors controlling hydrocarbon accumulation are the occurrence and distribution of source rock, the connectivity of carrier rock and the timing of trap formation and main expulsion stages. In a more profound respect, however, faults are the fundamental factor that ultimately determine the distribution of oil and gas reservoirs through controlling the distribution of source rock, trap formation, vertical and lateral connectivity,and formation of secondary oil and gas reservoirs from destruction of primary reservoirs.

Reservoiring patterns and favorable exploration targets in Mahai-Dahonggou salient in the northern edge of Qaidam basin

Wang Liqun, Pang Xiongqi, Dang Yuqi, Ma Dade, Ma Lixie

2004, 25(6):  696-702.  doi:10.11743/ogg20040619

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This paper summarizes the reservoiring patterns in Mahai Dahonggou salient in northern edge of Qaidam basin. Reservoiring pattern in the deeper part is of early primary accumulation with long period of hydrocarbon charging, which is characterized by multi stage (at least two stages) hydrocarbon migration and reservoiring. While Reservoiring pattern in the shallower part is of early expulsion and accumulation and late secondary reservoiring, which is characterized by late and once reserviring. Trap forming stages and reservoiring stages that are corresponding to the two major tectonic movements and two fault systems in the palaeo salient area are critical to the formation of these two different types of accumulations. Lunanshan piedmont structural zone, Mahai Huangnitan structural zone, Nanbaxian Donglingqiu anticlinal zone and the stratigraphic overlapping areas on the southwest and southeast limbs of Mahai structure are the major exploration targets in the future.

Characteristics of algal limestone reservoirs in Xiayoushashan-Shangganchaigou Formations in Huatugou oilfield, western Qaidam basin

Zang Zhencheng, Sun Jianmeng, Ma Jianhai, Huo Yuyan, Su Yuanda

2004, 25(6):  703-706,712.  doi:10.11743/ogg20040620

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Algal limestone reservoirs in Xiayoushashan Shangganchaigou Formations in Huatugou oilfield in western Qaidam basin mainly consist of algal laminated limestone,algal micrite,oosparite,sparry coquina,sparry algal lump limestone and terrigenous clastic micrite,of which the former two types of limestone have relatively good reservoir properties.The formation of algal limestone reservoirs has mainly been controlled by the stability of sedimentary environment,early eluviation and late cementation filling.Algal limestone reservoirs appear to have relatively high gamma ray value in well logs and strong reflectance signatures in seismic profiles.These characteristics can be used to determine lateral distribution of algal reservoirs.Formation testings in several wells have got oil/gas shows in algal limestone reservoirs,verifing that this type of reservoirs has good prospectiveness.

Characteristics of hydrocarbon accumulation in the northern edge of Qaidam basin

Gao Xianzhi, Ma Dade, Liu Zhen, Wang Liquan

2004, 25(6):  707-712.  doi:10.11743/ogg20040621

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Characteristics of hydrocarbon accumulation in typical oil and gas fields in the northern edge of Qaidam basin are analyzed by using homogenization temperature and compositions of inclusions and trap evolution history. Oil and gas in Jurassic reservoirs have been accumulated in 3 different stages, including the end of Jurassic, Oligocene and Pliocene; while that in the lower Tertiary reservoirs have been accumulated in 2 stages, i.e. Oligocene and Pliocene; and that in the upper Tertiary reservoirs are of secondary oil/gas reservoirs, accumulated through upward remigration of hydrocarbons from the adjustment and damage of the lower Tertiary reservoirs, due to post Pliocene tectonic movements. Faults have played a key role all the time during the formation and evolution of oil and gas reservoirs in the northern edge of Qaidam basin.