Table of Content

01 February 2022, Volume 43 Issue 1

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Petroleum Geology

Diversity in the lithofacies assemblages of marine and lacustrine shale strata and significance for unconventional petroleum exploration in China

Maowen Li, Xiaoxiao Ma, Zhijun Jin, Zhiming Li, Qigui Jiang, Shiqiang Wu, Zheng Li, Zuxin Xu

2022, 43(1):  1-25.  doi:10.11743/ogg20220101

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There is a strong correlation between the lithofacies and organofacies of fine-grained organic matter-rich sedimentary rocks，where the carbonate-， quartz-feldspar- and clay mineral-rich lithofacies often show high affinity respectively with type I-S/II-S，I/II and III/IV organic facies. Hence， whole rock mineral X-ray diffraction analysis （XRD） represents a straightforward and effective method to predict the lithofacies， organic matter type， depositional environment， hydrocarbon characteristics， and rock mechanics of geological samples concerned. Based on a thorough review of marine shale data from North America and the Sichuan Basin， a large collection of lacustrine shales from major non-marine sedimentary basins in China were characterized by whole-rock mineral XRD， core observation， thin section analysis and total organic carbon （TOC） measurement. The aims are to classify the lithofacies and organofacies， identify the key difference in lithofacies assemblages， and discuss their implication for unconventional shale oil and gas exploration. The results indicate that lacustrine shales in sedimentary basins in China were deposited in various tectonostratigraphic settings. In general， the fine-grained sediments in freshwater-brackish water lakes in sag stage are dominated by clay-quartz with minor carbonate， whereas the saline and alkaline lacustrine sediments in rift stage consist of carbonate or clay-quartz with carbonate minerals. The sedimentary systems of fine-grained lacustrine deposits are characterized by rapid facies change， complex lithologies， and variable reservior-caprock combinations. Thus， the strong heterogeneity in the lithofacies and their combinations often lead to multiple types of shale oil “sweetspots”； as different lithofacies correspond to varying organofacies， the variation in the thermal behaviors of different organofacies contributes to the change in the states of hydrocarbon occurrence among the various lithofacies combinations. These results confirm that every shale is unique. Therefore， the variation in lithofacies and organofacies of the fine-grained sediments indicates that it is essential to use quantitative and objective parameters in classifying the “sweetspots” and hydrocarbon states of occurrence in order to make the credible assessment of shale oil resources.

Discussion on key issues of shale oil/gas resource assessment

Qingfan Zhou

2022, 43(1):  26-33.  doi:10.11743/ogg20220102

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The potential of shale oil/gas resources has become the focus of social attention. In order to better understand the shale oil/gas resources potential， it is necessary to constantly improve the methodology and strengthen the work of shale oil/gas resources assessment. Shale oil/gas belongs to unconventional resources， the geological characteristics of which are obviously different from those of conventional oil/gas， and the assessment methods for conventional oil and gas resources are thereby not fully applicable. Developing resources assessment method suitable for shale oil/gas is a must. Shale oil and gas resources are usually large-size in place， though with quite low recovery factor. Therefore， the specific resources assessment is supposed to stress both resources in-place and recoverable resources. At present， there are two main basic methods for shale oil/gas resources assessment， namely resource in-place assessment based on shale oil/gas content per unit volume and technically recoverable resource assessment based on shale oil/gas EUR per well. The factors that affect shale oil/gas resources assessment mainly include geological understanding， technology progress and economic factors， with geological evaluation as the foundation and key.

A new trap type： Fault-controlled fracture-vuggy trap

Lu Yun, Xiuxiang Zhu

2022, 43(1):  34-42.  doi:10.11743/ogg20220103

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Sinopec Northwest Oilfield Company recently struck a billion ton-scale oil/gas discovery-the Shunbei oil and gas field-in the deep carbonates of Shuntuoguole low-uplift， Tarim Basin. A new trap type-fault-controlled fracture-vuggy trap-is found to be the major container for oil and gas in the field. Analyses show that the traps take the fracture-vuggy layers along strike-slip fault zone as reservoirs and the overlying mudstone and local tight carbonates as caprocks. The reservoirs are probably the result of a tectonic disruption of the Middle-Lower Ordovician carbonates under strike-slip faulting that induced the formation of a large number of fractures， vugs and caverns. Based on the style and fragmentation of strike-slip faults as well as the distribution of internal reservoirs in the field， this study classifies the traps into four sub-types： transpression type， translation type， transtension type and composite type. This new type of trap further enriches the theory of marine carbonate reservoirs for oil and gas and provides a scientific basis for future exploration and development in deep and ultra-deep carbonates.

Paleokarst fracture-vug types and their reconstruction in buried hill area， Tahe oilfield， Tarim Basin

Wenge Hu

2022, 43(1):  43-53.  doi:10.11743/ogg20220104

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The Tahe oilfield in the Tarim Basin contains typical karsr fractured-vuggy reservoirs with caverns and solution fractures and vugs as the major storage space. The classification and description of the paleokarst fracture-vug poses a challenge for the fine development of the oil reservoirs. This study takes the Block 4 in the Tahe area as an example to investigate the genetic correlation， controlling factors of distribution and productivity differences of these karst fracture-vug. Three types of karst fracture-vug， i.e.， epikarst type， underground river type and fault-controlled type， are then proposed based on whether they share unified karst water flow pattern and circulation path. Meanwhile， three processes that have modified the fracture-vug are recognized， i.e.， the architectural transformation， cave collapse and in-filling. The intrinsic architecture and spatial distribution of the fracture-vug are also characterized. Guided by the understandings， a producing reserve assessment and production-injection well pattern readjustment have been successfully carried out in the Block 4， verifying the guiding significance of the study.

Division and formation mechanism of fault-controlled fracture-vug system of the Middle-to-Lower Ordovician， Shunbei area， Tarim Basin

Cheng Huang, Lu Yun, Zicheng Cao, Haitao Lyu, Haiying Li, Yongli Liu, Jun Han

2022, 43(1):  54-68.  doi:10.11743/ogg20220105

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Heterogeneous fractured-vuggy reservoirs of diverse genetic mechanisms are widely developed in carbonate strata， and the pore generation and evolution of diverse types within is related to the geological evolution of rocks. Generally， the formation of heterogeneous fractured-vuggy reservoirs is associated with thermochemical action mode of various unsaturated fluids in the formation and the total volume of materials that can be carried away. However， the fault-controlled fractured-vuggy reservoirs in Shunbei area， Tarim Basin， are not subjected to dissolution modification of unsaturated fluid， and feature a space distribution mainly constrained by fault zone boundaries. Exploration experiences have suggested that those reservoir spaces can be of hydrocarbon reservoirs with commercial value. What we have to pay attention to lies in whether a reservoir system of fracture-vug type can be formed under large-scale volume adjustment of materials by fault activities. An integration of 3D seismic interpretation， analysis of abnormal drilling conditions， well logs， element logging， pressure buildup test， interference test， and production performance data， is applied to comprehensively characterize the fault-controlled reservoirs， with their fractured-vuggy units and reservoir system inverted. The strain mode of brittle strata， factors controlling fault sealing， and water-rock interaction mode are studied according to the rock physical characteristics， structural compatibilization mechanism， and fluid modification mechanism during the strike-slip fault activity. It is proposed that the material volume variation resulted from rock dislocation， damage， and mechanical-thermochemical action in the fault zone， is a factor determining the formation of fault-controlled fractured-vuggy reservoirs in Shunbei area， whose pore system formation and evolution is closely related to fault activities. As theoretical derivation， the fault-controlled reservoirs can be subdivided into two types given the property and action mode of fluids， that is， the reservoir of karst type and of hydrothermal dissolution type， if being ignorable effects in the process of formation and evolution of their fracture-vug system.

Internal architecture of strike-slip fault zone and its control over reservoirs in the Xiaoerbulake section， Tarim Basin

Qingyou Ma, Lianbo Zeng, Xuhui Xu, Zicheng Cao, Huashan Jiang, Haixue Wang

2022, 43(1):  69-78.  doi:10.11743/ogg20220106

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The fault zone outcropping the Xiaoerbulake section in the northwest margin of Tarim Basin is selected to conduct detailed field investigation and description， sampling identification， geochemical analyses and other studies with the aim of establishing an architecture-reservoir model for a better geological description of fractured-vuggy reservoir bodies in the basin. The results show that the fault zone comprises right-lateral strike-slip faults （according to the juxtaposition relationship between the fault walls） and vertically cuts through the Middle Cambrian Shayilike and Awatage Formations. A dual structure of fault core and damage zone can be observed within the zone. The fault core is mainly composed of fault breccia with calcite veins. The angular and sub-angular shaped breccia is consistent with surrounding rocks in terms of lithology， indicating a possible damage-collapse-accumulation genesis of near-source material. The calcite particles in veins vary in size but tend to be finer toward the wall of breccia， indicating a multi-stage activities and reformation of fluids. With δ¹³C values ranging from -8‰ to -2‰， ⁸⁷Sr/⁸⁶Sr values generally higher than those of synsedimentary seawater， and significant negative Ce anomalies in REE （rare earth elements） analyses， the calcite veins truthfully record the visit of a later atmospheric fresh water. Fractures are well developed in the damage zone， most fractures are half or fully open with enlarged caverns from dissolution occurring locally along the fractures， and some fractures are filled by calcite cement. Based on the above research， a model is set up to describe the genetic evolution of the internal architecture and its control over the reservoirs in the fault zone. It suggests that the initial shear activity of strike-slip fault zone led to a brittle shear fracturing of the Middle Cambrian carbonate stratum， which then experienced long-term friction and slide to form noncohesive breccia and fracture zone with highly porous and permeable reservoirs. However， this strike-slip fault zone with high-quality reservoirs was later degraded into a vug-cemented fault zone as a result of multi-stage activities of calcite cementation jointly triggered by materials brought by atmospheric fresh water migrated vertically from surface to the deep along the zone and changes in temperature and pressure.

Hydrocarbon accumulation process and exploration direction of the deep Cambrian in Bachu-Tabei area， Tarim Basin

Jianchao Zheng, Bin Li, Qian Yuan, Minghui Qi, Zhongshan Yin, Jun Peng, Yi Huang, Yeyu Zhang

2022, 43(1):  79-91.  doi:10.11743/ogg20220107

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Tarim Basin as a typical marine petroliferous basin， has undergone multi-stage structural transformation， with its Cambrian strata deeply buried， highly risky in exploration. No significant exploration breakthrough has been made in Bachu-Tabei area where two major palaeo-uplifts control hydrocarbon accumulation in the region. In this regard， the study focuses on dynamically revealing the hydrocarbon accumulation process in large area and wide time domain by applying basin modeling technology in combination of the basic data of drilling， seismic exploration and test analysis， with a view to deepening understanding and deciphering exploration direction. The research results suggest that： （1） The thermal evolution of hydrocarbon source rocks in the Yuertus Formation mainly underwent 4 key periods， namely the Middle Caledonian， Late Caledonian-Early Hercynian， Late Hercynian and Late Himalayan periods， in 3 patterns of successive evolution in depression area， relaying evolution in slope area and rapid evolution in peripheral piedmont area； （2） Carrier systems of fault type， carrier bed type and unconformity type are well developed in the study area， indicating favorable conditions for hydrocarbon migration； （3） Three hydrocarbon migration and accumulation modes are put forward， that is， reversed confluence mode as shown by east Bachu area， diffusion mode by west Bachu area and confluence mode by Tabei area； （4） The favorable exploration targets in the eastern part of Bachu area are the structural traps formed and well preserved before the Late Hercynian period， and the structural traps well preserved in the western part. The target zone favorable for hydrocarbon migration in Tabei area is situated inside the platform to the west of Shaya Uplift.

Controls on formation and development of large-sized high-quality dolomite reservoirs in the Tarim and Sichuan Basins

Zhanfeng Qiao, Shaonan Zhang, Anjiang Shen, Min She, Lili Huang, Wenzheng Li, Guanming Shao, Chuanrui Dai

2022, 43(1):  92-104.  doi:10.11743/ogg20220108

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Following a comparative analysis of the main mechanisms and periods of dolomitization and dissolution in typical dolomite reservoirs in Tarim and Sichuan Basins，we identified the factors controlling the development of large-sized high-quality dolomite reservoirs： penecontemporaneous dolomitization is crucial to the scale of the reservoirs and dissolution modification is necessary for the quality of the reservoirs，thus the coupling of dolomitization with dissolution is the prerequisite for the development of large and high-quality dolomite reservoirs.Three geological processes that facilitate the development of the reservoirs were also brought forward： the （pene-） contemporaneous dolomitization associated with dissolution that mainly formed high-energy mound/reef-shoal and tidal-flat reservoirs against the backdrop of sea level fall； the adoptive burial dolomitization that mostly created reservoirs through shallow dolomitization of preexisting reservoirs，with quality and scale conditioned by the facies and dissolution intensity of preexisting reservoirs； and the epigenetic modification by dissolution that formed reservoirs after a hydrothermal or meteoric karstification of preexisting dolomite reservoirs，with quality and size controlled by facies，faults and unconformities as well as the development of the preexisting dolomite reservoirs.The above understandings may shed light on the prediction of large-sized deep high-quality dolomite reservoirs.

Characterization and petroleum geological significance of deep igneous intrusions and related structures in the Shunbei area， Tarim Basin

Yuqing Liu, Shang Deng, Rong Zhang, Jun Liu, Cheng Huang, Tian Gao

2022, 43(1):  105-117.  doi:10.11743/ogg20220109

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The magma intrusion in the Shunbei area， Tarim Basin， is investigated based on newly acquired 3D seismic data. The igneous intrusions and related structures in the area are characterized and mapped in detail to establish a vertical intrusion model， determine lateral flow direction of magma， and reveal their petroleum significance. The igneous intrusions are saucer-shaped and distribute along the top boundary of the Lower-Middle Ordovician and the boundaries of strata above it in layered superimposition. Reverse faults and steep fractures occur above the saucer-shaped sills， leading to a vertical magma intrusion model of “magma pathway-superimposed sills-steep structure”. The actual distribution of the sills and related structures reveal that the magma in this area fisrt intruded from the deep to the top of the Middle-Lower Ordovician and then flew laterally from north to south in a radial pattern. All these suggest that igneous intrusions play a role in improving reservoir quality and the related reverse faults are the pathways for hydrocarbon to migrate. A deeper understanding of the sills and related structures can be a better guidance for oil and gas exploration in the Shunbei area or even the whole Tarim Basin.

Three-layer structure model of strike-slip faults in the Tazhong Uplift and its formation mechanism

Caiming Luo, Xinxin Liang, Shaoying Huang, Yuan Neng, Wei Zhang, Shi Chen, Shujuan Cao

2022, 43(1):  118-131.  doi:10.11743/ogg20220110

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The Tazhong uplift located in the central uplift zone of Tarim Basin， is one of the key oil and gas plays in the Tarim Basin. A detailed interpretation of high-quality 3D seismic data， is applied to study the geometry characteristics of the strike-slip faults on both plane and sectional views， active periods of faulting and its formation mechanism in the Tazhong Uplift. Seismological data show that activity of most strike-slip faults in Tazhong uplift finalized before the Carboniferous， and only a few still kept active after the Carboniferous. The strike-slip faults cut through the strata from the basement to the Silurian and Devonian systems with vertical fault planes， which are obviously different from each other in geometry as in a three-layer structure. The fault planes in the deep Middle and Lower Cambrian feature linear distribution， with the strata on both sides of the fault arching. Multiple branch faults are developed on the top surface of carbonate rock after the strike-slip faults growing upward into the Upper Cambrian and Ordovician， as shown in a positive flower-like structure on sectional view， and by a diagonal fault pattern on plane view. While the strike-slip faults in the shallow Upper Ordovician， Silurian and Devonian systems commonly appear as a negative flower-like structure on sectional view， and as NW-trending en-echelon faults on plane view， specific to this layer. These three-layer faults are superimposed vertically in a complex spatial pattern. The evolution of strike-slip faults in the Tazhong Uplift is divided into three stages， that is， the Middle Cambrian when a structural inversion event led to the development of small-scale strike-slip faults under transpressional stress； the Late Ordovician when the strike-slip faults got reactivated to form a positive flower-like structure； and the Silurian and Devonian during which the strike-slip faults kept active. The development of the strike-slip faults in the Tazhong Uplift can be attributed to the control of peripheral tectonic environment and evolution under the local transpressional stress in the Middle Cambrian， the transpressional setting in the Late Ordovician， Silurian and Devonian.

Characteristics of composite hydrocarbon accumulation in a superimposed basin， Junggar Basin

Yong Tang, Yong Song, Wenjun He, Long Zhao, Changyong Zhao, Menglin Zheng, Haibo Yang, Shuai Sun, Liying Fei

2022, 43(1):  132-148.  doi:10.11743/ogg20220111

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The Junggar Basin is one of the most important petroliferous superimposed basins in western China. Oil and gas discoveries made there in recent decades have confirmed its great hydrocarbon potential as a superimposed basin. Data gathered through the latest exploration breakthroughs and previous studies are used to summarize the enrichment patterns of oil and gas， analyze the characteristics of oil and gas discoveries， guide strategic selection of exploration targets， and provide theorical materials for a geological study on continental petroleum reservoirs in the basin. The result shows that the reservoir assemblages in the basin exhibit a vertical “three-story” pattern and horizontal superimposition of uplifts and depressions of different stages， areas and scales in various patterns and intensity during the multi-stage superimposition process， which results in a disparity in the accumulation and distribution of oil and gas. It also suggests that the oil and gas distribution is controlled firstly by source rocks and secondly by caprocks， faults， paleo-structures， unconformities and sedimentary facies. Four major strategic exploration targets are singled out： the mid-deep to deep layers in slope area of hydrocarbon-rich sag， the thrust belt of the southern margin， the Carboniferous of Lu-Liang uplift， and shale oil in the Middle-Lower Permian. However， finding oil and gas reservoirs in this multi-cycle tectonic basin is still a big challenge due to its complex evolution history.

Microscopic pore structure characterization and oil-bearing property evaluation of lacustrine shale reservoir： A case study of the Permian Lucaogou Formation in Jimsar Sag， Junggar Basin

Linsheng Wang, Yiping Ye, Jianhua Qin, Yang Gao, Yuan Deng, Yingyan Li, Dianshi Xiao

2022, 43(1):  149-160.  doi:10.11743/ogg20220112

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In view of the diverse types of shale oil reservoirs and great difficulty in sweet-spot prediction in the Lucaogou Formation， analysis on the mechanism influencing shale oil pore development and evaluation on oil-bearing properties， are supposed to be carried out. To this end， this study focuses on the microscopic pore structure of shale oil reservoirs in the Permian Lucaogou Formation， Jimsar Sag， through an integration of core observation， scanning electron microscopy （SEM）， mercury intrusion porosimetry （MIP） and nuclear magnetic resonance （NMR）， etc. As a result， the influential mechanism is disclosed and classification criteria for reservoir evaluation are set up. The results show that there are five types of reservoirs developed in the Lucaogou Formation： reservoirs of intergranular pore type， of intergranular-dissolved-intercrystalline pore type， of dissolved pore type， of dissolved-intercrystalline pore type and of intercrystalline pore type. Among others， reservoirs of intergranular pore type and dissolved pore type are the best in terms of physical property and mobility， and those of intergranular pore type are the worst. Second， larger amount of coarser grains， proper dolomite content and low level of cement， serve to jointly determine the formation of high-quality reservoirs， the distribution of which is controlled by near distance to provenance， high-energy environment and paleo-sag. Third， the oil-bearing property of source-reservoir alternating or integrated source-reservoir configurations is better than that of near-source thick reservoir type， with a pore throat of 15 nm and 70 nm as a boundary to determine whether the Lucaogou Formation contains oil and whether the reservoir has good oil-bearing property， respectively. Fourth， as guided by the correlation between pore throat structure and oil-bearing property， three types of oil layers （i.e.， Class Ⅰ， Class Ⅱ and Class Ⅲ） are identified. The second interval of the first member of Lucaogou Formation （Lu 1 Member） is characterized by well-developed reservoirs of high quality， good source-reservoir configuration， and large-scale distribution of sweet spots within. The research results are of a geological basis for the classification， evaluation and prediction of shale oil reservoirs in the Lucaogou Formation.

Genetic types and exploration potential of natural gas at northwestern margin of Junggar Basin

Deyu Gong, Changyong Zhao, Wenjun He, Long Zhao, Yumei Kong, Liya Ma, Ruiju Wang, Wei’an Wu

2022, 43(1):  161-174.  doi:10.11743/ogg20220113

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Although the Junggar Basin is one of the four major onshore petroliferous basins in China， the progress of natural gas exploration is poor with major discoveries confined to the eastern and southern parts of the basin. Based on gas composition and stable carbon isotopes， we find four types of natural gas at the northwestern margin of Junggar Basin. TypeⅠgas is primarily generated from the lacustrine source rocks of the Lower Permian Fengcheng Formation， and can be further subdivided into TypeⅠ_A （mainly derived from Shawai Sag） andⅠ_B （mainly from Mahu Sag）. Type Ⅱgas is mainly of coaliferous gas derived from the Carboniferous and Lower Permian Jiamuhe humic source rocks of high to over-mature type in the Shawan Sag. Type Ⅲ gas is a mixture of TypeⅠandⅡ. Type Ⅳ gas is of secondary biogenic gas with oil biodegradation under reservoir destruction. During the Cretaceous， a mass of coaliferous and petroliferous gases were derived from the Fengcheng， Jiamuhe and Carboniferous source rocks in the Shawan Sag， which then migrated to higher parts of the structure along the carrier system composed of faults and unconformities； while the Fengcheng source rocks in the sag was at oil generation peak during which only a small amount of petroliferous gas of low maturity could be generated and accumulate within or over margins of the sag. In this study， it is confirmed that two suites of effective gas source rocks of large scale were developed in the Upper Paleozoic， west of Junggar Basin， that is， the Carboniferous （including Jiamuhe Formation） and Fengcheng Formation source rocks， serving to shed lights on a new field of natural gas exploration. In all， the study is of a typical case to show gas-source correlation under complex geological conditions and hydrocarbon accumulation restoration， and also deepens the understanding of the exploration potential of natural gas in the Junggar Basin.

Geochemical characteristics and genesis of mid-to-shallow natural gas on the periphery of Shawan Sag， Junggar Basin

Erting Li, Jun Jin, Jian Wang, Wanyun Ma, Shijia Chen, Cuimin Liu, Haijing Wang

2022, 43(1):  175-185.  doi:10.11743/ogg20220114

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Natural gas on the periphery of Shawan Sag， is generally derived from mixed sources. A lack of holistic study on this area in the early stage has restricted the understanding of natural gas accumulation therein. In this regard， a systematic analysis of geochemical characteristics of the natural gas is carried out by means of source-rock thermal simulation， by which the genetic types of mid-to-shallow gas on the periphery of the Shawan Sag are identified. The research results show that the natural gas in the study area is dominated by methane， with a drying coefficient ranging from 0.73 to 1.00， and a δ¹³C₁ value from -56.0 ‰ to -31.5 ‰， indicating coexistence of mature and high-to-over-mature natural gas therein. A δ¹³C₂ value varying between -30.4 ‰ and -22.8 ‰ reflects a mixed distribution of coaliferous， petroliferous and mixed gas. In addition， the analysis of carbon isotopic characteristics of source-rock pyrolysis gas， shows that the natural gas in the study area is of 4 types in genesis： Type Ⅰ natural gas， featuring a very high δ¹³C₂ value greater than -25.5 ‰ and a methylcyclohexane content greater than 50 % in lighter hydrocarbon C7， is derived from source rocks of the Jiamuhe Formation， and mainly distributed in the Cretaceous in the middle of Hongche fault belt； Type Ⅱ natural gas of sparse distribution is mainly secondary biogas from crude oil degradation， with an abnormally negative δ¹³C₁ value and an extremely high drying coefficient； Type Ⅲ natural gas is originated from the Xiawuerhe Formation source rocks and mainly distributed in the Jurassic Formation in Xiaoguai area and southern Hongche fault belt， with a δ¹³C₂ value varying from -27.9 ‰ to -26.4 ‰， a characteristic of mixed source rocks； and Type Ⅳ natural gas is mainly derived from source rocks of both Xiawuerhe Formation and Fengcheng Formation， and mainly distributed in the southern Hongche fault belt and northern Jinlong area. Among others， the natural gas mainly derived from the Xiawuerhe Formation features a δ¹³C₂ value greater than -29 ‰， and that from the Fengcheng Formation features a δ¹³C₂ value less than -29 ‰.

Characteristic differences of glauconite formed in different geologic periods and related genetic analysis

Qin Zhang, Chen Zhou, Hanyun Tian, Kai Wang, Zeping Song, Qishi Dong

2022, 43(1):  186-195.  doi:10.11743/ogg20220115

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Glauconite with significance of indicating facies is complex and diverse in occurrence， composition and genetic mechanism， and widely developed in different geologic periods. The differences in respects of characteristics and genetic mechanism of glauconite developed in various geologic periods are investigated based on a review of previous researches from a large number of related case studies worldwide， with a view to improving the genetic theory of glauconite and providing a scientific basis for paleogeographic analysis. The results show that the occurrence of glauconite formed in different periods can be classified into three groups， that is the granular， cemented and halo edged groups. The granular group in turn can be subdivided into four types in terms of substrate. The Precambrian glauconite is mainly developed in substrate of quartz and K-feldspar， featuring high content of K₂O and high maturity， which can be interpreted via pseudo-shape replacement theory in genetic mechanism. The Phanerozoic glauconite is preferably developed in initial substrate containing fecal spherulite and bioclastic materials， featuring unstable K₂O content as its potassium component mainly derived from seawater， and its genetic mechanism is closely related to contents of K₂O and Fe₂O₃， to which layer lattice theory and granular glauconization theory are more appropriate for interpretation. An integrated analysis of sedimentary environments and ion sources of glauconite in the various geologic periods， indicates that areas adjacent to Fe-redox contact surfaces with sufficient supply of cations such as Fe， K， Al， Si and Mg are favorable for the formation of glauconite.

Methods and Technologies

Geochemical features and origin of the Cambrian formation water in Tabei Uplift，Tarim Basin and its mineral dissolution-precipitation simulation

Yaqian Gui, Guangyou Zhu, Zhuang Ruan, Yinghui Cao, Zhenhuan Shen, Qiuhong Chang, Guoping Chen, Bingsong Yu

2022, 43(1):  196-206.  doi:10.11743/ogg20220116

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The chemical compositions and ion ratio coefficient of the Cambrian formation water taken from 14 wells in Yingmai and Yaha areas and Well Luntan 1 in the Tabei Uplift， Tarim Basin， are analyzed and applied to carry out hydrogeochemical simulation， with the geochemical characteristics， origin and evolution of formation water therein clarified. The results show that the Cambrian formation water in the study area is of CaCl₂ type， high in total dissolved solids （TDS）， and dominated by Na⁺+K⁺ and Cl^-. The concentration of Na⁺+K⁺ and Cl^- is well correlated with TDS of the formation water. Concentration of ions like Na⁺+K⁺， Ca²⁺， Mg²⁺， Cl^- and SO₄^2- tends to decrease first and then increase with the increase of burial depth， consistent with the trend of TDS change with burial depth. Vertically， two hydrochemical zones can be identified in the study area： an overflow-discharge concentration zone and a deep percolation concentration zone. Its sodium-chloride coefficient ranges from 0.39 to 0.55， averaging 0.51； de-sulfuric acid coefficient varies from 0.07 to 1.91 with an average value of 0.35. The results of mineral saturation index simulation by Phreeqc software suggest that saturation index of dolomite ranges from -3.43 to 1.95 with an average of -0.10， and 50.91 % of the dolomite is precipitated； the saturation index of gypsum ranges from -1.77 to 0.12 with an average of -0.80， and 94.64 % of the gypsum is dissolved； the saturation index of calcite ranges from -0.88 to 1.62 with an average of 0.55， and 18.18 % of the calcite is dissolved. A combination of calcite dolomitization and gypsum dissolution in the study area leads to enrichment of Ca²⁺ and deficit of Mg²⁺ in the current formation water. The Cambrian formation water low in metamorphic degree in the study area， can be attributed to seawater evaporation and concentration， ion adsorption and water-rock interactions.

Hierarchy modeling of the Ordovician fault-karst carbonate reservoir in Tuoputai area， Tahe oilfield， Tarim Basin， NW China

Wenbiao Zhang, Yaxiong Zhang, Taizhong Duan, Meng Li, Huawei Zhao, Yan Wang

2022, 43(1):  207-218.  doi:10.11743/ogg20220117

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Reservoirs of fault-karst type， special to the carbonate reservoir of the fractured-vuggy type， feature complicated storage structure， irregular geometry and stochastic spatial distribution under the impact of large strike-slip fault zones， which is bound to bring new challenges to their 3D quantitative characterization and geological modeling. Based on diversified research data of multiple scales， the hierarchical scheme of fault karst is established in Tahe oilfield， Tarim Basin， focusing on the principles of hierarchical constraint， genetic control and hierarchical modeling. The results show that the fault-karst hierarchy can be divided into four levels according to genesis and scale， namely the strike-slip faulted fracture zone， the fault karst， the fractured-vuggy zone inside the fault karst， and the karst cavern filling. In detail， the strike-slip faulted fracture zone framework model is established with deterministic data of the main and secondary faults， envelope range and key geological horizons obtained from fine interpretation of seismic coherence. Based on the seismic fault likelihood （FL） attribute， the external envelope model of the fault karst is established with deterministic method through automatic attribute segmentation and drilling calibration. With the constraint of the external envelope， the internal architecture elements are classified and modeled. Besides， a large-scale karst cavern model and a mesoscale discrete fracture distribution model are established with deterministic method based on seismic attributes such as texture and ant tracking truncation. As controlled by well data and constrained by seismic-geological probability， the sequential indication simulation and object-based marked point process simulation methods are applied to obtain the dissolved pore distribution model and the small-scale fracture discrete distribution model respectively. A filling model of the karst cavern is built by sequential indication simulation as constrained by wave impedance and large-scale cavern. Finally， a typical fault-karst reservoir in Tuoputai area， Tahe oilfield， is studied as an example to test the modeling method mentioned above， and the 3D integrated model thereby built can reflect the spatial hierarchy of the fault-karst carbonate reservoir.

Probability-constrained identification of Ordovician small-scale fractured-vuggy reservoirs in Blocks 4-6， Tahe Oilfield， Tarim Basin

Juan Zhang, Min Yang, Runcheng Xie, Ming Wang, Hong Wang, Ziwei Luo

2022, 43(1):  219-228.  doi:10.11743/ogg20220118

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Various types of small-scale fractured-vuggy reservoirs occur in the Middle-Lower Ordovician of Blocks 4-6 in the Tahe Oilfield， Tarim Basin， and have been considered as the most potential targets for future oilfield development in the area. However， these reservoirs are difficult to identify due to their complex storage space that induces confusing responses during conventional well logging. To tackle the problem， this study starts with classifying the reservoirs according to drilling and logging data as well as core observation， continues with identifying logging responses typical of the reservoirs based on crossplotting well logging parameters， setting up a discriminant function by using the multivariate stepwise discriminant principle and ends with establishing a method for identifying the reservoirs under probability constraints by determining the parent posterior probability identification limits of all types of reservoirs. The results shows that the reservoirs can be classified into five types： unfilled vug， vug filled with sand， clay and gravel， fracture-vug， fracture-pore and dissolved cave-pore types， which can be recognized respectively with critical probability identification limits of 0.98， 0.98， 0.60， 0.60 and 0.60. These reservoirs are senstive to natural gamma， resistivity and density logging rays. The study suggests that reservoirs of fracture-vug， fracture-pore and cavern types are highly developed in the study area， but their development show great variation vertically in a single well and laterally in the study area. Block 4 develops more fractured-vug reservoirs than Block 6 and shows a vertical two-phase karstification. This identification method embodies well the idea of “step by step identification” and avoids the pitfall of confusing logging signals of these fractured-vuggy reservoirs. Using the method can effectively improve the identification of fractured-vuggy reservoirs， thus providing an effective way for a deeper understanding of the evolution and potential of the reservoirs.

Numerical simulation of evenly propagating hydraulic fractures with smaller cluster spacing in the horizontal well YYP1 for deep shale oil in the Shengli Oilfield

Haiyan Zhu, Xinqin Xu, Anhai Zhong, Qinxi Zhang

2022, 43(1):  229-240.  doi:10.11743/ogg20220119

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Shale oil in China is generally high in viscosity and stored in highly heterogeneous shale reservoirs， which often respond poorly to stimulation of staged volume fracturing （each stage） with multi?perforation clusters. Increasing the cluster number in each fracturing stage of a horizontal well to evenly facilitate fracture propagation has been considered an effective solution to the problem. This study is focused on the friction of fluids flowing through several thousand meters of fracturing strings and the perforation holes as well as the competition among fractures for the fluids， to establish a seepage-stress-damage model for the description of dynamic propagation of the fractures. The effectiveness of the model is demonstrated with actual fracturing data in the Shengli Oilfield， where numerical simulations were carried out to investigate the impact of different numbers of perforation clusters and perforation holes as well as varying fracturing parameters on fluid distribution， stress， and fracture geometry based on geological characteristics of shale oil reservoirs in the area. Findings include： The optimal propagation distance for crack?induced stress is ~10 m for fractures created by single perforation cluster. With a 10 m cluster spacing and three clusters， evenly propagated fractures can be achieved with a perforating density of 20 holes per meter with fluid displacement at 12 m³/min and fracturing fluid viscosity at 30 mPa·s. With four perforating clusters for each fracturing stage， the optimal cluster spacing is 10 m for an uniform distribution of fluid and even propagation of fractures being achieved. This study lies down a theoretical foundation for effective shale oil exploitation in the Shengli Oilfield.

Mini-frac test interpretation for shale reservoirs based on impulse injection theory and its application

Yanyan Wang, Weihong Wang, Xiaohu Hu, Hua Liu, Xiaorong Jiang, Cheng Dai, Sidong Fang

2022, 43(1):  241-250.  doi:10.11743/ogg20220120

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Mini-frac tests can be used to evaluate the physical property of shale reservoirs prior to large-scale fracturing， and provide key parameters for resource quantity calculation and evaluation of sweet spots both in geology and engineering. As of now， the well testing interpretation based on conventional shut-in pressure recovery has a very low accuracy in dealing with special issues like mini-frac test， featuring “short-term injection and long-term shut-in”， often accompanied by difficulties in flow pattern identification and parameter diagnosis. In view of the particularity of mini-frac test， the study proposes a well testing interpretation method based on pulse injection theory， where the process of injection and shut-in pressure drop is regarded as a whole， the shut-in pressure response is assumed to be the result of both injection and shut-in periods as considered at the inner boundary of mathematical diffusion-filtration model， and then a mini-frac test interpretation model is established. A pressure dynamic curve is calculated， and the influence of the parameters on the test curve is determined through sensitivity analysis. Pressure of key sections of the test curve is obtained through asymptotic solution， and 3 combined diagnostic curves are introduced to obtain the flow regime information. Finally， an interpretation method and process by mini-frac tests is established with parameter interpretation made for different flow regimes， including dominant flow regime identification and special straight line analysis. This method is then applied to analyze two wells located at different structural positions of Pingqiao anticline in Nanchuan block， Fuling shale gas field， through which its practicality is verified. The interpretation results serve to provide basis for understanding the gas-bearing potential of shale in the area， and predict the regional differences of fracturing effect successfully， which is of guiding value to future delineation of sweet spot zone and development zone of priority. The subsequent production tests also verify the reliability of the results obtained.