Oil & Gas Geology-2010, Volume 31 Issue 4

Deformation of the Late Palaeozoicaulacogen basin in the Great Nanpanjiang area

Ding Daogui，DengMo and ZhuWenli

2010, 31(4): 393-402. doi:10.11743/ogg20100401

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The Late Palaeozoic aulacogen（D-P 1 ）basin and later syneclise depression（P 2 -T 2 ）basin in the Great NanpanjiangˉYoujiang area are located mainly in Guangxi Province and also stretch into parts of Yunan，Guizhou，and Guangdong provinces. They experienced deformation due to the collision and closure of the AilaoshanˉHonghe prototethys ocean during the IndosinianˉEarly Yanshanian period. The over thrusting of three basal detachment bodies（MaguanˉHejiang，Xuefengshan and Yukansan）helped to form progressive tectonic deformation domains with strong to weak thrustingˉfolding in different places. The LiupanshuiˉZiyunˉDuan strikeˉslip belt acted as the deformation transform zone in the depression. These tectonic deformation domains controlled the generation，accumulation and distribution of marine oil/gas in the area.

3-D3-phase Darcy flow method and its application to the Kuqa Depression

ShiGuangren，Ma Jinshan and Chang Junhua

2010, 31(4): 403-409. doi:10.11743/ogg20100402

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The multi-phase Darcy flow method is the most complicated among all the modeling methods for secondary hydrocarbon migration. It has three major technical problems:（1）its model is too complicated to easily generate a solution，at least much difficult than that in reservoir simulation;（2）it is not easy to ensure the convergence and stability of the model and thus rendering a calculation of low accuracy or causing abnormal shut-down; and（3）it is very time-consuming as the calculation of a larger basin may take several days to complete. To tackle these problems，we tried the finite volume method based on PEBI（perpendicular and bisection）cells，the fully implicit solution based on the Newton iteration，and the ORTHMIN（orthogonal minimum）algorithm，and came to a conclusion that the key to deal with the problems is to correctly select the allowable maximum time step for a simulated block. Finally，we developed a program of3D3-phase Darcy Flow. An application of the software to the Kuqa Depression had gained positive results.

Comparison of the hydrocarbon accumulation patterns of petroliferous basins between the north and south parts of the West African coast

2010, 31(4): 410-419. doi:10.11743/ogg20100403

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The major petroliferous basins along Wes tAfrican coastline are typical superimposed basins of continental rift basins and passive continent almargin basins，therefore sharing the same regional tectonic evolution background. However，due to the difference of plate evolution process between the south and north parts of the West African coast，the basins vary in their form ing times，tectonic evolution characteristics，and deposition features，causing the basins in the south part significantly differs from that in the north in geological conditions，accumulation patterns and hydrocarbon abundance. The formation of basins in the north part is mainly connected with the breakup of the North Atlantic and the separation of the African from the North American plate. The structural evolution of North African also contributes to the formation of these basins. In the early stage of basin evolution，tectonic differentiation was strong and no unified depocenter formed. While in the late stage，the basins down warped as a whole and were influenced by the Atlas Orogeny. As a result，they vary greatly in height differences and their sedimentary thickness quickly thickens seaward. The formation of basins in the south and middle parts is chiefly related to the formation of South Atlantic and the separation of the African plate from the South American plates. In the early stage of basin evolution，a unified basin with similar sedimentary sequence and strata characters formed and its depocenter was located in central Gabon and lower Gongo basin. In the Tertiary，the unified basin began to differentiate and formed a series of delta and fanˉdelta depositional systems along the West African coast，such as the Niger delta basin and the Congo fan basin. However，the size or scale of these systems decreases progressively from north to south. This article is focused on the formation and evolution of the major petroliferous basins in the area and their controlling effects over the hydrocarbon accumulation. The comparative study can be used as a guide to screening of strategic petroleum provinces in West Africa.

A new approach to the exploration of lithologicstratigraphic reservoirs in the Hong-Che fault belt，the Junggar Basin

Chen Xuan,Zhang Changmin,Zhang Shangfeng and Zhang Liping

2010, 31(4): 420-427. doi:10.11743/ogg20100404

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Based on the relationship between sedimentary sequences and reservoir formation，the author puts for-ward a new approach to exploration of lithologic-stratigraphic reservoirs in the fluvial-delta sedimentary systems-“to look for intervals with low net-to-gross ratio outside a facies zone with high net-to-gross ratio，and to look for reservoirs with relatively high net-to-gross ratio within intervals with low net-to-gross ratio”.In combination with sequence stratigraphic theories and relevant target-evaluation techniques，this new approach is used to identify reservoirs in the1stmember of the Triassic Karamay Formation in the HongˉChefaultbelt in JunggarBasin. It is believed that the middle cycle-MSC2is the favorable“low net-to-gross ratio interval”，in which the channel sands with high curvature index are the favorable“exploration targets with relatively high net-to-gross ratio”.The exploration well（Hongshan-4）targeting at these channel sands was drilled and flowed commercial oil. Mean while，for the delta deposits in the Neogene Shawan Formation，it is believed that the transgressive system tracts in the third-order sequence-NSQ2-are the favorable“low net-to-gross ratio interval”，in which the underwater distributary channel sands at delta front are considered to be the“exploration targets with relatively high net-to-gross ratio”.The exploration well（Che-89）targeting at these sand stones was drilled and obtained discoveries. This indicates that，intervals with low net-to-gross ratio also can have well-developed lithologic-stratigraphic reservoirs. This new approach is of significance to exploration of lithologic-stratigraphic reservoirs in the terrestrial strata in China.

Dynamical mechanism and depositional responses of turbidity current sedimentation

JiangHui

2010, 31(4): 428-435. doi:10.11743/ogg20100405

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According to their density，turbidity currents can be classified into low density，sandy high density and gravelly high density currents. Their rheology mechanisms especially that of the highˉdensity currents，are still highly controversial. This article explored the distribution，triggering mechanism，suspension process and depositional velocity of the turbidity current，and identified five dynamicˉdeformation zones，including erosion，frontal adjusting，depositional offloading，rear adjusting and balanced zones. From the view of flow mechanism，the erosion zone keeps a steady state because of its perfect mobility. It swathes coarser grains and daggles finer grains，forming a nebulousˉlike trail behind. The depositional off loading zone presents only a slow and subtle change in flowing time and space under a mixing force. When the turbulence force by flow differentiation is less than the viscous dampening effect，the deposits start to offload and accumulate in sequence. Both the adjusting and balanced zones perform the functions of dynamic conversion，structural connection，and deposition and erosion balancing in currents. It is highly possible that turbidity current with high flowing speed become more structurally unstable and complicated in supporting mechanisms. And this may explain the phenomenon of transition，alteration and transformation between turbidity currents and other types of gravity currents.

Geological concept，method and application of sequence unit identification through wavelet analysis

ZhaoWei,Jiang Zaixing,Qiu Longwei and Chen Yan

2010, 31(4): 436-441. doi:10.11743/ogg20100406

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Wavelet analysis as amathematic method can be used to solve some problems in sequence stratigraphy research. This paper analyzed the concept of this method from a geological standpoint and come to a conclusion that the logs are reflections of certain characteristics of strata that reveal the superposition of the sedimentary cycles in different periods，while these sedimentary cycles can be identified through wavelet analysis and used for division of sequence stratigraphic units. In practice，we realize that choosing the right wavelets are essential to the division of sequence units. We applied the technique to the analysis of different intervals of two wells in Dongying Sag of BohaiBay and found out that the method is not only useful in division of conventional interbedded sandstone and shale stratum，but also capable of identifying subtle changes that tend to be ignored with con-ventional methods. Combined with Fischer Diagram，we can use the techniques to identify the sequence units in a large interval with mono lithology.

Depositional system of the Middle Permian Q ixia Formation in the western Sichuan Basin

WeiGuoqi,YangWei，Zhu Yonggang，Jin Hui，LiYuegang,Shi Zhensheng and Shen Yuhong

2010, 31(4): 442-448. doi:10.11743/ogg20100407

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Observation and analysis of numerous outcrops and cores as well as laboratory tests of samples from the Qixia Formation in western Sichuan Basin reveal that the formation mainly consists of bioclastic limestone，calcarenite， micrite，and leopard limestone. There are mainly three fossil assemblages，including brachiopodsˉalˉgaeforaminifera，corals and thin shell brachiopods. They were formed in platform，slope and slopeˉbasin environments respectively. Judging from geotectonic settings，bank features of platform edge and slopeˉbasin facies，and the relationship between slope facies and platform edge facies etc，we conclude that the Qixia Formation in western Sichuan Basin is the carbonateˉfringed platform depositional system with slope，platform edge bank and open platform as the main facies，of which the platform edge bank facies is widely distributed and the most favorable for hydrocarbon accumulations.

Diagenetic reaction pattern of the sequence boundary and its impacts on reservoir quality

Han Denglin,Zhang Changm in and Yin Taiju

2010, 31(4): 449-454. doi:10.11743/ogg20100408

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Cluster sampling was performed in the vicinity of the sequence boundary within deltaic sedimentary system and a detailed analysis of lithological and mineral composition was carried out on the samples gathered. The results reveal the diagenetic reaction patterns near the boundary. The constraints of the sequence boundary over diagenetic reaction are reflected in the following aspects.First，sequence boundary indicates a relative fall in lacustrine level，which led to flush and leach of meteoric waters and facilitate the kaolinitization of siliciclastic grains（mainly unstable feldspar），thus causing a higher kaolinite content in the bulk composition of the under-lying formation of the boundary. Secondly，the boundary means a relative longer residence time of the sediments，which increasing the content of carbonate cement in reservoirs under the boundary. Finally，the boundary is usually accompanied by a decrease in accommodation space，causing a large amount of fineˉgrained sediments being flushed and eroded by overlaying coarseˉgrained sediments and then leached into the underlying formation. With the increase of burial depth，these filling materials become pseudo matrix under compaction and reflected as a signal of higher content of claymatrix under the boundary. These diagenetic alterations have an obvious impact on reservoir quality. The kaolinite cement，for instance，has a positive effect upon the porosity of reservoirs; while clay matrix and carbonate cement，on the contrary，exert a negative effect upon reservoir porosity.

Genesis types and distribution pattern of the Cambrian and Ordovician dolomites in Tazhong area，the Tarim Basin

YangYufang，Zhong Jianhua,Chen Zhipeng and Chen Xin

2010, 31(4): 455-452. doi:10.11743/ogg20100409

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The Cambrian and Ordovician dolomites are important reservoir rocks of Lower Paleozoic carbonate in Tarim Basin，which can be divided into five genesis types according to various characteristics such as cores，microscopic features，textural components and macro-geology. In this paper，the micro textures of TypeⅡfine crystalline dolomite and TypeⅢmedium-coarse crystalline dolomite are described in detail，with the fine crystalline dolomite characterized by three attitudes and two textures. The TypeⅢdolomite can occur in bigger caves and fractures as a fill，which has a mosaic texture and may contain both TypeⅠand TypeⅡdolomites. The sedimentary combination and distribution pattern of the dolomites are discussed，and the favorable development zones of various types of dolomite，such as evaporatedˉseawater type，mixedˉwater type，karstification type，burial type，deep-dissolution type and syngenetic rudaceous type，are pointed out.

Lithofacies and reservoirs of allluvial fan in the Lower Keramay Formation in the blockˉ6of Karamay oilfield，the Junggar Basin

2010, 31(4): 463-471. doi:10.11743/ogg20100410

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On the basis of core description of11wells in the blockˉ6ofKeramay oilfield in the JunggarBasin，we recognize7types of lithofacies in the alluvial fan of the Lower Karamay Formation. In combination with sedimentary microfacies analysis，we study the quality differences and distribution of reservoirs within the alluvial fans by using core analysis and testing data. The pore space of the Lower Karamay Formation is dominated by primary intergranular pores. Depositional fabric controls the reservoir quality in the study area. The reservoirs of coarse sandstone facies and fine conglomerate facies are the best in the alluvial fans of the study area. The reservoirs of different microfacies are significant different in reservoir quality. The reservoirs of braided channel and gulley microfacies are the highest in quality，followed by the sandy gravel bars. The reservoirs of gravel bar and runoff channel microfacies are fair. The reservoirs of sheet flood microfacies are the worst. Gulley microfacies occur as narrow bands within gravel bars and sandy gravel bars，and act as the high trans mssibility pathways for reservoirs in the root fan and inner-middle fan.

Distribution of diagenetic facies and prediction of high-quality reservoirs in the Lower C retaceous of the Tanzhuang Sag，the southern North China Basin

ZhangQi，Zhu Xiaom in,Chen Xiang,Zhu Shifa，JiHancheng and JiangYongfu

2010, 31(4): 472-480. doi:10.11743/ogg20100411

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Using data such as ordinary thin sections，casting thin sections，scanning electronm icroscope observaˉtions and testing，we quantitatively calculate the diagenesis intensity parameters such as compaction，cementation and dissolution，which influence physical properties of the Lower Cretaceous reservoirs in the Tanzhuang sag，southern North China Basin.The apparent rates of compaction，cementation and dissolution porosity in different wells and intervals are obtained and the division standards of diagenesis intensity are determ ined.The Lower Cretaceous reservoirs are divided into five types of diagenetic facies.The typeˉⅠand typeˉⅡare good reservoir facies belts，the typeⅢis fair reservoir facies belt，while the typeⅣand typeˉⅤare poor reservoir facies belts.By predicting distribution ofdiagenetic facies and in combinationwith study ofdepositional systems and distribuˉtion pattern of secondary pores，we believe that the northern part of the southern fan delta and the southern part of the northern delta in the Tanzhuang sag，with moderate toˉrelatively high compaction，high cementation and moderate dissolution，are favorable reservoir facies belts for each interval.

Genesis and reservoir features of tuffaceous rocks in Wuerhe oilfield of the Junggar Basin

GongQingshun，NiGuohui，Lu Shuping，Ding Liangbo and Xin Yongguang

2010, 31(4): 481-485. doi:10.11743/ogg20100412

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Based on analyses of thin section，core observation，logging and seism ic data，we studied the genesis of tuffaceous rocks and their reservoir features in the Permian Wuerhe Formation in Wuerhe Oilfield，Junggar Basin. The rocks can be classified into tuff，tuffites and tuffaceous sandstone and conglomerate and show high resistivity and low interval transit time in logging curves. The study shows that the sedimentary environment of the Formation is enclosed or semi-enclosed lake bay，and the sediments are composed mainly of volcanic ash and terrigenous clastics. Syngenetic high-salinity and alkaline water body provided favorable conditions for hydrolysis and alteration of the deposited volcanic ash. During the process，dolomitic tuffites were formed and analcite was precipitated. These rocks have different reservoir space（dual porosity media），including dissolved vugs，sluiceˉway and fractures. The dual porosity feature of the rocks improves the poroperm characteristics and controls the accumulation and distribution of hydrocarbons inWuerhe oilfied.

Laws of mudstone compaction on the west slope of the Songliao Basin

2010, 31(4): 486-492. doi:10.11743/ogg20100413

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Based on the basic law of mudstone compaction，this paper does a systematic research on the mudstone compaction characteristics in the west slope of Songliao Basin by using various logging data. The overpressure is quantitatively calculated and analyzed. The results show that the strata in the western uplift zone of the study area are basically in normal compaction state，while the Qingshankou and Nenjiang Formations in the Longhupao terrace and QijiaˉGulongkou sag show under compaction. The causes of difference in compaction in different areas are analyzed through comparison of the key parameters of compaction curves and in combination with real geoˉlogic data. On the basis of this，we quantitatively calculate the overpressure in the study area，analyze the structural features and plannar distribution of formation pressures，and predict the features of subsurface fluid migration and accumulation.

Reservoir prediction with integrated information based on artificial neural network technology and geostatistics

Li Dongan,Ning Junrui and Liu Zhenfeng

2010, 31(4): 493-498. doi:10.11743/ogg20100414

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How to eliminate mistiness and improve the precision of seismic data interpretation is an everlasting topic in reservoir prediction. However，it is rather difficult to solve the problem due to the complexity of actual geological and geophysical conditions and low quality of seism ic data. Recent study shows that integration of information from different sources is an effective way to deal with the issue. Guided by this understanding，we tried to combine artificial neural network technologies and geostatistics in reservoir prediction of the D gas field. By doing so，we realized the integration of multiple seism ic attributes with logging data，and effectively reduced the mistiness and improved the precision of prediction.

Deliverability prediction for stepped horizontal wells

LiMiao，LiXiaoping and Guo Ping

2010, 31(4): 499-504. doi:10.11743/ogg20100415

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Based on the principle of the mass conservation law and the momentum conservation principle，we established a mathematic model of drawdown along the horizontal wellbore and coupled the inˉflow in the wellbore with seepage flow in the reservoirs by using specific productivity index. Reservoir filtration calculation in the model is expressed by the steadyˉstate flow model of JOSHI，and calculation of frictional drawdown in wellbore takes into account the impact of the radial inflow. We also wrote program for the mathematic model，so as to calculate and analysis the effect of relevant parameters of the well upon its deliverability. The results show that acceleration drawdown is negligible and frictional pressureˉdrop is insignificant in bigger borehole with lower deliverability，while that gravitational drawdown caused by inclination of wellbores has a bigger impact upon the deliverability. The article provides a simple，easy and effective way for predicting the deliverability of steeped horizontal wells.

Water-sensitive damage and its impacts on the water flood development of the Taizhou Formation reserviors in Caoshe oilfield，the Subei Basin

Zhang Xiongdong,Zhu Yushuang,Cao Haihong，Ma Huijie and Cheng Xiumei

2010, 31(4): 504-410. doi:10.11743/ogg20100416

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Using the experimental technique of natural sandstone micromodel，an emmersion experiment of water sentivity is conducted for the Taizhou Formation reservoir sandstone in the Caoshe oilfield，Subei Basin. The experiment results show that the sandstone is moderate in water sentivity. In order to fully understand the impact of water-sensitive damage on water flood development，the water-drive-oil experiments of natural sandstone m icro-model are conducted，including two experimen types，i.e.a single model type and a combined model type，which simulate m icro-water-drive and macro-water-drive respectively and compare water-drive-oil variances before and after waterˉsensitive damage. The results show that the water-sensitive damage does not change the style of water drive oil and the configuration of residual oil. For the single model experiment，however，the bypassing phenomenon is strengthened on the whole and the oil displacement efficiency is lowered，mainly controlled by several factors such as water-injection pressure，water-injection location and micro heterogeneity. For the combined model experimen，the oil recovery ratio tends to decline totally，controlled by both the micro-heterogeneity and macro-heterogeneity. To avoid the impact of waterˉsensitive damage on water flood development，it is suggested that the injected water should be around11.99g/L in salinity.

Relationship between reserve-production ratio and stable production: an example from SINOPEC’s producing oilfields

Zhao Qingfei，Fan Zheyuan and WeiHaifeng

2010, 31(4): 511-516. doi:10.11743/ogg20100417

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Currently，the common understanding of reserve-production ratio is contrary to the actual production in oilfields.To understand the relationship between reserve-production ratio and stable production，we analyze the existing views and their background and basis.We select38out of SINOPEC’s200producing oilfields and make a statistics on reserve-production ratio according to the type of oilfields.Their hypothetical reserve-produc-tion ratio at the end of stable production is also deduced.The results show that the relationship between reserve-production ratio and stable production depends on the development phase of an oilfield，oilfield type and the development policy.Whether an oilfield can keep stable production cannot be judged only from reserve-produc-tion ratio.

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