Table of Content

25 February 2010, Volume 31 Issue 1

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Development characteristics and main controlling factors of the Ordovician carbonate karst in Tahe oilfield

Qi Lixin, Yun Lu

2010, 31(1):  1-12.  doi:10.11743/ogg20100101

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The Middle and Lower Ordovician fractured-vuggy reservoirs in the Tahe oilfield experienced multi-stage karstification caused by multi-stage tectonic events from the Middle Caledonian to the Early Hercynian.Based on integration of geological,geophysical and geochemical data,we discuss the development characteristics and main controlling factors of multi-stage karstification of the Ordovician carbonates in the Tahe oilfield.Four main periods of karstification are identified as follows:the 1^st Phase,2^nd Phase and 3^rd Phase of the Middle Caledonian and the Early Hercynian.The origin,evolution and transformation history of the karst reservoirs are also discussed in this paper.According to this study,the major karstification periods are the Early Hercynian,the 1^st Phase and the 2^nd Phase of the Middle Caledonian,of which the Early Hercynian is the most important period.The main controlling factors of kastification are paleostructures,karst topography and geological conditions of paleo-drainage-system.

Features of deep-burial dolomitization of the Cambrian reservoirs in Tahe region

Lin Jun, Zhou Fangfang, Yuan Guofen

2010, 31(1):  13-21,27.  doi:10.11743/ogg20100102

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The Cambrian dolomites in the Tahe region have genetic features of transitional diagenetic environment.After a short-time early diagenetic period(sea-bed diagenesis and meteoric water diagenesis),the Camb-rian carbonate deposits went into burial diagenetic period,going through a diagenetic process from penecomtemporaneous dolomite to early diagenetic and shallow burial dolomite,to late diagenetic and deep burial dolomite,and finally to hydrothermal dolomite.Fractures and dissolved vugs(caves)are the main types of reservoir space of various Cambrian dolomites in the Tahe region.The major mechanism of the development of reservoir space of the Cambrian dolomites in the Tahe region is dissolution related to hydrothermal fluid flow through deep faults under deep-burial dolomitization background.And the main controlling factors responsible for the preservation of reservoir space are the intensity of filling and cementation in various periods,stable burial process after hydrothermal dissolution and relatively minor tectonic events.

A study of the paleo-cavern system in fractured-vuggy carbonate reservoirs and oil/gas development——taking the reservoirs in Tahe oilfield as an example

Lu Xinbian, Cai Zhongxian

2010, 31(1):  22-27.  doi:10.11743/ogg20100103

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A study of the Ordovician fractured-vuggy carbonate reservoirs in the Tahe oilfield is carried out to determine the concept and features of paleo-cavern systems in these reservoirs.Also included in the study are the formation,evolution,and deformation of these systems as well as their role in compartmentalization of reservoirs.The study points out that the compartmentalization of these reservoirs is mainly caused by collapse,filling,and fracturing processes of the pale-cavern systems.It is therefore suggested that these heterogeneous reservoirs formed by karstification and tectonic movements shall be described systematically and hierarchically from a karst system level(paleo-cavern system)and reservoir level(fracture-vug unit).The idea is applied to the study area and gains positive results.

A discussion on water flooding in the multi-well fractured-vuggy units of carbonate reservoirs in Tahe oilfield

Rong Yuanshuai, Liu Xueli, Yang Min

2010, 31(1):  28-32.  doi:10.11743/ogg20100104

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Based on a systematic evaluation of the effects of the early-stage water injection test,we studied water flooding schemes for multi-well fracture-cavern units of carbonate reservoirs in the Tahe oilfield through numerical stimulation.These schemes include the optimal timing of water injection,injection-production well network,the types of reservoirs and the relative vertical locations of injection and producing intervals,etc.For a specific type of reservoir,the optimal time to begin water injection should be determined according to its pressure maintenance levels.For cavernous reservoirs,water injection should begin at a pressure maintenance level in the range of 80% to 90%;for vuggy reservoirs,it should begin at a pressure maintenance level over 90%;while for fractured reservoirs,it should begin at a higher pressure maintenance level.An areal well pattern is preferable in water flooding.It is favorable to inject water in fractured reservoir interval and produce oil from cavernous reservoir intervals or from vuggy reservoir intervals,or to inject water in lower reservoir intervals and produce oil from higher reservoir intervals.

Water drive curves of fractured-vuggy carbonate rock reservoirs——taking the Ordovician carbonate reservoirs in Tahe oilfield as an example

Chen Qing, Yi Xiaoyan, Yan Changhui, Wang Xu

2010, 31(1):  33-37.  doi:10.11743/ogg20100105

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Although widely applied in China for years,the application of water drive curve method has been ge-nerally limited to the study of sandstone reservoirs both in theory and practice.This time,however,it is used in the study of the Ordovician carbonate reservoirs in the Tahe Oilfield.The water drive performance of carbonate reservoirs is much more complicated and changeable than that of sandstone reservoirs.Three types of curves are observed in the study of these reservoirs,including regular curve with single linear portion,regular curve with two linear portions(step-like curve),and irregular curve.Among them,the second one reflects such features of these reservoirs as well-developed fracture and vugs,unstable water body energy,and complicated water-driving pattern.It also indicates that the decline of energy affects different water bodies.An analysis of water from a typical well verifies the existence of several water bodies in one fractured-vuggy unit.

Frequency-divided inversion and its application to reservoir prediction in Block A of Tahe oilfield

Ji Yuxin, Liu Chunyuan, Cheng Dong, Wang Kangning

2010, 31(1):  38-42.  doi:10.11743/ogg20100106

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The Carboniferous Kalashayi Formation of Block A in the Tahe oilfield is one of the most important oil-bearing formations in the area.A thorough understanding of its spatial distribution,thickness and physical pro-perties is crucial to exploration and production.To tackle the problem,the researchers study the construction of target characteristic curves based on a fine analysis of petrophysical features and obtain a high-resolution inversion data volume for prediction of favorable sand bodies by adopting frequency-divided inversion method.As a result,an effective technical process is established for the prediction of the Carboniferous reservoirs in the area and its application has been proven successful.

Identification of low-amplitude structural traps in Luntai area, the Tarim Basin and its application

Han Qiang, Yang Zichuan, Zhao Yuan

2010, 31(1):  43-48.  doi:10.11743/ogg20100107

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This paper aims to identify low-amplitude structural traps in the Luntai area in the Tarim Basin.Regional structures in the study area are accurately located by means of precise 3D seismic interpretation and technologies such as fine calibration for horizon,automatic tracing,coherence attribute and variable velocity mapping.Analyses of geophysical response characteristics of logging and seismic data of the known oil and gas pools reveal that the velocity of reservoir sandstone is low while the velocity of mudstone cap rock is high,and the hydrocarbon-bearing zones have features of low frequency but high amplitude.Further,attribute analysis and hydrocarbon detection technologies are successfully used to analyze and predict the reservoir-seal combination and hydrocarbon potential of the identified structures.

Application of frequency division interpretation technique to reservoir prediction in Tahe oilfield

Zhu Ding, Sun Suqin, Jiang Jinyong

2010, 31(1):  49-53.  doi:10.11743/ogg20100108

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Sandbodies in the Carboniferous Kalashayi Formation in the Tahe oilfield are characterized by deep burial depth,small thickness and great lateral viariation,which makes it hard to predict them.In this paper,the frequency division interpretation tecnique is used to identify and characterize thin sandstones in the Kalashayi Formation.Boundaries of multi-stage deposits in the sandsonte unit are identified by using the phase of certain frequency.The distribution of sandstone is traced according to tuning "bright spot" and the thickness of sandstone is calculated according to the frequency of first peak.The frequency division interpretation tecnique can successfully identify thin-sandstone reservoirs and improves the accuracy of reservoir prediction.

Flow rate and pressure variation modeling for production wells in fractured-vuggy carbonates reservoirs

Zhao Yanyan, Yuan Xiangchun, Kang Zhijiang

2010, 31(1):  54-56,62.  doi:10.11743/ogg20100109

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A mathematical model is built for fluid flow in fractured-vuggy carbonate reservoirs with multiple fractures and vugs connected in series to analyze the patterns of flow rate and pressure changes.In reservoirs with ideal single fracture and single vug,both the flow rate and pressure decline exponentially.However,in reservoirs with multiple fractures and multiple vugs,the flow rate and pressure decrease much differently.For vugs near production wells,the decline of flow rate is the algebraic sum of several exponential decline functions,and the decline of pressure is the algebraic sum of several exponential decline functions and bottom hole flowing pressure.Taking the ideal triple fracture-triple vug reservoirs with the same sizes as examples,the pressure decline in vugs far away from the production well is much slower than that in vugs near the production well.The vugs near the production well contribute most of the oil flow in the initial stage and the vugs far away from the well join in progressively later.The study bears certain theoretical significances for analyzing and predicting well performance in fractured-vuggy carbonates reservoirs,understanding the patterns of fracture-vug connection and parameters of fractures and vugs,and reserve calculation.

Eopaleozoic inversion structures in the Tazhong Uplift and their significance to petroleum geology

Ning Fei, Tang Liangjie, Zhang Yu, He Chunbo, Zhu Chuanling, Wang Penghao, Chen Qun

2010, 31(1):  57-62.  doi:10.11743/ogg20100110

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By using typical seismic interpretation results,this paper focuses on a discussion of the development characteristics of the Eopaleozoic inversion structures in the Tazhong uplift and an analysis of their petroleum geological significance.The basement normal faults that were developed during the Sinian to the Early-Middle Ordovician controlled the later structural deformation styles,resulting in two inversion structural styles in the Tazhong uplift.The azimuth of the pre-exiting fault with larger dip angle might not be favorable for its further inversion and forced the inversion to the adjacent faults.The early faults might then serve as stress concentration point for later thrust faults,such as the Tazhong-Ⅱ fault belt and the Tazhong-10 fault belt.Or,they might be buckled by later compressional structures,such as the Tazhong-Ⅰfault belt.It is suggested that an inversion structure contributes in its early stage to hydrocarbon accumulation and forms in its later stage inverted anticlines with different amplitudes that are favorable traps for hydrocarbon accumulation.

Fluorescence characteristics of the Ordovician hydrocarbon inclusions in the Tazhong-Ⅰ slope-break zone and the timing of hydrocarbon accumulation

Zhang Nai, Zhao Ruihua, Zhang Dijia, Liu Keyu, Chen Yangui, Xin Yongliang

2010, 31(1):  63-68,75.  doi:10.11743/ogg20100111

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Hydrocarbon inclusions in the Ordovician in the Tazhong-Ⅰ slope-break zone can be divided into four episodes of development according to their fluorescence characteristics,intercalating relationship with calcite vein,and homogeneous temperature,and so on.Based on the tectonic evolution of the area,it is believed these episodes correspond to four episodes of hydrocarbon pooling:low maturity oil accumulation at the end of Silu-rian,oil generation in the late Hercynian,condensate oil and wet gas accumulation in the early Himalaya,and dry gas accumulation in the late Himalayan.The distribution of these four episodes of inclusions presents clear zonation in the Ordovician reservoirs in the Tazhong-Ⅰbelt.Four zones are recognized from the shallow oil reservoir in the southeast to the deep oil reservoirs in the northwest.It is suggested that the fossil oil from the episode Ⅲ and Ⅳ played a dominant role in the formation of present hydrocarbon accumulations.

Development characteristics and influencing factors of carbonate reservoirs in the Ordovician Lianglitage Formation at the northwest pitching end of the Katake Uplift, the Tarim Basin

Li Huili, Qian Yixiong, Sha Xuguang, Ma Hongqiang

2010, 31(1):  69-75.  doi:10.11743/ogg20100112

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Drilling results have revealed that fractured-vuggy carbonate reservoirs are developed in the Upper Ordovician Lianglitage Formation at the northwest pitching end of the Katake Uplift in the Tarim Basin.Based on analyses of core,thin section and logging data,this paper discusses reservoir development characteristics and influencing factors in the study area.The study shows that several factors have put effects on the development of reservoirs,such as karstification of the Middle Caledonian phase-Ⅱ weathering crust,penecontemporaneous karstification,burial dissolution and tectonic disruption.The penecontemporaneous karstification and hydrothermal fluid-related burial dissolution made limited contribution to the existing reservoir space.Further study is needed to identify the controlling factors for reservoir development in the study area.

Structural evolution and exploration significance of the Early Paleozoic palaeouplifts in Bachu-Maigaiti area, the Tarim Basin

L? Haitao, Zhang Zhongpei, Shao Zhibing, Zhang Genfa, Yue Yong

2010, 31(1):  76-83,90.  doi:10.11743/ogg20100113

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Based on integration of seismic and drilling data and in combination with the previous studies,we analyze the development,distribution and later tectonic evolution of the Early Paleozoic paleouplift in the Bachu-Maigaiti area by using the structural analysis method of superimposed basin.Its significance for hydrocarbon exploration is presented from the perspective of hydrocarbon accumulation conditions.A paleouplift favorable for karstification existed in the Middle Caledonian-Early Hercynian in the study area,and thus attentions should be paid to exploration of the Ordovician karst reservoirs.The northern part of the Maigaiti slope and the southern margin of the Bachu uplift were the northern slope zone of the paleouplift in the Early Paleozoic.They are the long-term target zone for hydrocarbon migration,and also are favorable for hydrocarbon accumulation.The Late Hercynian structural belt in the northern part of the Maigaiti slope is favorable for further exploration.

Geochemical characteristics of the Carboniferous oil pools in Maigaiti area, the Tarim Basin

Shao Zhibing, L? Haitao, Geng Feng

2010, 31(1):  84-90.  doi:10.11743/ogg20100114

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Analyses of geochemical characteristics and fluid inclusions of the Carboniferous oil pools in the Maigaiti area in Tarim Basin reveal that the natural gas here is dominated by marine sapropel-type gas with high-over maturity.In the Bashituo area,associated gas with high maturity is predominant,while in the Hetianhe gas field,a mixture of over-mature kerogen-cracking gas and crude-cracking gas occurs.Crude oil in the Bashituo area is dominated by highly-mature light oil with low content of sulfur and wax.Oil charging occurs at least in two stages and the preservation conditions are good.Oil-source correlation shows that the crude oil mainly originated from the Cambrian and has no genetic relations with the Carboniferous source rocks.In the Bashituo area,hydrocarbon pooling occurred mainly in two periods:one is the Late Hercynian when the oil and gas pools were destructed to some degree,and the other is the Himalayan when secondary condensate oil and gas were predominant.

The Paleozoic tectonic evolution and the Carboniferous source rocks in the northern Juggar Basin

Yu Tengxiao, Cao Zicheng, Xu Qinqi, Pan Quanyong, Xue Hui, Liu Shilin, Xie Guojun

2010, 31(1):  91-97.  doi:10.11743/ogg20100115

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The tectonic evolution history of the Northern Junggar Basin from the Sinian to the Carboniferous can be divided into two stages based on study of its fault evolution.The first is ocean basin spreading stage from the Sinian to the Middle Ordovician and the second is ocean basin subduction and plate suturing stage from the Late Ordovician to the Carboniferous.During the Carboniferous,the northern Junggar Basin was in bathyal-shallow marine and ocean-continent transitional environments,and therefore was suitable for the development of source rocks.Drilling data and field outcrops reveal that there are two sets of source rocks developed in the Lower and Upper Carboniferous respectively.Oil-source correlation and tectonic evolution analysis suggest that the oil/gas of the Lun-5 and DB-1 wells are mainly derived from the Carboniferous source rocks in the Wulungu depression.

Characteristics of the Carboniferous volcanic reservoirs in Che-91 wellblock in the Juggar Basin

Zhao Weijun, Yu Haoye, Li Yonghang, Ren Junmin, Dong Xuemei

2010, 31(1):  98-106.  doi:10.11743/ogg20100116

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A comprehensive reservoir study method is employed in the analysis of the Carboniferous volcanic reservoirs in Che-91 wellblock.The result indicates that the reservoirs here consist mainly of explosion-breccia and belong to pore-fracture dual porosity reservoirs.There are always fractures developed in formations overlying the reservoirs,showing clear features of excellent assemblage of upper fractured interval and lower reservoir.The reservoirs are irre-gular massive reservoirs controlled by both edge water and bottom water.The preserving conditions for hydrocarbons are favorable but get much poorer when near the major faults,resulting in heavier oil and oxidative environment.Identifying the area with well-developed fractures can guide the hydrocarbon exploration in the Carboniferous of the region.

Main controlling factors and patterns of gas accumulation in the Upper Triassic of the western Sichuan Depression

Su Yongjin, Tang Yuegang, Zhang Shihua, Fang Xinna

2010, 31(1):  107-113.  doi:10.11743/ogg20100117

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Based on the analysis of the Upper Triassic structural evolution features in the western Sichuan Depression,a study of sedimentary reservoirs and trap development features is carried out in this paper.By using the theories and methods of pool-forming dynamics,we identify the main controlling factors of gas accumulation,divide plays,and present several pooling patterns.The Upper Triassic Xujiahe Formation in the western Sichuan Depression possesses the conditions for the development of giant or medium-sized gas pool/fields.The main controlling factors of gas accumulation in the Upper Triassic are abundant sources,relatively high gas generation intensity,favorable migration conditions,good timing of paleouplifts and paleoslopes,well-developed traps,widely-distributed thick reservoirs and favorable preservation and sealing conditions.Two plays are recognized in the Upper Triassic,including an upper play and a lower play.The lower one is featured by high pressure driving and low pressure attraction,while the upper one is characterized by capillary imbibition,migration and release of water-soluble gas and transporting by buoyancy through preferential pathways.Different modes have different dynamic mechanisms of pool-forming and different distribution characteristics.

A study of structure styles between double thrust faults——taking the area between Yuejin-2 and Yuedong thrust faults in the Qaidam Basin as an example

Wang Xinguo, Wang Pu, He Jiankun, Li Dongan

2010, 31(1):  114-118.  doi:10.11743/ogg20100118

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The structure styles on both sides of a single thrust fault have been thoroughly probed.Nevertheless,detailed investigation into that between double thrust faults is still rare.Based on 3-D seismic data interpretation and stress field simulation with finite element,the paper analyzes the 3-D fault combination and the distribution features of stress and strain of the finite element simulation,recognize the existence of nearly vertical strike-slip faults with opposite occurrence of formations at their two sides.The study may be used to guide the research of structure style of complicate structural intersections as well as mineral resource exploration.

An analysis of point bar configuration of the Neogene Guantao Formation in the middle unit of Block 7 in Gudong oilfield, the Jiyang Depression

Zhou Weidong, Liu Zhenkun, Yue Dali, Zhou Yinbang, Zhong Xinxin

2010, 31(1):  126-134.  doi:10.11743/ogg20100120

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Hierarchy analysis and model fitting techniques are used to describe the point bar configuration of the meandering river deposits in the Neogene Guangtao Formation in the middle unit of Block 7 in the Gudong oilfield.According to the lithologic and electrical properties of sedimentary microfacies in the meandering river deposits and in combination with the modes of deposition,we divide the sedimentary microfacies and identify two point bars in the study area.With production performance and test data,quantitative models of point bar configuration unit are built,which includes the orientation,dip,space and horizontal width of a single lateral-accretion sandbody.Under the guidance of the models and constraints of well data,we analyze in detail the internal configuration of the point pars and build 3D models for point bar configurations of meandering river deposits.Furthermore,we analyze the remaining oil distribution and presents specific measures to tap the production potential.

Hierarchical subdivision and origin of single sandbody in the reservoirs of meandering river facies in the Yangdachengzi Formation of Fuyu oilfield

Li Jun, Song Xinmin, Xue Peihua, Zhang Hailong, Sun Jingmin

2010, 31(1):  119-125.  doi:10.11743/ogg20100119

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It is acknowledged that distribution of sandbodies in reservoirs of meandering river facies is very complex,which makes it hard to rebuild the subsurface system.Based on the analyses of formations in terrestrial basins and the development of meandering river reservoirs,we divide the meandering river reservoirs of the Yangdachengzi Formation in the Fuyu oilfield into 4 different sand groups and 21 subzones by using the hierarchical subdivision method.Based on the subdivision of subzones,we focus on the configuration of internal architecture of single sand body in meandering river reservoirs.With data of cores,dense well log and production perfor-mance,we systematically study the hierarchical interfaces,lithofacies combinations and spatial distribution and assemblage of single point bar sandbodies in the study area.The genesis and distribution of remaining oil are also discussed in this paper.The results show that,the single point bar sandbody of meandering river reservoirs in the Yangdachengzi Formation deposited in an environment with weak hydrodynamic force is controlled by two hierarchical sedimentary interfaces.The number of lateral accretion bodies is mainly 3 or 4;the included angle of la-teral accretion face ranges from 4? to 7?;the horizontal scale varies greatly.The main types of spatial combination of single sandbody are isolated type,lapping type,overlaying type and cutting-piling type.Influenced by la-teral-accreted mudstones of point bars and injection-production strategies,the remaining oil is mainly distributed in point bars,isolated and cutting-pilling sandbodies,which lays the foundation for secondary recovery.