Table of Content

20 June 2008, Volume 14 Issue 2

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Article

On the Wenchuan Earthquake

LU Hua-fu

2008, 14(2):  133-138. 

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The triggering fault of Ms 8.0 Wenchuan earthquake on May 12th, 2008 is the Guanxian-Anxian fault, the frontal thrust of the Longmenshan thrust belt, or a newly created fault near the Guanxian-Anxian fault with the similar attitude. The triggering fault is a NE trending, NW dipping and mainly of thrusting with dextral strike slip fault. The Wenchuan earthquake is resulted form the tectonic stress accumulation and release in the frontal fault of Longmenshan thrust belt caused by the SEE ward horizontal compression from the eastern Tibet plateau including the Longmenshan toward the northwest margin of Sichuan basin, and the SEE ward horizontal compression is led by the indentation collision and subducton of India continent against the Tibet plateau after the continent-continent collision 50 Ma B.P., which resulted to the crust movement from NNE to NE to NEE to SEE gradually from middle Tibet to eastern Tibet. The Wenchuan earthquake leds to a tectonic model of uplifting of the eastern margin of Tibet plateau, the Longmenshan Mountians, through the upper crust thrusting and shortening in the Longmenshan thrust belt. That is basically different from Burchfiel's model about the Longmenshan uplifting resulted from the ductile lower crust flow eastwart barred by strong Sichuan basin crust and become upwelling. Probably both models worked in the different stages of the Longmenshan uplifling respetively, but our model is essential because we have seen it worked in the Wenchuan earthquake.

Sequence Stratigraphy and Subtle Reservoir Exploration of Triassic System in Northwestern Margin of Junggar Basin

XIAN Ben-zhong

2008, 14(2):  139-146. 

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Sequence stratigraphy and seismic data interpretation based on sequence stratigraphic principle provide useful theory and technology for exploration of stratigraphical and lithological oil and gas reservoirs. In order to fulfill the needs of exploration changes from hanging wall of Ke-Wu fault zone to its foot wall and from structural reservoirs to subtle stratigraphical and lithological pools, sequence stratigraphy and sedimentary characteristics were studied carefully with continental sequence stratigraphy theories and geophysical data. As a result, the Triassic system was divided into one second order sequence, five third order sequences and eight system tracts. Two sequence styles of depressions, i.e. wholly developed depression and partially developed depression, were recognized. The architectures of sandstone and development features of five subtle trap types were discussed according to sequence stratigraphic principle, logging constrained seismic interpretation and sedimentary strata correlation of wells. Based on the analysis of oil and gas accumulation conditions, the hydrocarbon accumulation model was established. It is indicated that the effectiveness of traps and recognition of oil/gas transporting faults are key constraints in exploring the lithological oil and gas reservoirs in the slope zones.

The Relation between Slope-Break Belts and Lithological Stratigraphic Traps in Middle Permian in Ke-Bai Area, Junggar Basin

ER Chuang

2008, 14(2):  147-156. 

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The slope-break belts are well developed in Middle Permian in Ke-Bai Area. Characterized by multiple orders along the source direction, they can be divided into basin-margin slope-break belts and inner-basin ones according to their respective natural geographical locations. Due to the persistent activities of boundary dominant thrusting faults and secondary faults in the basin, multiphased, episodic, differential activities and heritability are major features of the evolution of slope-break belts. Slopebreak belts play an important part in controlling stratigraphic overlapping, sequence architecture and distribution of sedimentary facies. In this paper, a reasonable model that slope-break belts control the formation of lithostratigraphic traps has been established. It is considered that the lithological traps, stratigraphic overlapping traps and unconformity surface-seal traps may be developed near the slope-break belts.

High Frequency Base-Level Cycles of Alluvial Fan and Distribution Patterns of Sandbodies of Lower Karamay Formation（ Triassic ）in Middle 6th District of Karamayi Oilfield, Xinjiang

WU Sheng-he

2008, 14(2):  157-163. 

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Using the data of dense well net in the oil development area, the features of the high resolution sequence stratigraphy of alluvial fan and the distribution patterns of sandbodies in different base-level cycles of Lower Karamay Formation (Middle Triassic) in the Karamay Oilfield of Junggar basin are studied. The results show that the hierarchy base-level ascending hemicycles are well developed, including 3 middle-term cycles, 7 short-term cycles and several super short-term cycles. It indicates the hierarchy change of the accommodation space to sediment supply ratios (A/S) in a vertical direction. Several smaller cycles with increasing A/S ratios are observed on a general increasing A/S background. The distribution patterns of sandbodies changed from sheet pattern at the bottom, through broad band pattern in the middle, to the narrow band pattern on the top of the Lower Karamay Formation. With the increasing A/S ratios which control the migration of braid channels, the width and thickness of sand bodies decreased.

Characteristics of Volcanic Reservoirs in Jiamuhe Formation of Permian in the 5th and 8th Districts, Northwestern Margin of Junggar Basin

KUANG Li-chun

2008, 14(2):  164-171. 

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A detailed analysis of volcanic rocks reservoirs in the 5th and 8th districts of the northwestern Junngar Basin was done in this paper. The volcanic rocks comprise lava and volcanoclastics and are mainly distributed in the Jiamuhe Formation of the Permian. Based on the study on the well cores, rock thin sections and FMI it is shown that the average porosity and permeability of lava is 6.94% and 6.68×10-3μm2, respectively, and those of the volcanoclastics are 10.01% and 7.93×10-3μm2, respectively.The diagenesis of volcanic rocks includes cooling consolidation and devitrification at the earlier stage, and structural fracturing,dissolution and filling replacement at the later stage, which formed the secondary pores and fractures. The main influencing factors of reservoir quality include lithology, diagenesis and tectonics.

Jurassic Diagenesis and Its Control on Reservoir Quality in Ke-Bai Area, Northwestern Margin of Junggar Basin

ZHU Shi-fa

2008, 14(2):  172-180. 

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Analysis of laboratory data indicates that the Jurassic reservoir of Ke-Bai area in northwestern margin of Junggar basin experienced compaction, cementation, dissolution and other diagenetic processes. Compaction and dissolution are main diagenetic processes that control the reservoir quality. The compaction is the major reason for reducing storage space. It is here dominated by the mechanical compaction with little significance of compaction-dissolution. The cementation is mainly of carbonate character,with little pyrite-cementation and less analcidite- and kiesel-cementation. The dissolution comprises cabonate cement dissolution,lithic grains dissolution and detrital feldspar dissolution. The reservoir pores evolved from primary to secondary pores. The pore space was affected by the dissolution of calcite and feldspar, as well as the precipitation and cementation of carbonates. The cementation by early calcite is favorable for the development of secondary pores in the later stage. The difference between the reservoir characteristics for different walls of the fault belt is obvious. On the hanging wall of the fault belt, the burial depth of strata is shallower, less than 1800 m, and the storage space is characterized by primary pores; However, on the foot wall of the fault belt, the burial depth of strata is deeper. Because of strong compaction, the proportion of the primary pores become obviously less, and the secondary pores are the major storage space. The characteristics of the secondary pore distribution are controlled by the distribution of acid fluid and fault.

Pore Types and Genetic Analysis of Shan-2 Member of Shanxi Formation in Eastern Ordos Basin

JI Han-cheng

2008, 14(2):  181-190. 

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This paper is concerned with the reservoir characteristics and genesis of secondary pores of the Shan-2 Member of Shanxi Formation (Lower Permian) in the eastern of Ordos basin. The main types of pores are the primary intergranular pores and the dissolution pores. The scanning electron eastern analysis of thin sections shows that the secondary pores were mainly formed by dissolution of matrix. Besides the sedimentation processes, the intensity of dissolution was affected by the rock types and matrix content. Dissolution was strong in the deltaic sandbodies where the coal-seams are found and the hydrocarbon generating center is located nearby, and therefore the pores are well developed.

Cenozoic Geotectonic Evolution of the Bohai Basin

TANG Liang-jie

2008, 14(2):  191-198. 

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Choosing one well separately in eight main sags of the Bohai basin and analysing the subsidence history of each well,we come to a conclusion that the subsidence characteristics of different sags were different. The southern part and the northern part of the basin subsided first, and the central part subsided at a later time. The subsidence center migrated from outside towards the basin center, which reflects the difference of tectonics in different parts of the basin. We made a balanced section analysis for the Liaodong Bay. Combining the subsidence history, we divided its Cenozoic tectonic history into three stages: (a) Rift stage.The time from the Kongdian Formation to the fourth member of the Shahejie Formation is the early rift stage, and the time of the third member of Shahejie Formation is the intensive rift stage; (b) Fault depression stage. The time from the second member of the Shahejie Formation to the first member of the Shahejie Formation is the early fault depression stage, and the period from the third member of the Dongying Formation to the second member of the Dongying Formation is the intensive fault depression stage; (c) Depression stage. This stage lasted from the first member of the Dongying Formation to the Quaternary.

Major Unconformities and Their Dynamic Conditions in Ke-Bai Area,Northwestern Margin of Junggar Basin

CHEN Shu-ping

2008, 14(2):  199-205. 

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The recognition of unconformities and their properties and intensities is of significance for understanding the tectonic history and petroleum accumulation regularities in Ke-Bai area, northwestern margin of Junggar basin. Seismic data show that there are several unconformities, reflecting several times of tectonic movements, of which, four times of tectonic movements played important roles in the formation of the structures in Ke-Bai area. The first occurred after sedimentation of the Middle Permian Lower Wuerher Formation, showing the lower and Middle Permian titling toward the southeast to be eroded; The second occurred at the end of the Triassic, affecting the area from the Dazhuluogou fault to Baikouquan in the hanging wall of Ke-bai fault; The third occurred at the end of Middle Jurassic, showing thrust faulting with gentle folding in the Ke-Bai fault and its hanging wall; The fourth occurred at the end of the Early Cretaceous, showing titling with weak faulting and folding. The differences in deformation of various periods and different areas in Ke-Bai area are closely related with the geodynamic conditions.

Tectonic Evolution of the Bohai Bay Basin and the Palaeozoic Original Oil and Gas Reservoirs

PENG Chuan-sheng

2008, 14(2):  206-216. 

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Based on the refined interpretation of over 200 seismic profiles in Bohai Bay basin, the tectonic styles developed in the basin and the tectonics and movement times they reflect are systematically analyzed. Tension, compression, strike slip and superimposition are four major tectonic styles developed in this area. The correlation of sequence and structural layers related with the tectonic styles indicates that during the Palaeozoic this area experienced two stages of compression, two stages of tension or strike slip motion and two stages of tectonic inversion. The balanced cross section analysis for 20 seismic profiles throughout the area shows, although different structural tracts experienced different tectonic activities, their total mechanical characters, development times and tectonic styles are consistent. Generally speaking, it is mainly expressed in: compression in T3, relatively stable in J1+2, tension in J3+K1, compression in K2, and tension in E-Q. The diverse evolutions in different tracts during the Meso-Cenozoic time controlled the Palaeozoic remains and burial. An analysis on the Palaeozoic original reservoirs indicates that for the superimposed basin having experienced polyphase tectonic disturbance reconstruction, one of the key factors for oil and gas accumulation is whether the Palaeozoic strata can produce hydrocarbon again or not at a later time. Therefore, the inversion tectonic transition zone with effective Palaeozoic hydrocarbon source sequences, and with shallow burial or weak denudation during the Mesozoic and deep burial during the Cenozoic, is favorable for accumulations of the Palaeozoic original reservoirs.

Characteristics of Strike-Slip Faults in the Northwestern Margin of Junggar Basin and Their Geological Significance for Petroleum

XU Huai-min

2008, 14(2):  217-222. 

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The foreland thrust belt has complex and various structural styles. Besides the large-scale reverse faults, there are a series of north-west strike-slip faults. Based on high-resolution seismic data, the foreland fault systems in the NW margin of Junggar basin were analyzed. It reveals that the strike-slip fault system of Ke-Bai area can be divided into two types, namely the flower strike-slip faults and the simple strike-slip faults, which have obvious differences in genesis and section features.The strike-slip faults have dual influence on hydrocarbon migration and accumulation. They can not only be effective migration channels for petroleum, but also can seal petroleum.

Tectonothermal Evolution of the Northeastern Sichuan Basin: Constraints from Apatite and Zircon (U-Th)/He Ages and Vitrinite Reflectance Data

QIU Nan-sheng

2008, 14(2):  223-230. 

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The tectono-thermal evolution since Triassic for the northeastern Sichuan Basin was modelled by using combined vitrinite reflectance and apatite and zircon (U-Th)/He dating methods. Corresponding to a transformation of the foreland basin during the Late Triassic to the interior cratonic basin in the Cretaceous, the paleo-heat flow was 51-66 mW/m2 during the Early Triassic. It decreased gradually to the present-day's 44.5 mW/m2. However, the heat flow increased a little at the late Cretaceous and Paleoene due to the late Yanshanian tectonic movement. At the same time, the apatite and zircon (U-Th)/He age dating revealed an uplifting of northeastern Sichuan Basin at the Late Cretaceous and the erosion could be up to more than 3000 m. The apatite and zircon (U-Th)/He ages can also provide the details of cooling history for the study area.

Some Thoughts on the Enrichment/Dilution Mechanisms of Oil and Gas in Superimposed Basins

ZOU Hua-yao

2008, 14(2):  231-236. 

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Petroleum accumulation mechanisms for two oil/gas fields, the Puguang gas field in the Sichuan basin and the MXZ oil field in the Junggar basin, were analyzed and compared, and factors controlling petroleum accumulation and occurrence in superimposed basins were discussed. In view of petroleum accumulation, the superimposed basins are characterized by the multiple petroleum sources (multiple source kitchens, multiple source rock intervals and multiple petroleum precursors such as kerogen, accumulated oils and various bitumens), the spatial and temporal diversities in fluid conduit systems and pressure, temperature and stress fields, and the complicated post-accumulation physicochemical processes such as chemical alteration of the petroleum reservoirs and fluid re-migration and re-accumulation. The manners and styles of the superimposition of accumulation processes during multiple-stage tectonic movements control the enrichment or dilution of petroleum. Successive or enhanced focusing of petroleum originated from different kitchens or source rock intervals is the important mechanism for petroleum enrichment in superimposed basins, whereas dispersal migration and adjustment of accumulated petroleum may cause petroleum dilution.

Evaluation of Oil-Gas Geological Anomalies Based on the Hierarchical Analysis for Chuxiong Basin, Yunnan Province

ZHOU Ting-quan

2008, 14(2):  237-242. 

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Hierarchical analysis is a mathematical method. It can gradually decompose a complicated issue which is hard to be quantified into a series of quantifiable simple issues, and then to be solved. This method has a feasible significance for quantitative evaluation of hydrocarbon resources. Based on the qualitative study of oil geology in Chuxiong basin and adopting the hierarchical analysis, we selected special information such as hydrocarbon conditions, reservoir conditions, preservation conditions, trap conditions, migration conditions, oil-gas micro-leakage conditions etc. and evaluated the favorable exploration zone. It is regarded that the borderland between the Mouding fault-bend anticlinal zone and the Heijing sag nappe area, the Pianjiao sag and the Huijiguan domal anticlinal zone are optimum exploration areas in Chuxiong basin. The profitable reservoirs, long-term developed low fluid potential localities and excellent trap conditions are the most favorable controlling factors for hydrocarbon reservoirs in this area.

The Sealing Properties of Ke-Bai Fault and Its Control on Oil Accumulation in Junggar Basin

JIANG Xiang-qiang

2008, 14(2):  243-249. 

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The Ke-Bai fault is a large overthrust fault. Its sealing properties control the distribution of oil accumulation in the northwestern area of Junggar basin. By analysis of the tectonic stress, fault zone characteristics, logging response and formation water features, we come to a conclusion that the Ke-Bai fault is typical of sectional sealing. The fault seals well where the fault strike is vertical to the tectonic stress, the tectonic mudstone rock is developed, the thickness of the damage zone, change of AC and increment of SDT are relatively small, the formation water is of CaCl2 type near the fault zone, and the Na+ to Cl- ratio is less than 0.7. The fault seals badly where the intersection angle of fault strike with the tectonic stress is less than 45°, the thickness of the damage zone, change of AC and increment of SDT are relatively large, the formation water is of NaHCO3 or Na2SO4 type, and the Na+ to Cl- ratio is more than 0.85. The sector of the fault where it seals badly is the major migration pathway, while the foot wall part where the fault seals well is the preferential place for oil accumulation.

The Lower Limit Values of Physical Properties for the Mesozoic HydrocarbonCarrier Beds in the Karamay-Baikouquan Area and Their Relation with Petroleum Accumulation

ZHANG Yue-qian

2008, 14(2):  250-255. 

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The porous permeable strata are called carrier beds. They are the pathways for hydrocarbon migration. The transporting capacity of the carrier beds is related with the physical properties of the rocks. The better the physical property is, the stronger the transporting capacity is. Otherwise, the worse the physical property is, the weaker the transporting capacity is. Based on the analysis of the Mesozoic carrier beds in Karamay and Baikouquan area it is determined that the lower limit values of physical properties for the hydrocarbon carrier beds are not the fixed numbers. They are related with the burial depth, oil properties and the deposit conditions. The distribution of the carrier beds has a direct control on the distribution of petroleum reservoirs. Only if the traps are distributed on the carrier beds nicely, they can become the reservoirs.

Research on Relation between Oil Properties and Petroleum Pool Formation of Permian in the 5th & 8th Districts, Northwestern Margin of Junggar Basin

WANG Xu-long

2008, 14(2):  256-261. 

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By analysis of oil physical properties and organic geochemical features of the Permian System in the 5th and 8th districts,northwestern margin of Jungar basin, the authors come to a conclusion that the initial oil pools here were formed in an earlierstage, and afterwards they were viscosified and destroyed. At the middle and later stages the oil pools were formed again and preserved. The total organic geochemical characteristics of different generations of oil were similar and mainly originated from the Fengcheng Formation. Because of difference of reservoir formation times and secondary recreation roles, the crude oil can be divided into A1 and A2 types. The type A1 oil is of limited distribution. It was generated, migrated and accumulated in the earlier stage, and the oil pool formation time was early too. Afterwards, the oil pools were destroyed, and the crude oil was biodegradated and experienced water washing and oxidation, and therefore its density and viscosity are higher. The type A1 crude oil pools were formed in the middle to late Hercynian time, and its densification time was chiefly in the late Hercynian to early Indosinian time.The type A2 crude oil is of wide distribution in this area. Its generation, migration, accumulation and pool formation time was later,chiefly in the Mesozoic and even still later time. Most oil pools found in the Permian sediments in this area belong to this type.

Differences between Oil and Gas Accumulations in Zhongguai, 5th & 8thDistricts of Karamay Oil Field, Northwestern Margin of Junggar Basin

YANG Hai-feng

2008, 14(2):  262-268. 

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There are significant differences between accumulations of crude oil and accumulations of natural gas in their distribution, hydrocarbon source, accumulation time and caprock thickness in Zhongguai, 5th and 8th districts of Karamay Oilfield,Junggar basin, Northwest China. The natural gas accumulations occur in the localities far away from the Ke-Bai thrust fault and are mainly sourced from over-matured humic kerogen in the Jiamuhe Formation of the Permian. However, the crude oil reservoirsare distributed in the localities near the fault, and are chiefly derived from the matured sapropelic kerogen in the Fengcheng Formation of the Permian. The charging time of the natural gas was later as compared with that of the crude oil, although both of them were accumulated in the Late Triassic to Jurassic. The cap rocks less than 10 m thick could hardly seal natural gas, but could effectively seal small scale crude oil reservoirs in this area.

Distribution Regularities and Appraisal of Unproved Reservoirs for the Jurassic in the Northwestern Margin of Junggar Basin

HOU Jia-gen

2008, 14(2):  269-275. 

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The Huwan area in the northwestern margin of Junggar basin is a highly prospected area for more than 50 years. Since 1998 several new oil reservoirs have been discovered in this area including KQ-10 and KQ-109 reservoirs. In this paper, 835 old well logging data and observations of cores from 8 wells and microfacies analysis were used to finely study the reservoirs. It is concluded that the Qigu Formation and Badaowan Formation belong to the braided river sedimentation. The high permeable reservoirs mainly belong to the batture and channel sedimentary microfacies. They are distributed in a wide banded shape. By using two quantitative oil identifying models, 56 oil layers were determined from the old drilled wells for Qigu Formation. The types, controlling factors and distribution of hydrocarbon accumulations in the Badaowan Formation and Qigu Formation were summarized. It is shown that the reservoir of Qigu Formation belongs to the lithologic oil pool and the reservoir of BadaowanFormation belongs to the stratigraphic oil pool. The latter reservoir has a further exploration potential and the former reservoir may have a continuous distribution.

Favorable Target Evaluation for the Jurassic Reservoirs in the 6th and 7th Districts at the Northwestern Margin of Junggar Basin

XUE Xin-ke

2008, 14(2):  276-282. 

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In view of reservoir heterogeneity and incomplete subsurface data during reservoir evaluation of nonmarine facies,according to rock cores, well logging and seismic data of Jurassic in 6th and 7th districts at the NW margin of Junggar basin, taking sequence stratigraphy, sedimentology and logging geology as theoretical guidance, the sedimentary microfacies were analyzed, the logging evaluation of oil reservoirs was made, and the reserve distribution was illustrated. The J3q3 sand layer of Qigu Formation was evaluated comprehensively. The study shows that the braided river is dominated in the study area. Three microfacies, i.e. channel bar, braided channel and floodplain were identified, and the last is very limited. The oil reservoir distribution is controlled by fault and sedimentary microfacies. The favorable oil reservoirs are located in the channel bars of structurally high part close to the fault. According to the microfacies analysis, the reservoir logging evaluation and related reserve calculation, it was determined that the new incremental productive area is 8.37 km2, and the new incremental controlled reserve is 719×104 t. Two favorable target districts were optimized, four appraisal wells were proposed, and the scientific bases were provided for further reservoir exploitation in the study area.