Table of Content

08 October 2016, Volume 37 Issue 5

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Organism assemblages in the Paleozoic source rocks and their implications

Liu Wenhui, Hu Guang, Tenger, Wang Jie, Lu Longfei, Xie Xiaoming

2016, 37(5):  617-626.  doi:10.11743/ogg20160501

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The accumulation and preservation of the Paleozoic organisms provided a solid organic matter foundation for hydrocarbon generation during the Paleozoic.As different types of organisms were considered to contribute differently to hydrocarbon-generation,a study on organism assemblages (HGOs) started to prevail in source rock researches and has been used a new means of assessing the quality of matured source rocks.From geological,historical,and organic evolution perspectives,algae are the most important material providers for hydrocarbon generation during the early Paleozoic.They can be classified into two types according to their living habit:the benthic and the planktonic algae.The former lived near coast in a zonal distribution in accordance with light intensity and nutrient conditions.The latter lived in a photic zone and their distribution was controlled by their sizes and wind wave strength.Study on the HGOs and the organic geochemical characteristics of the source rock samples from seven formations of four Paleozoic outcrop sections shows that the TOCs in the source rocks dominated by benthic algae are higher than those controlled by planktonic algae.Together with depositional settings,the digenesis and hydrocarbon generation processes were all thought to have played some parts in shaping the organic carbon isotopic compositions of the samples.However,the processes seemed to have exerted lesser effect.Comparison of the kerogen carbon isotopic composition and the HGOs assemblages of the samples indicates that the δ¹³C_kerogen values of samples dominated with benthic algae are less than -34‰,whereas the δ¹³C_kerogen values of source rocks dominated with planktonic algae are more than -30‰.Taking the fact that the δ¹³C values (all more than -30‰) of known oil samples in the Tarim Basin into consideration,this article concludes that the crude in the basin is mostly sourced from rocks with planktonic algae and the source rocks with lower TOCs and planktonic algae from the Paleozoic may have greater exploration potential.

Simulation experiments on crude oil cracking and carbon isotopic evolution in carbonate reservoirs

Liu Wenhui, Luo Houyong, Tenger, Wang Wanchun, Wang Jie, Lu Longfei, Tao Cheng, Wang Ping, Zhao Heng

2016, 37(5):  627-633.  doi:10.11743/ogg20160502

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To study the characteristics of crude oil cracking and isotopic evolution in carbonate reservoirs,crude oil samples were collected from Well TK772 in the Ordovician Yingshan Formation,Tahe oilfield,Tarim Basin.We carried out the simulation experiment of oil cracking into gas process under simulated carbonate reservoir conditions by using the semi-open experiment system "hydrocarbon generation and expulsion simulation instrument under the conditions of stratum-porosity and thermo-compression",and then analyzed the geochemical characteristics of gaseous products.The experiment results showed that oil cracking process in a carbonate reservoir was mainly influenced by temperature,and the occurrence of the TSR was caused by the adding of magnesium sulfate.TSR affected the oil cracking process and the characteristics of its products,resulting in the drop of temperature for the large scale cracking of heavy hydrocarbons,and the generation of large amount of non-hydrocarbon gases in addition to the consuming of large amount of heavy hydrocarbons.The alkane gas carbon isotopic composition is mainly controlled by thermal evolution degree.The carbon isotopes get heavier progressively along with the increasing of temperature,but the alkane gas carbon isotopic sequence partially reverses (δ¹³C₁< δ¹³C₂ >δ¹³C₃) at certain temperature intervals when sulfates exist.The study shows that TSR not only could alter the chemical composition of natural gas,but also might be an important factor for alkane gas carbon isotopic sequence reversion that is common in the carbonate gas reservoirs with high content of sulfur.

Research advances on hydrocarbon sealing properties of gypsolyte/saline rocks

Li Yonghao, Cao Jian, Hu Wenxuan, Lu Xiancai, Fan Ming, Zhang Dianwei, Hong Dongdong

2016, 37(5):  634-643.  doi:10.11743/ogg20160503

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Gypsolyte/saline rocks are one of the most important hydrocarbon cap rocks worldwide.More and more exploration cases indicate that their sealing properties are highly variable from well to well.To better understand the rocks and its implication as cap rocks,we present in this article an overview of the research advances in the industry.It shows that these rocks are mostly originated from evaporitic basins and sabkhas,largely presented themselves as annular rings or bands parallel to shorelines.The rocks of the former origin have been considered ideal cap rocks as they are usually widely distributed and big in thickness.The sealing mechanisms of the gypsolyte/saline rocks may be summarized as favorable physical properties,overpressure and plasticity.The dissolution of the rocks was recorded vertically by four zones:top dissolution zone,fault dissolution zone,intra-layer water dissolution zone and bottom dissolution zone,and horizontally by three zones:karst uplift,karst slope and karst basin.Many factors,such as the distribution area,integrity,thickness and etc.(the internal controlling factors for sealing properties),shall be taken into consideration during the evaluation of sea-ling properties of the rocks.The impacts of dissolution and tectonism (external controlling factors for sealing properties) on the sealing properties of the rocks are then analyzed.Current research is focused more on quantitative evaluation of the sealing capacity of the rocks,especially the impacts of dissolution and tectonism.The research is still in a stage of exploration and it is both challenging and exciting.

Characterization and genesis of fault-controlled karst reservoirs in Ordovician carbonate karst slope of Tahe oilfield,Tarim Basin

Han Changcheng, Lin Chengyan, Lu Xinbian, Ren Lihua, Wei Ting, Zhang Xianguo, Duan Hongzhen

2016, 37(5):  644-652.  doi:10.11743/ogg20160504

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The internal karst reservoirs in the slope area of Tahe oilfield,Tarim Basin have attracted more and more attention as further exploration and development were carried out in the weathering crust karst reservoirs in the buried hills of the main blocks in the field.It is generally agreed that faults control the formation and distribution of karst reservoirs.Based on reservoir classification,the paper analyzed control effect of faults upon karst reservoirs and their distribution and discussed forming mechanism and evolving characteristics of the reservoirs through study of core,thin sections,logging and drilling as well as seismic data.A coupling relationship between extent and phases of development of faults and karst reservoirs was revealed during the analyses.Multi-phased and inherited faults of grade Ⅱ and Ⅲ-1 seemed to have a more prominent controlling effect upon the karst reservoirs.Horizontally,the reservoirs distributed as belts along faults; and vertically,they were observed to occur within intervals 0-50 m or 100-150 m below the top of the Ordovician Yijianfang Formation.The paper also suggested that these reservoirs were formed mainly by meteoric water dissolution,TSR (Thermochemical Sulfate Reduction),hydrothermal dissolution,and hybrid dissolution.The Early Hercynian was believed to be the main stage for the development of the fault-controlled karst reservoirs in the field.

Characterization of hypogenic karst systems in the Middle-Lower Ordovician of Shunnan area,Tarim Basin

Zhu Xiu, Zhu Hongtao, Chen Honghan, Qi Lei, Li Peijun, Yun Lu

2016, 37(5):  653-662.  doi:10.11743/ogg20160505

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By contrasting two opposite seismic reflection features of hydrothermal conducting pathways (low continuity and weak seismic amplitudes) and hydrothermal zones (high continuity and strong seismic amplitudes) of data gathered from hypogenic karst systems in the Middle-Lower Ordovician of Shunnan area in Tarim Basin,we developed a new seismic characterization workflow consisting of seismic amplitude thresholding,fracture system interpretation,pick up,and merge display,to effectively describe the coupling relationship between spatial distribution of hydrothermal zones (hypogenic karst system' zones) and hydrothermal conducting pathways (fracture systems),and then to establish development modal of the hypogenic karsts.Characterization of the hypogenic karsts from the Yingshan Formation of Well SN4 in central Tarim Basin shows a close relationship between the formation and distribution of the areas affected by hydrothermal fluids and the migration pathways of the fluids.The migration pathway system is usually composed of primary and secondary faults,and lateral carriers (epigenic karst systems,porous layers and vuggy layers),among which,the primary and secondary faults together form vertical migration pathways.Fluids from the deep usually move upwards along the vertical pathways first and then flow along the lateral carriers-a criss-cross migration system.The accumulation of fluids from the deep at the top of the secondary faults makes it possible for the scattered epigenic karst systems of individual bead string type to communicate with one another and form a massive hypogenic karst system of "bead string commplex" type with such features as branched carriers,layered distribution and interfingered bead strings.

Controlling mechanism of two strike-slip fault groups on the development of the Ordovician karst reservoirs in the Tazhong Uplift,Tarim Basin

Zhang Yanping, Lyu Xiuxiang, Yu Hongfeng, Jing Bing, Zhang Chunlin, Cai Jun

2016, 37(5):  663-673.  doi:10.11743/ogg20160506

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Controlling mechanisms of strike-slip faults on development of the Ordovician karst reservoirs in the Tazhong Uplift,Tarim Basin are poorly understood.Seismic attribute of root mean square amplitude was used to predict karst reservoir distribution in the shallow,middle and deep Ordovician on the North Slope of Tazhong Uplift.The results show that most of the karst reservoirs distribute with inheritance in the northeastern and southwestern ends of blocks which are bounded by the NW- and NE-trending faults,especially the deep Ordovician in ZG5 and ZG10 blocks.Such distribution characteristics cannot be interpreted by sedimentary facies and palaeogeomorphology,while strike-slip fault activity could provide a possible interpretation:Right-lateral compress-shear activity of the NW-trending faults induces left-lateral strike-slip activity of the NE-trending faults.The northeastern and southwestern ends of blocks bounded by the NW- and NE-trending faults are located in the intersection area of the NW- and NE-trending faults,where crossed fracture zones induced by faults are well developed.In addition,the strike-slip movements led to local extension in the northeastern and southwestern ends of blocks,while local compression in the northwestern and southeastern ends.Extensional fracturing of rock in local extension area is much easier than compressional fracturing of rock in local compression area for that the tensile strength of rock is much smaller than its compressive strength.The extensional fractures provide migration pathway to dissolution fluid and increases contact area of dissolution fluid and rock,so the northeastern and southwestern ends of blocks have well developed karst reservoirs.The research suggests that the strike-slip fault is one of controlling factors on development of karst reservoirs in the Tazhong Uplift.

Hydrogeomorphologic characterization and evolution of the Early Hercynian karstification in Tahe oilfield,the Tarim Basin

Li Yuan, Lu Xinbian, Wang Yingying, Zhang Heng, Cai Zhongxian, Li Daze

2016, 37(5):  674-683.  doi:10.11743/ogg20160507

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Complex karst fracture-vuggy systems formed during early Hercynian of Tahe Oilfield show greatly disparities in scales and distributions in the east (blocks of interests) and west (peripheral area) parts of the oilfield.Karst water was thought to be the key factor during the karstification under varying hydrogeomorphology and hydrokinetic conditions against different geological settings.To deeper understand and interpret the mechanisms behind the development of karst reservoirs in western China,the authors restored paleokarst geomorphology and hydrology networks buried in a depth less than 5000m with modern karst theories as guidance,and high-precision 3D seismic data,seismic attribute abstraction and seismic images,as basis.A fine description of hydrogeomorphology of the east and west parts of the oilfiled were also carried out to help define the evolution stages of the karstification.The results show that the east part was developed into karst plateau with high-relief peak clusters and karst mounds as the primary landform units.Typical landscape combinations consisting of dry valleys,blind valleys,swallet steams,canyons and natural bridges,were formed by strong vertical erosion from surface water.Integrated drainage systems made up of both surface and underground hydrologic networks were also formed there.These features reveal that the part had gone through a long and stable period of karstification.The results also point out that the west part is mostly a karst depression holding gentle topography gradient and karst mounds with relatively lower relief.The surface water network was observed to erode largely laterally into snaking networks in the part.However,continuous underground water networks failed to come into being,indicating a shorter period of karsitification in the part.Combining these observations with regional tectonic evolution data,the authors suggests that the karst re-servoirs in the east of the oilfield are still in their stage of maturity and those in the west are during their adolescence phase.

Types and origin of pore-fillings from the 5^th member of the Ordovician Majiagou Formation in Ordos Basin

Zhang Juntao, Jin Xiaohui, Li Shujun, Li Wei, Sun Yipu

2016, 37(5):  684-690.  doi:10.11743/ogg20160508

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Mold pore of anhydrite nodule is the major pore type in the 5^th member of Ordovician Majiagou Formation,Ordos Basin.The fillings of the pore consist mainly of calcite,very fine crystalline dolomite,and coarse crystalline dolomite,as well as a small amount of quartz,anhydrite,kaolinite,etc.The calcite,fully filling the pores,is mostly coarse crystalline and shows yellow color under cathodoluminescence.Its moderate Mn content,a relatively low Fe content and the obviously negative δ¹⁸O values indicate that the calcite deposited in atmospheric environment.The very fine crystalline dolomite,usually partly filling the pores,is similar to matix dolomite in mineralogy and origin.It shows two color types (one is light orange and the other is dark red similar to matix dolomite) under cathodoluminescence.The very finecrystalline dolomite can be divided into two parts under electron probe backscatter:the core and the cortex.The Fe and Mn content of the core are close to that of the matrix dolomite,while the cotex has the highest Fe and Mn content in carbonate minerals.Furthermore,its δ¹⁸O and δ¹³C values are more negative than that of the matrix dolomite.These features illustrate the change of diagenetic fluids.The coarse crystalline dolomite,blue after dyeing,has higher Mn and Fe content,relatively negative δ¹⁸O and δ¹³C values,suggesting a possble late hydrothermal fluid origin.Different pore-fillings in the reservoir are the results of different fluids.The calcite is possibly the result of meteoric water filling in hypergenic stage and has certain relatonship with karst paleogeomorphology.It commonly occurs in low-lying areas with surficient fluids.In constrast,the very fine crystalline dolomite is related to sedimentary environments and alterated by hydrothermal fluids.The fromation of coarse crystalline dolomite as a result of hydrothermal fluids associated with fractures.

Characteristics and genesis of Ordovician Ma5¹⁺² sub-member reservoir in northeastern Ordos Basin

Xiong Ying, Li Ling, Wen Caixia, Hou Yundong, Xiao Di, Zhong Yuan, Nie Wancai, Cao Jian, Tan Xiucheng

2016, 37(5):  691-701.  doi:10.11743/ogg20160509

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Characteristics and genesis of Ma5¹⁺² sub-member reservoir (the 1^st and 2^nd sub-member of the 5^th member of Ordovician Majiagou Formation) in the northeastern Ordos Basin are studied based on core,thin section and physical property analysis.The result shows that diverse types of reservoir rocks and reservoir spaces occur in Ma5¹⁺² sub-member in the study area.The reservoir rock types include gypsiferous micritic to very fine crystalline dolomite,grain dolomite,saccharoidal medium-coarse crystalline dolomite,microbial carbonate and sandy rubble dolomite with caverns filled.And the corresponding reservoir space types are gypsum mould pore,micropore between pore fillings,intergranular (dissolved) pore,intragranular dissolved pore,intercrystalline (dissolved) pore,fenestral pores,inter-breccia residual vuggs.The reservoirs in Ma5¹⁺² sub-member are typical porous reservoirs with low porosity and low permeability as a whole.The micritic and very fine crystalline dolomite with gypsum mold pores have the best reservoir properties but fair frequency and scale of occurrence.In contrast,the grain dolomite and sandy breccia dolomite have poorer reservoir properties but larger frequency and scale of occurrence than the former.Reservoir genesis analysis indicates that the formation and distribution of the Ma5¹⁺² sub-member reservoirs in this region are mainly controlled by sedimentary microfacies and multi-stage superimposed karstification,thus they are typical facies-controlling karst reservoirs.The psammitic banks and the gypsiferous dolomite flat provided the material foundation for the reservoir development and later karst reformation.The dissolution of gypsum clumps via epigenic karstification in the immature stage,the dissolution caused by layer-parallel flowing of karst water along the porous beds of early psammitic banks played a key role in the formation of the reservoirs.However,the overprint of epigenic karstification in the mature stage on that in the immature stage and the hybrid filling destroyed the earlier reservoirs or made them disappear.

Characteristics and geological significance of gas chimney of the Sinian Dengying Formation in the Weiyuan Structure,Sichuan Basin

Liang Xiao, Liu Shugen, Xia Ming, Sun Wei, Deng Bin

2016, 37(5):  702-712.  doi:10.11743/ogg20160510

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Fullness coefficient of the Upper Sinian Dengying Fm gas reservoir in the Weiyuan Structure is only 25%,while that in the Gaoshiti-Moxi Structure separated with the former by the Cambrian intracratonic sag is approximately 100%.Though the Dengying Fm in the two structures have similar regional background and source-reservoir-cap assemblage conditions,they have large differences in the fullness coefficient of natural gas reservoir.Abnormal tubular characteristics can be found in the Sinian-Triassic on 2D and 3D seismic profiles of the Weiyuan Structure.Some of them expose on the surface or are subtle under the shallow layer.They are supposed to be the characteristics of "gas chimney".Their major features are as follows:①Affected by the gas chimney,a large amount of gas seepages can be found in rivers near the Lower Triassic Jialingjiang Formation outcrops of the Weiyuan Structure; ②Non-hydrocarbon gases components such as nitrogen,argon and helium of the gas reservoirs in the Upper Cambrian Yuxiansi Formation,Xixiangchi Formation and Lower Permian Yangxin Formation are similar to the Upper Sinian Dengying Formation in Weiyuan,indicating the upward channeling of gas in the Upper Sinian Dengying Fm; ③ Although the pressure coefficient is 1.0 in the Dengying Formation of Gaoshiti-Moxi Structure,the overlying strata have overpressure.The existence of gas chimneys in Weiyuan shows that the seal system of the Upper Sinian Dengying Fm gas reservoir is basically failed,thus the sealing conditions are poor.The vertical migration of natural gas directly affects the scale of the current gas reservoir of the Weiyuan Structure.

Characterization of high-frequency sequences in the epicontinental carbonate platform of Jia 2 member in Moxi gas field and its controls over reservoirs

Liu Hong, Wang Gaofeng, Liu Nan, Qiao Lin, Ding Wei, Ding Xiong, Duan Hongzhen, Tan Xiucheng

2016, 37(5):  713-720.  doi:10.11743/ogg20160511

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High-frequency sequences,generally formed in basins with stable subsidence settings and correspond well to astronomical Milankovitch cycles,are vivid records of high-frequency eustatic flucturation.Such sequences developed in epicontinental platforms of non-coastal sedimentary environment are both representative and special in certain ways.By combining core observations,thin section analyses and logging facies studies with such factors as energy environment,eustatic fluctuation and sea water salinity cycle as well as reservoir genesis types under high-frequency sequential framework,we classified high-frequency sequences in the epicontinental carbonate platform of the Jia 2 member of the Moxi gas field into the following three types:platform flat type,intraplatform shoal type and lagoon type.The sequences are found to be characterized by widely-distributed gypsiferous rocks and grainstone with all kinds of exposure marks.As a whole,they consist of stratigraphic assemblages of shallowing upwards sequences.Within the framework,sequences are separated by clear boundaries and stratigraphic assemblages are stable and highly comparable.The paper also discussed the mechanisms behind the formation of early reservoirs in the high-frequency sequence framework and proposed that the high-frequency sequences controlled the order of lithofacies development and the syngenetic karstification,and thus controlled the development of different genetic types of reservoirs.

Geochemical characteristics and fluid origin of the Dengying Formation dolomites in southern Sichuan Basin

Wu Tie, Xie Shuyun, Zhang Dianwei, Li Pengtao, Bao Zhengyu, He Zhiliang, Qian Yixiong, Jiao Cunli

2016, 37(5):  721-730.  doi:10.11743/ogg20160512

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Dolomite of the Dengying Formation in southern Sichuan Basin has been studied a lot as an important kind of reservoir rocks for oil and gas.However,origin of the dolomite is still in controversy.This paper studied the geochemical characteristics and fluid origin of the dolomite in the Dengying Formation in southern Sichuan Basin through analyses of C-O isotopes and trace elements.The results show that the main types of dolomites are micritic dolomite,fine-medium dolomite and breccia-dolomite.The δ¹³C and δ¹⁸O values are 0.52‰~2.06‰ (PDB) and -11.9‰~-2.14‰ (PDB),respectively,which are lower than the original values of δ¹³C (4.43‰) and δ¹⁸O (-0.62‰) of carbonates of the same period.The dolomite has high U/Th ratio,low Sr and high Eu anomalies.It is believed that the dolomite formed in a burial diagenetic environment and was altered by hydrothermal fluid later.REE distribution patterns of the dolomite is similar to the dolomite originated from seawater.Salinity indexes(Z)of the dolomite is higher than those of normal seawater.Therefore,it can be inferred that the dolomitization fluids mainly originate from the condensed seawater trapped in the Formation.

Characteristics and development mechanism of dolomite and dolomitic quartzite reservoirs of the Middle Permian Maokou Formation in eastern Sichuan Basin

Tang Xuesong, Tan Xiucheng, Liu Hong, Ma Teng, Su Chengpeng, Cheng Xueying, Chen Hongyu, Cao Jian

2016, 37(5):  731-743.  doi:10.11743/ogg20160513

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Unique dolomite and dolomitic quartzite reservoirs occur in the Middle Permian Maokou Formation in eastern Sichuan Basin.They are commonly believed to be of "fault-controlled hydrothermal"origin.However,most of the explorary wells targeting at these reservoirs are unsuccessful,thus further study needs to be performed.The characteristics and development mechanism of these reservoirs are studied by means of comparative analysis of the outcrops on the periphery of the basin and cores inside the basin.The results indicate the main reservoir rocks in the Maokou Formation are crystalline dolomite,siliceous dolomite and dolomitic quartzite,whose distributions are mainly related to the karst systems,thus are uneven both laterally and vertically.Most of the crystalline dolomite occurs on the periphery of the karst systems,and their reservoir properties are poor.On the contrary,the siliceous dolomite and dolomitic quartzite are mostly distributed within the karst systems and have favorable reservoir properties.The development of reservoirs obeys the law of "facies-controlled karst".Specifically,grain-beach facies provided the material basis for the reservoirs,and facies-controlled eogenetic karst was the key for the formation of high-quality reservoirs.The following hydrothermal dolomitzation made it possible for the preservation of reservoir pores.The siliceous hydrothermal activities and later calcite cementation resulted in the filling of some pores.The unique siliceous dolomite and dolomitic quartzite finally came into being.According to these results,it is proposed to further strength study on "facies-controlled karst" reservoirs in addition to "fault-controlled hydrotherm"reservoirs,so as to improve the accuracy of reservoior prediction.

Controlling factors and development models of biohermal reservoirs of the Changxing Formation in Yuanba area,Sichuan Basin

Li Hongtao, Xiao Kaihua, Long Shengxiang, You Yuchun, Liu Guoping, Li Xiupeng

2016, 37(5):  744-755.  doi:10.11743/ogg20160514

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Based on detailed observation of cores,thin sections and analysis of seismic data,this paper studied the high frequency sequence,sedimentary facies,reservoir types and diagenesis of the biohermal reservoiors in the Changxing Formation in Yuanba area,the Sichuan Basin,analyzed the architecture of the biohermal reservoir and the factors controlling its development,and formulated its development models.The results show that the bioherms in the Changxing Formation were favorable facies and were developed mainly on platform margin of the SQ2.Vertically,the bioherms consist of two distinct reef cycles.Laterally they are in asymmetrical distribution featuring in "early in the east and later in the west".The quality biohermal reservoirs are dominated by dissolved dolomite with low porosity and moderate-low permeability and mainly occur in reef caps of the upper fourth-order sequences,and feature in typical "double layer reservoirs architecture".Meteoric dissolution and dolomitization in the early diagentic stage were the most important constructive diagenesis in the Changing Formation.A comprehensive analysis indicates that the biohermal facies controlled by high-frequency sequence is the foundation of reservoir development,and also is the key factor controlling penecontemporaneous meteoric dissolution and dolomitization of the bioherms in Changxing Fm.Burial dissolution,burial dolomitization and fracturing further improve the physical properties of reservoir rocks.The development model of the biohermal reservoirs in the Changxing Fm of Yuanba area is ‘lithofacies-controlling reservoir,early formation and late reformation’.

Relationship between reservoir development in the Middle Permian Maokou Formation and paleostructure evolution in the Sichuan Basin

Li Dajun, Chen Hui, Chen Hongde, Liang Hong, Peng Cai, Xia Ming, Duan Hongzhen

2016, 37(5):  756-763.  doi:10.11743/ogg20160515

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The development characteristics and main controlling factors of reservoirs in the Middle Permian Maokou Formation in Sichuan Basin were studied based on drilling and logging data,framework seismic sections and regional geolo-gical data.The relationship between paleostructure evolution and formation of geomorphology,karst landform and dolomitization zone of the Formation,was also discussed systematically to predict potential reservoir zones there.Study shows that the Caledonian Movement defined and shaped the fundamental landforms of the Middle & Lower Permian in the Basin and the Emei mantle plume's uprising exerted a continuous influence on the paleogeomorphology evolution in the whole Permian.As a result,southwestern Basin turned to an inherited paleogeomorphological high.It is not only a sedimentary paleogeomorphological high for the development of high energy bank depositsin the Formation but also a paleogeomorphological high for the development of weathering crust karst on top of the Formation.Emei tafrogeny provided migration pathways of hydrothermal fluid and acid diagenesis fluids for the overprint of multi-stage dolomitization and dissolution on early band deposits and karstification,thus largely controlled the distribution of dolomite reservoirs in the Formation.

Rock types and characteristics of the Middle-Upper Cretaceous carbonate reservoirs in Halfaya oilfield,Iraq

Wang Yuxiang, Zhou Wen, Guo Rui, Fu Meiyan, Shen Zhongmin, Zhao Limin, Chen Wenling

2016, 37(5):  764-772.  doi:10.11743/ogg20160516

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The Middle-Upper Cretaceous carbonate reservoirs are the main pay zones in Halfaya oilfield of Iraq.Comprehensive analyses of cores,thin sections,physical property test results,mercury injection data and scanning electron microscopy (SEM) images were carried out on samples from typical coring wells in the reservoirs of the field to figure out rock types and characteristics in them.The results show that the mineralogy of the rocks is dominated by calcite and clay,with small amount of dolomite scattered locally.Components and their relatively contents in rocks vary greatly.Based on rock component types and characteristics,we constructed a three end-member framework with micritic matrix,foraminifera skeleton and bioclast/calcarenite limestone as each end-member,respectively,and proposed a classification scheme for rock types in the field.According to the scheme,the rocks can be categorized into three major classes (micrite,foraminifer limestone and bioclastic limestone/calcarenite) and nine sub-classes based on the contents of certain grains in the rocks.Different types of the rocks are observed to have various pore structure and physical characteristics-footprints of different sedimentary settings.Horizontally,the distribution of different types of rocks shows clear evidence of being controlled by sedimentary settings.And vertically,foraminifera limestone of greater thickness seems to be concentrating more on the Upper Cretaceous carbonate reservoirs.Bioclastic limestone/calcarenite was rather confined within certain areas and the Middle Cretaceous carbonate reservoirs contained mostly bioclastic limestone/calcarenite.Micrite is seen interbedded as thin layers in bioclast/calcarenite.And foraminifera limestone is rare.

Characteristics of globigerinid limestone reservoirs of bottom current deposition in Gas Field A of Madura Strait, Indonesia

Ni Jun'e, Sun Lichun, He Juan, Jiang Baizhao, Wang Hongqiang, Wang Long, Guo Lina

2016, 37(5):  773-778.  doi:10.11743/ogg20160517

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Globigerinid limestone in Gas Field A of Madura Strait in Indonesia might be deposited by bottom current in outer shelf to upper slope.Petrological and reservoir space characteristics as well as well logging and seismic data were integrated to describe its sedimentary characteristics.According to Dunham's classification,the reservoir consists of grainstone,packstone and wackestone.The former two lithologies are grain supported and well sorted,and the reservoir space was dominated by intragranular dissolved pore,followed by intergranular dissolved pore and vuggsand a small amount of moldic pores and fractures.The reservoirs feature in large thickness,good physical properties and high NTG,and have great petroleum-bearing potential.There has been much debate over the depositional environment of the Pliocene globigerinid limestone in the Madura Strait.We propose three provenances for the reservoir.

Characterization and genesis of the Middle and Late Eocene tectonic changes in Zhu 1 Depression of Pearl River Mouth Basin

Hu Yang, Wu Zhiping, Zhong Zhihong, Zhang Jiangtao, Yu Weigao, Wang Guangzeng, Liu Yiming, Wang Shouye

2016, 37(5):  779-785.  doi:10.11743/ogg20160518

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Comprehensive analyses of unconformities,faulting systems and basin tectonic framework were carried out to characterize tectonic transformation mechanisms during the periods of Early-Middle Eocene and Late Eocene-Early Oligocene in the Zhu 1 Depression of Pearl River Mouth Basin.The results show that the Indochina block's rotation and extrusion and southward subduction of the proto-South China Sea resulted in a regional stress field clockwise change from NW-into SN-extension,and a shift in fault-controlled basin from NE- and NEE-trending to near EW and NWW-trending,with numerous new faults of EW-trending developed.The lithosphere extensional deformation transformed from homogeneous wide rift mode to centralized narrow rift mode,and migrated to north margin of the South China Sea.As a result,the Basin expanded as the isolated half-grabens or narrow grabens tended to get connected.Under a SN-trending stretching,the stretch boundary at the north of the depression was defined by basin-controlling faults.As a pre-existing weak zone with concentrated stress,the northern margin experienced strong and northward faulting and significantly spread northwards,causing the sedimentary center migrating northward as well.The tectonic changes during the faulting activities of the Basin may explain the characteristic differences of the two source rocks:the Wenchang and the Enping Formations,as well as the disparity of oil and gas distribution in the area.

Seismic prediction of fault damage zones in carbonates in Halahatang area,Tarim Basin

Wan Xiaoguo, Wu Guanghui, Xie En, Zhang Yintao, Gao Lianhua

2016, 37(5):  786-791.  doi:10.11743/ogg20160519

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Carbonate fault zone often has relatively wide damage zone,which is favorable for the development of fracture-cave system,and its identification and prediction is of great significance for oil/gas target optimization.On the basis of well-seismic responses analysis of the Ordovician carbonates in Halahatang area of the Tarim basin,comparative analysis of various seismic techniques reveals that the carbonate fault fractured zone can be identified and described by using coherent enhancement technique(AFE) and fracture seismic facies analysis technique,combining with the seismic root-mean-square amplitude,seismic curvature and fracture-cave system description.The results show that the carbonate fault damage zones are in stripped,fan-shaped distribution along the fault zone,generally with a width of 200-2 500 m,which have good agreement with the well data.The fault fractured zone controlled the distribution of large scale fracture-cave system,and the wider the damage zone is,the larger the fracture-cave system is.

Dynamic simulation of 3-D multiple-scale wormhole propagation in carbonate rocks

Xue Heng, Zhao Liqiang, Liu Pingli, Cui Mingyue, Jiang Weidong, Liang Chong, Ye Jiexiao, Xu Bin

2016, 37(5):  792-797.  doi:10.11743/ogg20160520

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Three dimensional (3-D) simulation of wormhole propagation is crucial for predicting stimulation effects of acidizing or acid fracturing in carbonate reservoirs.This paper presented a 3-D multiple-scale wormhole propagation model built based on the double-scale model,and its solving details.Meanwhile,geological modeling method was used to establish the geology model of porosity correlation distribution in 3-D space,which deals with the saltation of porosity successfully and makes the simulation results more realistic and reliable.Through the simulations,we found that the injection rate is the key factor affecting wormhole patterns.With the increase of the injection rate,five dissolving patterns may be observed,namely face dissolution,conical wormhole,dominant wormhole,ramified wormhole and uniform dissolution.When the convection and dispersion are comparable,the dominant wormhole may form with a thick root and a thin tip.The do-minant wormhole propagation can be divided into the following successive four stages,including competition stage,dominant stage,breaking stage and broadening stage.Most importantly,we found that the wormhole propagation pathway is highly correlated with the spatial correlation of porosity,since the wormhole breaks the core through those large pores with well connectivity.Finally,the field simulation results verified that the models and methods proposed in this paper are feasible and reliable for wormhole simulation of acidizing and acid fracturing in carbonate reservoirs.Those works could provide theoretical support for fine simulation and prediction in the relevant research areas.