Table of Content

08 December 2014, Volume 35 Issue 6

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The first marine petroleum discovery in China and its significance

Kang Yuzhu

2014, 35(6):  749-752.  doi:10.11743/ogg20140601

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The Well Shacan-2 in northern Tarim Basin was the first marine petroleum discovery in China, indicating a strategic breakthrough in the oil and gas exploration of marine sequences in the country. The exploration histroty of the Tarim Basin can be traced back to the 1970s when a strategic project was launced to assess the petorluem potential of the basin. In 1984, the Shacan-2 was tested with a high rate of oil flow in the Ordovician dolomite in northern Basin. In 1990, the giant Ordovician oil field was discovered in Tahe area. More than 30 years of exploration practice had not only inspired us grealty but also produced technology series for developing oil and gas from fractured-vuggy reservoirs in marine carbonates. A review of the history may serve to help us to be more brave and innovative in our future path of finding oil and gas for the country.

Natural gas source and deep gas exploration potential of the Ordovician Yingshan Formation in the Shunnan-Gucheng region, Tarim Basin

Wang Tieguan, Song Daofu, Li Meijun, Yang Chengyu, Ni Zhiyong, Li Huili, Cao Zicheng, Zhang Baoshou, Feng Zihui

2014, 35(6):  753-762.  doi:10.11743/ogg20140602

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Shunnan-Gucheng region is located at the convergence of three structural units: Shuntuoguole Low Uplift (Aman low Uplift), Katake Uplift (Tazhong Uplift) and Guchengxu Uplift (Gucheng Low Uplift), in Tarim Basin. In recent years, gas flows have been tested in 11 exploratory wells drilled into the carbonate reservoirs in the Middle-Lower Odovician Yingshan Fm. or the Middle Odovician Yijianfang Fm. -Middle-Lower Odovician Yingshan Fm. (mainly the Yingshan Fm.)in Shunnan-Gucheng region and the Lower Cambrian Xiaoerbulake Fm. in the Tazhong No. 1 Structure of the basin. Among the 11 wells, the well Shunan-5 and Gucheng-9 tested the highest flow rate of 165. 8×10⁴ m³/d (open flow)and 107. 8×10⁴ m³/d, respectively, showing great gas potential in Shunnan-Gucheng region. Composition analyses of hydrocarbon and stable carbon isotopes of gas samples from 11 wells in Shunnan-Gucheng region and Tazhong No. 1 Structure indicate an over-matured dry gas, with dry coefficient between 0. 99 and 1. 00 and carbon isotopic values (δ¹³C₁-δ¹³C₄) bigger than-42. 5‰. Correlation with the typical gas samples with different sources in Tarim Basin reveals that the gas in Shunnan-Gucheng region originated from source rocks in the Cambrian. Combining fluid inclusion microthermometry with single well numerical modeling of stratigraphic burial as well as thermal histories and measurement of vitrinite reflectance R_o profile on samples from Well Shunnan-4, we determined that the studied gas reservoirs were typical late-stage gas accumulations with gas-charging occuring druing the Miocene (22-10 Ma).

A study on the distribution of oil and gas reservoirs controlled by source-cap rock assemblage in unmodified foreland region of Tarim Basin

Jin Zhijun

2014, 35(6):  763-770.  doi:10.11743/ogg20140603

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The marine sequences in the unmodified foreland region of Tarim Basin had experienced multiple stages of hydrocarbon accumulation with alternative later adjustment, destruction and preservation processes. Consequently, the distribution of oil and gas reservoirs in the study area today is very complicated. The article proposed that both source rocks and cap rocks had played roles in controlling the distribtuion of oil and gas reservoirs and suggested that the interaction between favorable timing of hydrocarbon generation with formation of cap rocks as well as the sealing capacity of cap rocks are the key factors that had affected the distribution of oil and gas reservoirs. Macroscopically, different collocation of source/cap rocks had determined where and how the oil/gas-bearing formations were located. Based on this understanding, the paper also discussed the distribution pattern of oil and gas reservoirs and predicted potential exploration targets for middle-and large-sized oil and gas fields in the study area.

Exploration practice and prospects of giant carbonate field in the Lower Paleozoic of Tarim Basin

Qi Lixin

2014, 35(6):  771-779.  doi:10.11743/ogg20140604

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This paper reviews the exploration history of large oil/gas fields in the Lower Paleozoic carbonates in the Tarim Basin, sums up experiences and draws the necessary lessons, analyzes the geological conditions of the Shunnan gas reservoir, and predicts the exploration potential of the Lower Paleozoic carbonates, so as to provide references for exploration in the Cambrian and Ordovician carbonates. Exploration practice shows that the discovery of giant oil and gas fields are closely related with theory innovation, mentality transformation and technological progress. The exploration of the Lower Paleozoic Carbonate reservoirs has experienced three stages including buried hill (karst monadnock) trap stage, karst fractured-vuggy trap stage and interior fractured-vuggy trap stage. The well SN4 in Shunnan gas reservoir discovered an interior fractured-vuggy reservoir of multiple origins and tested high gas flow in a dry gas reservoir in Yingshan Formation, indicating the exploration of the Lower Paleozoic carbonates entering the stage of interior fractured-vuggy trap with multiple origin. The main target areas of the future exploration are the southern slope of Shaya Uplift, the northern slope of the Tazhong Uplift and the southern slope of central uplift zone.

Hydrocarbon accumulation characteristics in Lower Paleozoic Carbonate reservoirs of Tarim Basin

Huang Taizhu, Jiang Huashan, Ma Qingyou

2014, 35(6):  780-787.  doi:10.11743/ogg20140605

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The first major breakthrough of Well Shacan 2 in exploration of the Lower Paleozoic carbonates promotes the successive discoveries of Tazhong, Tahe and Hetianhe oilfields and continuous innovation of exploration theory of large fields. And the hydrocarbon accumulation theory of carbonate reservoir is also gradually put forward. The researches on hydrocarbon accumulation conditions and typical reservoirs in the Lower Paleozoic show that the formation and distribution of large carbonate fields are controlled by the temporal-spatial combination of stable paleo-uplifts and slopes, several heterogeneous carbonate reservoirs, abundant oil sources and multiple periods of hydrocarbon accumulation. The south slope of Shaya uplift is a complex fractured-vuggy reservoir system formed due to the modification by the Caleton-Hercynian karstification and multi-staged hydrocarbon charging from Manjiaer and Awati depressions. Single fractured-vuggy reservoir has re-latively independent oil, gas and water system. These reservoirs are superimposed vertically and connected laterally with inhomogeneous oil enrichment. Katake uplift in the central of Tarim Basin possesses the typical characteristics of composite hydrocarbon accumulation. The gas oil ratio decreases from the Cambrian to the upper formations and the patterns of hydrocarbon enrichment are more complicated than that in Tabei. The hydrocarbon accumulation is characterized by lateral zonation, vertical layering, incharging in multi-places and accumulation in relative structural highs. Hetian paleo-uplift is one of three lower Palezoic uplifts with favorable hydrocarbon accumulation conditions. Yubei Ordovician reservoir was discovered recently in the Hetian paleo-high and it was an uneven karst fractured-vuggy carbonate reservoir system. However, the hydrocarbon accumulative conditions and patterns are still poorly understood. The south slope of Shaya uplift and its surroun-ding slope areas, Katake uplift and the northern slope Shunnan-Gucheng areas are potential target areas for exploration of the Ordovician, while Shuntuoguole uplift, Maigaiti slope, Madong fault belt of Tanggubasi depression, and Tabei, Tazhong-Bachu Cambrian-Ordovician deep formation area are the important areas for future exploration.

Hydrocarbon enrichment pattern and exploration potential of the Ordovician in Shunnan area, Tarim Basin

Yun Lu, Cao Zicheng

2014, 35(6):  788-797.  doi:10.11743/ogg20140606

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Shunnan area lies in the north slope to the down-thrown blocks of Tazhong No. 1 fault in Katake Uplift, and is adjacent to the hydrocarbon kitchen of Manjiaer Depression. The study area has regionally good cap rocks, and remains a slope for a long geological period, thus it is the main area where hydrocarbons migrate to and accumulate in. Multi-group NE-trending faults developed here were conducive to reservoir development and hydrocarbon accumulation, all such provide the favorable conditions for the formation of large oil and/or gas fields. Multi-period tectonic movements from the Caledonian to Hercynian controlled development of the multilayer and multiphase carbonate reservoirs. With tectonic movements and sea level fluctuations, the Ordovician carbonate rocks experienced multi-period exposure and led to the formation of karst reservoirs. The reservoirs were reconstructed by meteoric water and deep thermal fluids through faults, resulting in fractured-vuggy reservoirs. In brief, reservoirs determined the scale of hydrocarbon accumulation, and the strike-slip faults restricted the hydrocarbon accumulation zones. Geochemical analysis shows that the natural gas is dry gas with high maturity in Shunnan, and its accumulation period was the Himalayan. The natural gas in Shunnan area features in highly variable ln(C₁/C₂)value but low variable ln(C₂/C₃) value, a character obviously differing from that of cracked gas. The inclusions in several wells are dominated by methane gas inclusions, but rare bituminous gas phase inclusions and bitumen inclusions, indicating kerogen cracking origin. Analyses of drilling and test data reveal that the Ordovician carbonate gas accumulation is likely to be a large fractured-vuggy gas reservoir cluster featuring in vertical superimposition, lateral connection, high gas column and strong control of reservoir heterogeneity.

Hydrocarbon accumulation patterns and favorable exploration areas of the Ordovician in Tarim Basin

Lü Haitao, Ding Yong, Geng Feng

2014, 35(6):  798-805.  doi:10.11743/ogg20140607

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Oil and gas shows are common in the Ordovician carbonates in Tarim Basin, but few oil and gas fields (reservoirs) have been found. Based on the current Ordovician oil and gas fields (reservoirs)in Tarim Basin, summarizing the hydrocarbon accumulation patterns and main controlling factors of enrichment is helpful to identify potential exploration target areas. According to the analysis of the Ordovician geology conditions, reservoir types, sealing conditions and petro-leum system (source rocks, the main hydrocarbon generation and expulsion period, petroleum system division), we analyzed the areal and lateral distribution characteristics of the Ordovician reservoirs. The results show that fractured-vuggy carbonate reservoir is the main reservoir type of the Ordovician in Tarim Basin, which are characterized by large scale continuous distribution or zonal distribution. In combination with the previous understandings, we summarized the regularity of the Ordovician carbonate reservoirs in Tarim Basin: fractured-vuggy karst reservoirs that matched well with hydrocarbon generation and expulsion period and were development on paleo-uplifts and paleo-slopes, and/or fractured-vuggy carbonate reservoirs of multiple origins that were controlled by deep faults are characterized by large scale hydrocarbon enrichment and the development of reservoirs is the key factor controlling differential hydrocarbon accumulation. It is proposed to understand the hydrocarbon accumulation process from a dynamic perspective and to identify carbonate exploration targets via dialectical thinking. According to the comprehensive analysis, the deep interval of the Ordovician in Tahe oilfield and the Ordovician in south Shunnan-Gulong area are the most favorable target for discovering large and medium sized gas fields (reservoir).

Timing and chronology of hydrocarbon charging in the Ordovician of Tahe oilfield, Tarim Basin, NW China

Chen Honghan, WuYou, Feng Yong, Lu Ziye, Hu Shouzhi, Yun Lu, Qi Lixin

2014, 35(6):  806-819.  doi:10.11743/ogg20140608

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It is of great significance for study of the temporal-spatial coupling of hydrocarbon migration and accumulation with trap formation to investigate the timing and chronology of hydrocarbon charging in multi-stages superimposed basins. This study, firstly, employed the analytic methods of crude oil and oil inclusion molecular geochemistry, single fluid inclusion microspectrofluorimetric and thermal maturity assessment, and microthermistry to determine the timing of hydrocarbon charging in the Ordovician strata of Tahe oilfield. The results show that the hydrocarbons in the Ordovician were charged in three periods. Secondly, the indirect dating method of aqueous inclusion and the direct dating method of stable radioactive isotope of Re-Os and K-Ar were used to establish the chronological sequence: the first charging event took place during the Middle to Late Caledonian (463. 2-414. 9 Ma), the second charging event happened during the Late Hercy-nian (312. 9-268. 8 Ma), and the third charging event occurred during the Himalaya (22-4. 8 Ma). All the three events occurred in the main Tahe oilfield, but only the first and third events developed in the peripheral areas of Tahe oilfield, such as Aiding, Tuoputai and South Tahe areas. Obviously, a deep understanding of the heterogeneity of hydrocarbon charging events and chronology in Tahe oilfield, which were controlled by differential structure evolution, will be helpful for hydrocarbon exploration in the Ordovician of other structural belts of Tarim Basin.

Characteristics comparison and origin of natural gas in Tahe oilfield

Gu Yi, Huang Jiwen

2014, 35(6):  820-826.  doi:10.11743/ogg20140609

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The geochemical features of natural gas in the Ordovician vary obviously from east to west in Tahe oilfield. After years of exploration, a large amount of testing data for natural gas accumulated. In combination with analyses of the geologic conditions and stages for natural gas accumulations in Tahe oilfield, studies of natural gas compositions and C/H isotopic features, led to the following conclusions. In the western part of Tahe oilfield, the natural gas in the Ordovician reservoirs was mainly dissolved or associated gas, featuring in lighter methane carbon isotopes, and mainly originated from the initial cracking of crude oils. In the central part of Tahe oilfield, the natural gas in the Ordovician reservoirs was mainly associated gas and partially condensate gas, and was mature/high-mature gas which was charged at the same time as crude oils. In the eastern part of Tahe oilfield, the natural gas in the Ordovician reservoirs feature in highly dry coefficient and heavy methane C/H isotopes, with higher maturity than those samples from both the western and the central Tahe oilfield. For some natural gas samples, δ¹³C₁ >δ¹³C₂, indicating that the natural gas were charged and accumulated in stages. The first stage of natural gas charging and accumulation took place when hydrocarbon accumulated in the central part of Tahe oilfield. In the second stage of natural gas charging, the cracking of over-mature hydrocarbon played an important role. The nature gas mainly was sourced from the cracking of crude oil and kerogen, and charged during the late Himalayan period. The different origins of natural gas and the mixing of natural gas during different stages explained the obviously different features of natural gas among different regions in the Tahe oilfield.

Distribution of major hydrocarbon source rocks in the Lower Palaeozoic, Tarim Basin

Yun Jinbiao, Jin Zhijun, Xie Guojun

2014, 35(6):  827-838.  doi:10.11743/ogg201406010

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Analysis based on outcrop and drilling data indicates that source rocks can be formed in different tectonic settings. Transgressive process is benefit for source rocks development in various tectonic environments. Source rock development environment is increasingly limited from basin stretching to stable subsidence and finally to returning stage. The extensive transgressive overlap in stretching process is most favorable for the deposition of source rock in a large area. Source rocks are mainly distributed in the platform edge shelf and internal depression in stable subsidence stage. The closed environments such as intra-platform depression and regions behind high energy facies are favorable for source rock development in the returning stage. Sedimentary sequence and tectonic evolution show that the Cambrian-Ordovician source rocks in Tarim Basin experienced a whole process from basin stretching to returning stage, i. e. basin stretching in the Early Cambrian, stable subsidence in the Middle Cambrian to Middle Ordovician, and tectonic return in the Late Ordovician to Silurian. So that, 3 types prototype basins were formed. Stretching process in the Early Cambrian was the best period for source rock development. The northern depression, shelf in the southern basin edge and basin facies were the best zones. The basin showed difference in east and west during the Early and Middle Ordovician, and platform edge and limited basin developed in Manjiaer depression Aman transitional belt, being favorable for the development of source rocks. In the Late Ordovician, besides the high energy belt on the platform edge, Awati Depression and Aman transitional belt were intra-platform depressions, where limestone was deposited and was favorble for source rock development. Therefore, the major source rocks at different positions have different characteristics.

Dynamic evolution assessment of the Paleozoic hydrocarbon cap rocks in Bachu-Magati area, Tarim Basin

Zhang Zhongpei, Wang Yi, Li Jianjiao, Liu Shilin, Yun Jinbiao, Li Jingchang

2014, 35(6):  839-852.  doi:10.11743/ogg20140611

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With the aim of assessing the dynamic evolution of marine hydrocarbon cap rocks experienced multi-cycle tectonic movements, the Paleozoic in the Tarim Basin was taken for case study. Based on the experimental test results of large amount of cap rock samples, this paper reconstructed the evolutionary process of its sealing capability by taking geological evolution as a clue and in combination with its macroscopic change, physical property evolution and stress variation. The results show that the polyphase burial changes due to uplift-depression conversion obviously controlled the sea-ling property of cap rock. Most of the Carboniferous mudstone caps had good sealing capacity in the late Hercynian in most areas except for the northwestern part where the burial depth of cap rocks was less than 2100m due to differential uplifting. Later, the sealing capacity of the Carboniferous mudstone deteriorated in the northwestern part, but enhanced in the eastern and southern parts. Salt-gypsum rocks in the Middle and Lower Cambrian established their sealing capacity in the Early Ordovician, but it became ineffective in the southwest of Yubei area where its burial depth was less than 1230m in the early Hercynian. The Carboniferous mudstone caps were regarded as the most important regional seal. Magiati slop, structural belt at the southern margin of Bachu uplift and stable area between large fault zones are the favorable areas for oil and gas preservation.

Types of sequence boundaries and their control over formation and distribution of quality carbonate reservoirs

He Zhiliang, Gao Zhiqian, Zhang Juntao, Ding qian, Jiao Cunli

2014, 35(6):  853-859.  doi:10.11743/ogg20140612

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Carbonate sequences contain sequence boundaries of various orders and genetic types. Accordingly, the development of the carbonate reservoirs was controlled by various geological processes in association with those sequence boundaries. In general, the sequence boundaries could be categorized into six orders according to the magnitude of tecto-nic movement, and the relative range and the duration of sea level change. Among these boundaries, the sequence boundaries of higher orders of (Ⅰ to Ⅲ) correspond to various unconformities, while that of the lower orders correspond to flooding surfaces or local deposition events. The formation of the scaled karst reservoir was controlled by the I- and II-order sequence boundaries, and the ancient geomorphic location of the karst landform had the most significant effect on the formation and preservation of the reservoir. There was a clear deviation between the degree of the reservoir development and the controlling factors of karstification in various ancient geomorphic locations. As a whole, the karstification was relatively weak in the third-order sequence boundary with a limited reservoir distribution, but was still closely related to the formation and distribution of the platform margin and the reef flat face. There was a dual-control on both the deposition and diagenesis by the third-order boundary as well. The exposure times were relatively short in the fourth-order and fifth-order sequence boundaries. As a result, large scale reservoirs were rare due to the lack of post-diagenesis effect. The types and orders of the sequence boundaries have various influences on the formation and distribution of the carbonate reservoir owing to different geological processes. Thus, the prediction and evaluation of the carbonate reservoirs could be carried out by the fine division of sequences, recognition and description of the sequence boundaries. However, the deposition filling pattern, facies belts distribution, structural deformation and fluid movement should also be taken into consideration.

Characteristics, origin and geological implications of the Cambrian microbial dolomite in Keping area, Tarim Basin

Hu Wenxuan, Zhu Jingquan, Wang Xiaolin, You Xuelian, He Kai

2014, 35(6):  860-869.  doi:10.11743/ogg20140613

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Reefal (biohermal) carbonate rocks are important petroleum reservoirs. In Tarim Basin, the occurrences of reefal facies in the Ordovician sequences have been well documented, whereas that in the Cambrian strata have been rarely reported. In this study, the occurrences of stromatolite dolomite and reefal dolomite in the Cambrian Qiulitage Formation at Keping area, Tarim Basin were reported. Based on careful field observations, the micro-structures of these microbial dolomite were investigated by using conventional microscope and scanning electron microscope (SEM). It is suggested that the microbial reefs formed in shallow subtidal environment where the water was turbulent, the deposition rate was relatively high and the platform was relatively open. In contrast, the stromatolite dolomites were developed in tidal flat-intertidal facies. Nanoscale spherical dolomite formed nanocrystal aggregats including dumbbell and chain-like structures. In addition, tubular and sheet-like aggregates were suggested to be in close association with the mineralization of extracellular polymeric substances (EPS). All the above nanostructures resemble the morphology observed in microbial culture experiments. These evidences support the conclusion that microbial mediation might have played an important role in the formation of the Cambrian dolomite. During the formation of microbially mediated dolomite, sulfate reducing bacteria did not play an important role because no pyrite was observed, then SO₄^2- may not serve as an important kinetic inhibitor for the precipitation of dolomite. Thus it is proposed that the hydration of Mg²⁺ should be emphasized in the future study on the origin of dolomite.

Lithofacies, diagenesis zone and reservoir origin of the Ordovician in eastern tectonic belt of the Maigaiti slope

Qian Yixiong, Yu Tengxiao, Zhou Linfang, Yue Yong, Qiao Guilin, You Donghua, Shao Zhibing, Jing Ting

2014, 35(6):  870-882.  doi:10.11743/ogg20140614

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Through the detailed research of fracture style, strata feature, sedimentary facies (microfacies), diagenisis process and reservoir characteristics of two exploratory wells in carbonate rocks of Maigaiti slope, the following conclusions can be obtained. The Yingshan Formation consists of a set of intra-platform beaches-inter-beach seas of open platform, and the Penglaiba Formation consists of dolomite flat, dolomite-limestone flat and granule shoal. The diagentic processes in Lower and Middle Ordovician mainly include submarine cementation, early atmospheric fresh water dissolution and cementation, compaction and pressure solution, two periods of atmospheric water karstification, burial and hydrothermal modification. Due to the influence of multiphase tectonic movements, open and relative closed diagenetic facies could coexist in the same limestone. The open diagenetic facies feature in small thickness and multiple intervals. In contrast, the closed diagenetic facies is characterized by large thickness and less intervals. The quality of reservoir was controlled by muti-phases unconformities, faults, fractures and water-rock interaction, so the effective reservoir diagenetic sequences were submarine cementation, compaction and pressure solution-two periods atmospheric water karstification, buried dissolution and hydrothermal reformation. Karst reservoir facies zones were mainly distributed in the superimposed tectonic belt between fault fold top, folding wing or fracture with high frequency sequence boundary, and show a karstification model featuring in superimposition of vadose zone and water hole under the control of multiphase tectonic movements.

Tectonic-sedimentary evolution of Tazhong-Bachu-Maigaiti area and its control on the Ordovician reservoir

Li Huili, Liu Shilin, Yang Shengbin, Zhang Jibiao, Gao Xiaopeng

2014, 35(6):  883-892.  doi:10.11743/ogg20140615

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Tectonic-sedimentary evolution history of the Early Paleozoic was further studied in the Tazhong-Bachu-Maigaiti area by using the drilling data and seismic data obtained recently. The results show that a unified large "paleo-uplift" was formed in the Tazhong-Bachu-Yubei area in the Middle Caledonian movement, which controlled the distribution of the sedimentary facies during the early and middle stage of the Late Ordovician. Thenceforth, regional differentiation and development occurred in this large "paleo-uplift" from the Late Caledonian to the Early Hercynian, which was characterized by the development and finalization of the Hetian paleo-uplift, by the formation of the NE-trend buried hill structure zones in the East Yubei area and the Tanggubasi Depression, and by the development of the NE-trend strike-slip faults in the Tazhong North Slope. The macroscopic geologic conditions for the development of the Ordovician carbonate reservoir in these areas were different due to the control of the tectonic-sedimentary evolution characteristics of the Early Paleozoic. The Tazhong-Bachu area was located in the main part of the large middle-Caledonian paleo-uplift, in which the karst reservoirs of Lower Ordovician were developed in this stage. During the Late Caledonian to the Early Hercynian, the karst reservoirs of Lower Ordovician might be developed continually in the Middle and West of Yubei area, the western Bachu area, and the NE-trend buried hill structure zones of the Tanggubasi Depression. Fractured-vuggy Ordovician reservoirs, related with the multi-period activities of the NE-trend faults, possibly developed in the Tazhong North Slope.

Fault and fracture characteristics of the Middle-Lower Ordovician in Yubei area, Tarim Basin

Li Yingtao, Yuan Xiaoyu, Ye Ning, Huang Qingyu, Su Bingrui

2014, 35(6):  893-902.  doi:10.11743/ogg20140616

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The Yingshan Formation marine carbonate reservoirs in Yubei Area were examined by applying various data such as cores, FMI, 3D seismic attributes and thin sections. Although extensive erosion has led to the absence of the Upper Ordovician in the study area, the carbonate reservoirs are not typical weathering-crust reservoirs, but fault-controlled karst reservoirs. The reservoir spaces are mainly small-scale solution vugs and fractures, the distribution of which is strongly controlled by fault systems. This paper first studied the timing of fault, structural styles and the azimuth of fractures in a macroscopic view, then analyzed the features of fracture in a microscopic view, and finally discussed the relationship between the fractures and reservoir reconstruction. The results show that meteoric corrosion downward along faults is critical to reservoir reconstruction. Whereas the burial diagenesis (deep hydrothermal erosion or organic acid) also has influences on reservoir pores but is not the main factor. Faulted reservoirs occur in the whole Ordovician in Yubei Area, especially in the top weathering crust and the bottom transition from dolomite to limestone. The overthrust system of NE-trending cap rock detachment type in the middle structural layer of east fault-fold structural belt is adjacent to the NE-trending vertical strike slip fault in the lower structural layer. They together consist of the hydrocarbon migration pathway in the eastern faulted horst belt, which is the most favorable zone for fractured or fractured-vuggy reservoirs.

The Silurian hydrothermal clastic reservoirs in Tarim Basin: evidences, mineral assemblages and its petroleum geological implications

Shi Zhiqiang, Wang Yi, Jin Xin, Xiao Kai

2014, 35(6):  903-913.  doi:10.11743/ogg20140617

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Comprehensive analyses of various data including core, scanning electron microscope, energy spectrum, thin section, fluid inclusion mineral assemblages and authigenic mineral forming temperature reveal evidences that hydrothermal clastic reservoirs exist in the Tarim Basin. The petrological evidences were found in the Silurian Kepingtage Formation in Well Zhong-1 as interlayers of ultrabasic intrusive rock, hydrothermal breccias, thermal burning phenomena and hydrothermal corrosion caverns. Three hydrothermal mineral assemblages were identified in the clastic reservoirs of Kepingtage Formation in Tazhong and Shuntuoguole area in Central Tarim Basin. Thermal fluids invasion occurred mainly in the Permian, causing the ionic disturbance of pore fluid. The dissolution in the clastic reservoirs accelerated the development of secondary porosity and the precipitation of hydrothermal minerals, which improved the overall reservoir quality and was an ignorable factor for the the formation of the clatic reservoirs in the Kepingtage Formation. The infusion of hydrothermal fluid would destroy the paleo-oil reservoirs, thus might play a key role in the formation of the Silurian asphalt sandstones in the Tarim Basin.

Discovery of large kink-band structures and petroleum exploration implications in Bachu area, Tarim Basin

Wang Yi, Zhang Zhongpei, Zhang Bo, Yun Jinbiao, Liu Shilin, Song Haiming

2014, 35(6):  914-924,949.  doi:10.11743/ogg20140618

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Outcrop observation, reprocessed seismic reflection profiles and drilling data were integrated to describe the characteristics of structural deformation of Bachu uplift. The previously interpreted structural style was "one uplift between two fault zones". However, the previously interpreted fault zones with deep angle were not faults but a pair of conjugate kink-band structural zones. It is believed that the large kink-band structural zones and the fold structures in the area are the dominant styles of structural deformations in the abdomen of the cratonic basin under compression. The widely developed gypsum-salt bed and discontinuous bed boundary and regional along-bedding compressional deformation are the vital geological factors controlling the formation of the large kink-band structural zones. Marks like geometry, seismic reflection and stress were presented for identification of large underground kink-band structural zones based on their formation mechanism and differences with large fault zones. On the basis of research on history of Bachu uplift, we discussed the implications of the Paleozoic kink deformation to reservoir improving and traps forming, analyzed their exploration prospect and outlined the favorable zones for discovering large anticlinal reservoirs of kind fold type.

Superimposition, evolution and petroleum accumulation of Tarim Basin

Zheng Menglin, Wang Yi, Jin ZhiJun, Li Jingchang, Zhang Zhongpei, Jiang Huashan, Xie Daqing, Guo Xin

2014, 35(6):  925-934.  doi:10.11743/ogg20140619

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According to previous research and petroleum exploration data, Tarim Basin has experienced four evolution stages: the Cambrian-Devonian faulted basin, the Carboniferous-Permian basin, the Triassic-Jurassic intra-continental basin and the Cretaceous-Cenozoic large basin. The evolution of the Cambrian-Devonian basin was controlled by the peripheral plates splitting and convergence and was characterized by clastic, evaporite and carbonate build-ups. The basement had a structural pattern of west uplifting and east depressing from the Cambrian to Ordovician. The Late middle Ordovician tectonic events lead to the formation of the Shaya uplift, Tazhong uplift and Hetian paleo-uplift. The Late Silurian tectonic events led to the uplift and erosion of the basin as a whole. The Carboniferous - Permian basin was characterized by epicontinental sedimentary and intra-continental depression evolution of carbonates, gypsum-salt and clastic rocks. The basement had a structural pattern of southwest depressing and northeast uplifting in this stage. At the end of Permian, tectonic events led to the uplift and erosion of the basin. In the Triassic, the southern and northern piedmont depression and an intra-platform depression were formed. The Jurassic basin only existed on the front tectonic belt. From the Cretaceous to Paleogene, the Tarim gradually formed a large-scale intra-continental basin. The sea water invaded Basin again. At the end of Late Cretaceous, tectonic events led to the demise of the basin and the overall uplifting. The Late Cretaceous deposits were limited in the southwest and east Tarim. Kuche and southwest Tarim foreland basin formed because of tectonic event of the Late Neogene. Evolution of the superposed basins led to the formation of several sets of organic-rich source rocks, such as the Lower-Middle Cambrian, Middle-Upper Ordovician, Triassic, Jurassic. The gypsum, salt, mud and coal-bearing rocks developed in the Cambrian, Ordovician, Carboniferous, Triassic, Jurassic, Paleogene and Neogene are good seals. These seals and source rocks consist of high-quality combination within the basin. Carbonate karst reservoirs and sandstone reservoirs and the structural traps consist of favorable reservoir-trap combinations. The lower, middle and upper assemblages are favorable sequences for exploration in the basin. Exploration results show palaeohigh, paleoslopes and ancient source rocks control oil and gas distribution of the basin

3D seismic exploration technology of carbonate gas reservoir in Shunnan desert area of Central Tarim Basin

Chen Mingzheng, Li Zongjie, Wang Baocai

2014, 35(6):  935-943.  doi:10.11743/ogg20140620

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In Shunan desert area of central Tarim Basin, large sand-dunes with large relief cause great attenuation and absorption of seismic signals. In addition, the deep Ordovician carbonates have strong heterogeneity. All these unfavorable conditions give rise to poor imaging of the fractured-vuggy reservoirs and small faults, bringing significant difficulties to reservoir prediction and trap identification. Therefore, in this paper, a comprehensive research of 3D seismic acquisition, target-oriented processing and interpretation is carried out to solve the above problems. First, a better acquisition scheme based on fractured-vuggy carbonate reservoir imaging is designed, in which the shooting and receiving parameters are optimized to improve seismic data quality. Then, a series of imaging techniques like pre-stack time migration and RTM migration imagining with suitable key parameters for the Ordovician carbonate reservoirs are established to improve imaging precision of the fractured-vuggy carbonate reservoirs and small faults while preserving true amplitude. In addition, accor-ding to the seismic reflection features and types of carbonate reservoirs in the Ordovician, a set of reservoir prediction and hydrocarbon detection techniques for fractured-vuggy reservoirs and fractured traps is developed including seismic pattern recognition of reservoir in different stratigraphic section and sensitive attributes analysis. Finally, special techniques of traps prediction and characterization are developed and they divide different zones by using faults and define borders of traps by using attributes. Techniques featuring in five factors including type, genesis, model and technique are developed for target optimization and evaluation. Several wells discovered natural gas in this area, providing a solid proof for the effectiveness and applicability of 3D seismic exploration technology of carbonate gas reservoir in Shunnan Desert Area.

Numerical simulation technology and its application to fractured-vuggy carbonate reservoirs

Kang Zhijiang, Zhao Yanyan, Zhang Yun, Lü Tie, Zhang Dongli, Cui Shuyue

2014, 35(6):  944-949.  doi:10.11743/ogg20140621

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Fractured-vuggy carbonate reservoirs have various types of reservoir space with significantly different sizes including vugs, fractures, and caverns. Fluid flow in this type of reservoirs is complicated by the coexistence of flow through the dissolved pores, flow in the fractures and flow in the caverns. The common numerical simulators are no longer suitable for fluid flow in this complex medium. In order to study the development mechanics and the remaining oil distribution in the reservoir, a mathematical model that couples cavern flow and fracture flow with seepage flow is established and discretized with the finite volume method. The equations are solved by using the self-adaptation method. A numerical simulation software for the fractured-vuggy reservoir is developed. The flows within a well group and a fractured-vuggy unit in Tahe oilfield were simulated respectively. The results show that the remaining oil in this kind of reservoirs mainly include the remaining oil at the top of caverns, marginal remaining oil, attic oil, the remaining oil at the bottom of caverns, the remaining oil around high permeable channel and the remaining oil in filled caverns. The results are consistent with the actual production data, verifying effectiveness of the fractured-vuggy carbonate reservoir numerical simulation model. This model is useful for extracting remaining oil and enhanced oil recovery for fractured-vuggy carbonate reservoirs.

Logging evaluation of carbonate reservoir effectiveness in Tahe oilfield, Tarim Basin

Liu Jianhua, Lin Xuemin, Zhang Weifeng, Zhang Xiaoming

2014, 35(6):  950-958.  doi:10.11743/ogg20140622

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The exploration and development of the Ordovician in Tahe Oilfield have been undertaken for more than ten years. In addition to the fine exploration in Tahe Oilfield, exploration is also performed outside of Tahe at present. In recent years, the exploration of the Ordovician carbonate reservoirs in Yubei and Tazhong shows that the reservoir effectiveness evaluation has been becoming more and more important. On the basis of the logging response characteristics and the mutual calibration of the core analysis, imaging logging and conventional logging, the dissolved pore reservoirs, fractured reservoir sand fractured-vuggy reservoirs are further subdivided, and well logging identification and evaluation methods are put forward for the effectiveness evaluation of the Ordovician carbonate reservoirs in Tahe Oilfield. The effectiveness of vuggy reservoirs is evaluated through cross-plotting of natural gamma and apparent formation water resistivity. Additionally, the effectiveness of fractured-vuggy reservoirs is evaluated by using the electric imaging local resistivity method. The practices show that these two methods can better solve the problem of reservoir effectiveness evaluation caused by filling of clay. For the different types of fractured-vuggy reservoirs whose effectiveness is influenced by the strength of fracture development, we studied the relationships between lithology, electrical property, porosity and oil-bearing property and performed mathematical simulation. Based on these studies, we put forward the evaluation model featuring in constant permeability, mutative vuggy porosity and fracture porosity. The application of this model shows that the coincidence rate of the logging interpretation has been improved. It provides evidences for the reservoir effectiveness evaluation, the formulation of completion scheme and the calculation of reserve parameters.