Oil & Gas Geology-2008, Volume 29 Issue 2

Property differences and genetic mechanisms of hydrocarbon in each oil unit of the Carboniferous in Tazhong-4 oilfield

Jiang Zhenxue, Yang Jun, Pang Xiongqi, Yang Haijun, Li Zhuo

2008, 29(2): 159-166. doi:10.11743/ogg20080202

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The three oil units of the Carboniferous in Tazhong-4 oilfield feature in a hydrocarbon distribution of gas-bearing formation being clipped between oil-bearing formations.The Carboniferous reservoirs in Tazhong-4 oilfield were formed mainly in two stages,the earlier oil-charging stage and the later gas-charging stage.By combining the oil and gas charging times with the analysis of mudstone compressive strength,fluid residual pressure and normal pressure on fault surface in different periods,and the relationship among these three parameters,it is suggested that the fault cutting through the three units were open in the late Hercynian,leading to accumulation of oil and gas in the Unit C_Ⅲ and C_Ⅰ;while in the Himalayan period,fluid residual pressure of Unit C_Ⅲ was greater than the normal pressure on fault surface,resulting in late accumulation of gas in the Unit C_Ⅱ rather than Unit C_Ⅰ.Moreover,the gas charged in C_Ⅱ unit could not migrate upward through the fault into Unit C_Ⅰ due to the large thickness of the upper shale interval.The analysis of the whole process of the pooling in the Carbonife-rous reservoirs in Tazhong-4 oilfield indicates that the western section of the central fault-horst zone is favorable for finding secondary oil pools in Unit C_Ⅰ and C_Ⅱ.

Characteristics of hydrocarbon enrichment in the Lower Ordovician carbonate rock weathering crust on the northern slope zone of Tazhong area

Han Jianfa, Yu Hongfeng, Zhang Haizu, Luo Chunshu, Jing Bing, Huang Guangjian, Ji Yungang, Dong Ruixia

2008, 29(2): 167-173,188. doi:10.11743/ogg20080203

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A large-scale Ordovician carbonate weathering crust was developed on the north slope of Tazhong area in Tarim basin.Studies on the weathering crust reservoirs and its reservoiring conditions show that the reservoirs feature in low porosity and low permeability dominated by secondary solution pores and cavities.They are mostly developed on the top of the Lower Ordovician and have an excellent correspondence relationship with the morphology of the unconformity surface.Reformed by the development of faults and fractures,the weathering crust reservoirs were widely distributed along the karst slope vertical superimpositon and horizontal connection.The hydrocarbon carrier system which is composed of faults and unconformities controlled the migration and accumulation of hydrocarbons;and the development of reservoirs controlled the enrichment of oil and gas in the weathering crust.Multiphase charging and heterogeneity of the reservoirs caused the differences of oil and gas occurrence.

Reservoir characteristics of the 3^rd upper sub-member of the Silurian Kepingtage Formation in Tazhong area and their relations to oil and gas distribution

Wangping, Liu Luofu, Li Yan, Jiang Zhenxue, Chen Zhijun, Pang Xiongqi

2008, 29(2): 174-180. doi:10.11743/ogg20080204

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The poroperm characteristics of reservoirs in the 3rd upper sub-member of the Silurian Kepingtage Formation in Tazhong.Tarim Basin are described in detail in this paper based on petrology study.The results show that the pores of the reservoirs are mainly of primary residual intergranular pores and the lithologies are dominated by siltstone and fine lithic sandstone.The reservoir poroperm characteristics are affected by carbonate cementation,and the carbonate content presents an evident negative correlation with porosity and permeability.The porosity is more than 9% and permeability is over 2×10^-33×10^-3 μm²(it is generally accepted that strata with porosity≥9% and permeability≥2×10^-3 and 3×10^-3 μm² are reservoirs)and the permeability is in positive linear correlation with the porosity.The horizontal distribution of poroperm characteristic indicates that this reservoirs interval has high porosity and permeability in the southeastern Tazhong areas and its oil/gas distributions are closely related to the poroperm characteristics.Furthermore,favorable plays for petroleum exploration in this reservoir interval are predicted on the basis of relationship between oil/gas distribution and reservoir cha-racteristics.These prediction results point out the directions for further exploration in the area.

Reassessment of the Carboniferous oil pooling in Tz-4 field,the Tarim Baisn

Yang Chupeng, Zhang Baomin, Wang Feiyu, Lu Yuhong, Xiao Qilin, Zhang Haizu, Liao Zewen, Geng Ansong

2008, 29(2): 181-188. doi:10.11743/ogg20080205

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The oil pooling process in the Carboniferous Tz-4 field is quite typical in the Lower Paleozoic petro-leum system,Tarim basin.The molecule and carbon isotope geochemical behavior of oil in the field was analyzed systematically and the pooling process was reassessed based on regional sedimentology and tectonics as well as geochemical data of reservoir bitumen and fluid inclusions.The results indicate that two kinds of source kitchens(i.e.the Middle-Upper Ordovician and the Middle-Lower Cambrian) with different thermal evolution history in the central Tarim basin might have contributed to the formation of the Carboniferous Tz-4 oil pool.The C_Ⅰ reservoir is thought to be formed by one hydrocarbon charging from a single oil source and suffered almost no geochemical alteration,whereas the C_Ⅲ reservoir is characterized by diversified hydrocarbon sources,multiphase charging,and late modification.The results also suggest that the C_Ⅱ reservoir has a close relationship with C_Ⅲ reservoir,and is partly the product of C_Ⅲ reservoir adjustment and modification.

Paleohigh geomorphology and paleogeographic framework and their controls on the formation and distribution of stratigraphic traps in the Tarim Basin

Lin Changsong, Yang Haijun, Liu Jingyan, Cai Zhenzhong, Pengli, Yang Xiaofa, Yang Yonghen

2008, 29(2): 189-197. doi:10.11743/ogg20080206

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The palaeotectonic framework of the Tarim basin in the Early Paleozoic was characterized by development of a series of large-scale,NWW or NEE trending uplifts.The configuration of the uplifts and the depressions varied significantly and resulted in the change of the tectonic paleogeography of the basin.The paleohigh belt in the central basin was formed when the basin shifted from divergence to compression since the Middle Ordovician.It comprises the Hetianhedong and Tazhong palaeo-uplifts with complex paleogeomorphology.Several typical geomorphologic units have been recognized in the early Paleozoic paleohighs,including the uplift high,marginal slope and slopebreak,shelf gentle slope and lower rise or platform,shelf slopebreak and deep-sea basin or plain.The southern marginal slopebreak zone of the Tabei paleohigh,and both the northern and southern slopebreak zones flanking the Tazhong paleohigh controlled the development of high-energy facies of reef and shoal deposits.From the paleohighs to the paleo-depression belt,the marginal slopebreak zones of carbonate platforms,triangle zones of truncated and onlap unconformities are favorable for the formation of lithologic-stratigraphic traps.

Controlling effect of the fractures in Upper Ordovician fractures upon reservoir formation at the platform edge of Tazhong oilfield,the Tarim Basin—an example from the Tazhong-Ⅰ slope break zone

Zhou Bo, Jia Chengzao, Gu Jiayu, Li Qiming, Wu Guanghui, Ju Yan

2008, 29(2): 198-203,209. doi:10.11743/ogg20080207

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Observation and analysis of cores and thin sections,combined with cathodeluminescence identification technique,reveal that at least 3 stages of structural fractures were developed in the Middle-Upper Ordovician limestone formation at the platform edge,Tazhong oilfield.Among the fractures,the one formed in Himalaya pe-riod had greatly improved the quality of reservoirs.It is suggested that the numerous tiny to small fissures in cores could enhance permeability and connect larger fractures with matrix pores.The authors present in this paper the model of reservoir formation which emphasizes the controlling roles of fractures to reservoir formation,and regard burial dissolution as the main mechanism of matrix pore growth.They also point out that the future exploration targets in the basin shall be fractures in the favorable sedimentary facies belt.

Distribution and genesis of natural gas in Lunnan area

Wang Xiaomei, Zhang Shuichang

2008, 29(2): 204-209. doi:10.11743/ogg20080208

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Oil and gas distribution in Lunnan area is very complicate in Tarim basin.Component analysis of the natural gas from the commercial wells in Lunnan area indicates that the gas here contains mostly methane and is typical dry gas.The dry coefficient and the gas/oil ratios decrease from east to west,suggesting a westward gas invasion from the east slope.Combining the fact that abundant pyrobitumen exists in the reservoirs with the ana-lysis of gas characteristics from other areas in Tarim basin,we conclude that gas in Lunan area is cracked gas from crude oils generated by the mature to over-mature Middle-Lower Cambrian source rocks.

Geochemical characteristics and implication of high thermal maturity oils in Tazhong-Ⅰ faulted slope break zone

Li Sumei, Pang Xiongqi, Yang Haijun, Gu Qiaoyuan, Li Yujiang

2008, 29(2): 210-216. doi:10.11743/ogg20080209

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Considerable quantity of oils and gas with diverse characteristics and origins has been discovered in the Tazhong-Ⅰ faulted slope break zone,Tarim basin.Totally 45 oil samples were gathered and analyzed with conventional geochemical approaches.The results show that the oils are characterized by significant predominance of light fractions and lack of heavy fractions,suggesting high thermal maturity of source rocks and/or thermal cracking of the oils.High absolute abundance of dibenzothiophene was detected in the oils,indicating a marine source.Several indexes(including relative distribution of gammacerane/C₃₀ hopane,C₂₇-to C₂₉-steranes,and C₃₀-steranes) and biomarker fingerprints indicate that the oils in the area were generated from at least two kinds of primary source rocks.It was suggested that there is a strong gas invasion point at the eastern end of Tazhong-Ⅰ faulted slope break zone according to the abnormal distributions of diamondoid hydrocarbons and n-alkanes.There are evident differences among those oils even from closely adjacent wells,inferring different hydrocarbon generation and accumulation processes.All the features for the oil samples analyzed indicate the common characteristics of the oils in the superimposed basins in western China,i.e.,high thermal maturity,multiple sources and multiphase pooling,heterogeneity of chemical composition,and relative compartmentalization.This study is helpful for investigating the origin and accumulation mechanisms of the oils.It was suggested that great petroleum potential could be expected in the Tazhong-Ⅰ faulted slope break zone.

Hydrothermal fluid activity and thermochemical sulfate reduction in the Upper Ordovician,Tazhong area

Li Kaikai, Cai Chunfang, Cai Liulu, Zhang Chunming

2008, 29(2): 217-222. doi:10.11743/ogg20080210

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Measurements of rare earth elements,value of ⁸⁷Sr/⁸⁶Sr,and fluid inclusion,had been carried out on facture-filling minerals(such as calcite,fluorite and barite) in Tazhong area.Results show that the ⁸⁷Sr/⁸⁶Sr va-lues of all the filling minerals(0.708 89-0.710 36) and formation water are higher than that of the whole-rock samples of fracture-carbonate rock and sea water in the same period,and the homogenization temperature of fluid inclusion is about 20℃ higher than that of the highest temperature ever experienced by the Upper Ordovician,signaling the presence of hydrothermal fluid and a high concentration of ⁸⁷Sr.The hydrothermal fluid with high ⁸⁷Sr/⁸⁶Sr value is most likely derived from the Cambrian or pre-Cambrian clastic rocks.Its Eu²⁺ and Sr²⁺ are of similar origin,resulting in a positive Eu anomaly and a good linear relationship between Eu and Sr in the fracture fillings.It induced TSR and generated HSS and pyrite.Consequently,the process led to the replacement of barite by calcite.The presence of fracture-filling ⁸⁷Sr-rich fluorite with relatively low temperature and low salinity as indicates that beside the influence of hydrothermal fluid,the area might also be affected by fresh water and basin fluid of low temperature.

Chemical characteristics of formation water in Tazhong area of the Tarim Basin and their petroleum geological significance

Zeng Jianhui, Wu Qiong, Yang Haijun, Qian Shiyou, Kong Xu, Ma Zhongliang

2008, 29(2): 223-229. doi:10.11743/ogg20080211

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Study on the chemistry of the Ordovician,Silurian and Carboniferous formation waters in Tazhong area suggests that these waters are of CaCl₂ type with high TDS and contains mainly cations K⁺+Na⁺ and anion Cl.Their average sodium-chlorine coefficient is in the range of 0.73-0.87,the de-sulfuric acid coefficient averages at 1.54-9.29,and the metamorphic coefficient ranges from 5.54 to 12.80.The chemical composition and ionic ratio of these waters present positive,negative and steady variations along with depth.While horizontally,the chemical compositions of Carboniferous formation water remain relatively stable,but the Ordovician formation water is complicated in chemical changes.Generally speaking,the Ordovician,Silurian and Carboniferous formation waters are in a closed fluid dynamic environment now.However,the Ordovician,Silurian and Carboni-ferous unit C_Ⅲ are quite different in their fluid dynamic environments and conditions of oil & gas accumulation and preservation.While the C_Ⅰ and C_Ⅱ units have similar fluid dynamic environments and are favorable for oil & gas accumulation and preservation.The TDS of the formation waters is low and the fluid dynamic environment is less closed in the paleo-uplift and areas near faults that cut into the shallow formations.The Ordovician and Silurian formation waters suffered from strong leaching of meteoric water in the geologic history,thus their fluid dynamic systems were open.Especially,the Silurian formation water was exposed to significant open fluid dynamic environment,extremely unfavorable for hydrocarbon accumulations and preservation,while the Carboniferous formation water was,in the contrary,excellent for hydrocarbon accumulation and preservation due to its closed fluid dynamic environment.

Seismic reservoir prediction of reef-flat complexes in the Upper Ordovician,Tazhong area

Yang Haijun, Hu Taiping, Yu Hongfeng, Li Xinsheng, Zhao Kuanzhi, Dong Lisheng

2008, 29(2): 230-236,251. doi:10.11743/ogg20080212

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Tazhong-Ⅰ slopebreak zone is located at the north margin of the large carbonate platform developed on the northern slope of Tazhong paleo-uplift.The northwest trending slope break is 220 km long and 8-18 km wide.It developed large reef-flat complexes in the Upper Ordovician and harbors giant condensate gas reservoirs with burial depth of 3 700-7 200 m.High-quality seismic profiles are hard to acquire here because of the undulate dunes at the surface and complex structures underground.And the accuracy of lateral reservoir prediction is low due to the deep burial depth of the Ordovician carbonate reservoirs,high heterogenity,and multiple controlling factors upon reservoir development.However,a set of methods have been developed and applied successfully to the prediction of the reef-flat carbonate reservoirs through analyzing geophysical responses to various types of Ordovician reef-shoal complex reservoirs based on geologic log,core,and test data.The application of these methods has accelerated the exploration of large condensate gas reservoirs in the slope break zone.

Research on sequence architectures of continental foreland basins——a case study of the late stage of the southern Junggar foreland basin

Ji Youliang, An Aiqin, Zhu Rukai

2008, 29(2): 237-243,251. doi:10.11743/ogg20080213

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The sequence stratigraphy of continental foreland basins is an important domain in current research.The southern Junggar basin is a regenerated compound foreland basin resulted from two-phase superimposition.The sequences of the late phase foreland basin can be divided into 1 first-order sequence,3 second-order sequences,5 third-order sequences and 15 fourth-order sequences in accordance with analyses results of outcrops,cores,seismic data,and well logs.Four systems tracts are recognized in each second-order sequences based on the cyclicity of tectonic and sedimentary evolution and the internal architectures of the sequences.The sequence stratigraphic model of southern Junggar foreland basin is established and sequence-stratigraphic responses to tectonics are discussed in this paper.Tectonic activity controls the formation of sequence boundary,which is formed mainly by the resilient uplift at the end of a thrusting and the deformation caused by the following thrusting.With continuous advancement of the orogenic belt front,the center of subsidence and depocenter of the foreland basin migrate toward the ramp region.The tectonic sequences are developed cyclically and periodically,and each sequence begins with a speeding up of subsidence.

Sedimentary characteristics of the shallow Jurassic braided river delta,the Junggar Basin

Zhu Xiaomin, Zhang Yina, Yang Junsheng, Li Dejiang, Zhang Nianfu

2008, 29(2): 244-251. doi:10.11743/ogg20080214

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Braided river delta is an important reservoir type in continental sedimentary basins and the potential exploration target for the future.Core,log,and geophysical data indicate that the braided river delta in the Jurassic Sangonghe Formation of Junggar basin was deposited in an environment with gentle slope and shallow water.As a result of continuous progradation from the north to the center of the basin,arenaceous-conglomeritic deposits in the delta showed typical progradational seismic reflectance signatures,large cross bedding,and non-continuous positive rhythm.Its characteristics of sedimentary subfacies and microfacies as well as the favorable areas for exploration of lithologic traps are also discussed in this paper.

Characteristics of tectonic deformation within transitional belt between the Junggar Basin and the northern Tianshan Mountain

Qi Jiafu, Chen Shuping, Yang Qiao, Yu Fusheng

2008, 29(2): 252-260,282. doi:10.11743/ogg20080215

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The transitional belt between Junggar basin and northern Tianshan Mountain is a NWW-trending or near EW-trending complex tectonic deformation zone.According to the differences of tectonic deformation styles,the configuration within the transitional belt could be divided into several parallel zones from south to north,se-veral sectors from west to east,and 2-3 levels from basement to shallow cover.The northern Tianshan is a "thrust-upheaval basement zone" consisting of parallel alignment of basement that involves thrust faults and folds.There are 2-3 major thrust faults of high-angle dipping to the south within the "thrust-upheaval basement zone.The transitional zone is a "basement-cover" that involves thrust flexure zone consisting of basement-involved thrust faults and sedimentary cover of monocline dipping toward basin.There are several branch thrust faults of middle-and low-angle dipping to the south of the basin within the "basement-cover" and some of them have associated anticlines in the hanging-wall.The inner part of Junggar basin is a "delaminated contraction and vertical stack zone" consisting of 2 lines of detachment anticline and basement-involved high-angle faults.Structural styles are similar in the same zones,sectors,or levels,but vary in different zones,sectors and levels.The transitional belt was influenced by superpositional tectonic deformation from regional extension during the Early Permian,trans-contraction during the Late Jurassic,and contraction during the Pliocene.However,it was mainly the result of the upheaval of northern Tianshan Mountain since the Pliocene.The main factors influencing the tectonic style of the transitional belt include the pre-existing faults,weak beds within sedimentary basin cover,and upheaval and compression of northern Tianshan.

Distribution and sequence stratigraphic framework of the Jurassic strata on the northwestern margin of the Junggar Basin

Qi Liqi, Bao Zhidong, Wu Boran, Li Weifeng, Yuan Lichuan

2008, 29(2): 261-267. doi:10.11743/ogg20080216

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The sequences in Junggar basin were influenced by the major fault on the northwestern margin.The Jurassic is relatively thick in the footwall of the major fault,but is relatively thin or even absent in the hanging wall.However,based on available drilling,log,and seismic data in the study area,this paper recognized 2 structural sequences(JTS1,JTS2),7 sequences(Ⅰ-Ⅶ),and 10 system tracts in the Jurassic by employing classical sequence stratigraphy theory and approaches.It suggests that the occurrence of coal layers bears important significance in sequence demarcation and that the controlling factors for the Jurassic sequence stratigraphy on the northwestern margin of Junggar basin include tectonic movement,sediment supply,lake level rise and fall,and climate.Among them,the tectonic movement and lake level rise and fall are considered the most important.

Distribution patterns of the end of the Middle Devonian tectonic unconformity and their constrain on the development and distribution of favorable stratigraphic traps in the Tarim Basin

Liu Jingyan, Lin Changsong, Peng Li, Chen Qingqing, Zhang Xiangxiang, Zhou Xiaojun

2008, 29(2): 268-275. doi:10.11743/ogg20080217

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As a result of the early Hercynian Movement,the latest Middle Devonian tectonic unconformity is one of the most important unconformities in Tarim basin.It is widely distributed in the basin,occurring in the forms of superimposed unconformity,high angle unconformity,parallel unconformity,and conformity from the uplifted area to the depression.The underlying strata in Tabei and Tazhong areas were eroded severely and were accompanied by deposition and onlapping of the Donghe sandstone along the unconformity in Late Devonian,forming several erosional unconformity triangle zones and onlapping unconformity triangle zones.The distribution and structural styles of the unconformity are the results of tectonic movement and controls the distribution of the favorable stratigraphic traps.The erosional unconformity triangle zones are the potential areas for the Silurian lithologic traps,while the onlapping unconformity triangle zones are regarded as the favorable areas for the Devo-nian Donghe sandstone lithologic traps.

Study on the application of hydropyrolysis technique to the description of organic matter in highly mature source rocks

Sun Yongge, Will Meredith, Colin E Snape, Chai Pingxia

2008, 29(2): 276-282. doi:10.11743/ogg20080218

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Hydropyrolysis is a newly-developed technique used to set free small molecular from macromolecular of organic matters by covalented-bond cracking.Our study has been focused on the possibility of employing the technique in extracting primary organic matters from highly mature hydrocarbon source rocks.The Lower Palaeozoic source rocks of four sedimentary facies in Tarim basin were selected to run the hydropyrolysis experiments.The results show that molecular compositions of hydropyrolysis products(n-alkanes and steranes) from kerogens match well with that of the selected source rocks,proving that the technique works effectively in extraction of primary organic matters from highly mature hydrocarbon source rocks.The study paves the way for further application of the technique to oilsource correlation.

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