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02 September 2022, Volume 43 Issue 5
Petroleum Geology
Progresses and directions of unconventional natural gas exploration and development in the Carboniferous-Permian coal measure strata, Ordos Basin
Xusheng Guo, Dehua Zhou, Peirong Zhao, Zengqin Liu, Dianwei Zhang, Dongjun Feng, Fengcun Xing, Wei Du, Gang Chen, Fan Yang, Chuanxiang Sun
2022, 43(5):  1013-1023.  doi:10.11743/ogg20220501
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With the development and breakthroughs made in sweet spot assessment of unconventional gas, multi-stage hydraulic fracturing in horizontal wells, and simultaneous development of multiple pay zones, the exploration and development paradigm of unconventional gas has shifted from single mode to composite mode. The joint exploration and commingled production of unconventional gases from coal measure strata of marine-to-continental transitional facies is expected to become a new “unconventional revolution” following the shale revolution. The Carboniferous-Permian is of the most important source rock sequences in the Ordos Basin, featuring huge potential in unconventional gas resources, though at the initial stages of exploration and development. Based on exploration progresses made and research results achieved during the 13th Five-Year Plan period, we summarize the geological features of the Carboniferous-Permian coal-measure unconventional gas in the Ordos Basin, that is, “two sources, three highs, and three gases”. The Carboniferous-Permian is dominated by multiple types of lithology or lithological components, and the coal-rich components(“two sources”) are major targets in exploration,among others. The organic macerals of the source rocks are dominated by vitrinite, and organic pores and micro-fractures are the main reservoir spaces, characterized by high total organic carbon content, high thermal maturity, and high gas content (“three highs”). The gas occurrence is markedly differentiated between deep and shallow layers, with adsorbed gases dominating the shallow coal seams, and free gases occurring in the deep coal seams, resulting in the coexistence of coalbed methane, shale gas and tight sand gas (“three gases”), and the existence of their various combinations. Regarding challenges such as tightness of reservoirs, large difficulty in sweet spot identification, and relatively high stresses, we have developed drilling and completion technique of “long lateral, large pumping rate, large liquid volume and high sand load”, which could be of effective supports to breakthroughs made in unconventional gas exploration in deep coal-measure reservoirs. It is suggested to further strengthen the comprehensive research on coal-measure source rocks and lithological assemblages, establish theory and technology for integrated evaluation of geological and engineering sweet spots, and develop supporting technologies in exploration, development, and engineering, in an effort to accelerate high-quality development of unconventional gas in the Ordos Basin during the 14th Five-Year Plan period.

Sequence stratigraphy and lithofacies characteristics of fine-grained deposits of Wufeng-Longmaxi Formations in the Sichuan Basin and on its periphery
Zongquan Hu, Wei Du, Tong Zhu, Zengqin Liu
2022, 43(5):  1024-1038.  doi:10.11743/ogg20220502
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This paper takes the shale samples from the Wufeng Formation and the first member of Longmaxi Formation in the Sichuan Basin and its periphery as the research subject. Through the comparative analysis of drilling data, core and outcrop analysis, and test data, a study on the sequence stratigraphic division and correlation, lithofacies classification and identification, and sedimentary microfacies-lithofacies distribution of the rocks is carried out. The results show that the fine-grained deposits of study interval can be divided into three third-order sequences (SQ1, SQ2, and SQ3), which are subdivided into seven systems tracts. High-quality shales are mainly distributed in the transgressive systems tracts (TST) of the three sequences, of which high-quality shales of SQ2-TST are the most widely distributed. The lithofacies are mainly organic siliceous shale, organic-rich limy siliceous shale, and medium-high organic siliceous argillaceous shale. The regional distribution of different types of inorganic minerals is clarified. The contents of carbonate minerals are relatively high near the central Sichuan paleo-high and northern Guizhou Uplift. With the increase of water depth, the contents of clay minerals and quartz minerals gradually increase. The sedimentary-lithofacies of SQ2-TST is mapped by using the contour maps of the content of three terminal principal minerals (quartz, clay, and carbonate) and the content of total organic carbon (TOC), in combination with the contour maps of parameters such as geochemical parameters (Th/U) and isopachous maps. On the periphery of the central Sichuan paleo-high, central northern Guizhou paleo-high, and northern Sichuan paleo-high, medium-low organic argillaceous shales and medium-low organic limy siliceous argillaceous shales deposited. In the deep-water shelf of southern Sichuan Basin enclosed by the central Sichuan paleo-high and central northern Guizhou paleo-high, organic-rich limy siliceous argillaceous shales are mainly deposited. organic-rich siliceous shales are mainly developed in the Fuling area extending to the northeast of the Sichuan Basin.

Review on provenance, transport-sedimentation dynamics and multi-source hydrocarbon sweet spots of continental fine-grained sedimentary rocks
Zaixing Jiang, Yunzeng Wang, Li Wang, Xiangxin Kong, Yepeng Yang, Jianguo Zhang, Xinyu Xue
2022, 43(5):  1039-1048.  doi:10.11743/ogg20220503
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With the rapid development of unconventional oil/gas industry, more and more attention has been paid to fine-grained sedimentary rocks. The study expounds the rock types, provenances, transport-sedimentation dynamics and hydrocarbon sweet spots of fine-grained sedimentary rocks of continental lacustrine facies in a systematic manner, based on the researches of fine-grained sedimentary rocks in recent years both at home and abroad. The accumulation of fine-grained sedimentary rocks in the continental lacustrine basin is mainly a result of multi-source supply of terrestrial, intrabasinal authigenic, volcanic and mixed-source sediments. The deposition mechanisms of fine-grained sediments include suspended sedimentation, gravity flow, volcanic, hydrothermal and intrabasinal biochemical activities, and the sediments may have undergone autochthonous re-deposition or re-deposition after short-distance transport before becoming consolidated rock. The provenance of fine-grained sediments plays a major role in determining the sedimentary characteristics, physical qualities, and hydrocarbon enrichment mechanism of the fine-grained sedimentary rocks. The study of provenance and transport-deposition dynamics of fine-grained sedimentary rocks is of great guiding value to shale oil/gas exploration and development.

Key factors and directions of exploration in the Cambrian pre-salt sequence, Tarim Basin
Haitao Lyu, Feng Geng, Kai Shang
2022, 43(5):  1049-1058.  doi:10.11743/ogg20220504
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The Cambrian pre-salt sequence has been an important strategic successor play for oil and gas exploration with the greatest exploration potential in the Tarim Basin. However, no significant discoveries have been made for the past many years, with some key problems yet to be solved. Based on the summary and review of the achievements of key exploration wells in the Cambrian in the Tarim Basin, the conditions of hydrocarbon accumulation in the Cambrian pre-salt sequence are recognized, while pointing out the play fairways for exploration and the directions for research in the near future. The study shows that the Cambrian pre-salt sequence in the Tarim Basin is of better geological conditions for hydrocarbon accumulation such as source rocks, reservoirs and cap rocks, but there are differences in the configuration of play elements in different zones. Generally speaking, there are two hydrocarbon accumulation patterns in the pre-salt sequence, namely the “accumulation via vertical migration of hydrocarbons from indigenous source rocks” and the “accumulation via lateral migration of hydrocarbons from allochthonous source rocks”. It is pointed out that the Tazhong Uplift facing Manjiaer Sag, the Selibuya-Haimiluosi-Mazhatage structural belt facing Maigaiti Slope and the Akekule Salient of Tabei Uplift are the key areas for further exploration, and the key research directions are put forward respectively.

Hydrocarbon accumulation pattern and exploration prospect of the structural traps in lower play of the western Central Depression in the Junggar Basin
Jianzhong Li, Xiaojun Wang, Fan Yang, Yong Song, Ablimiti, Baoli Bian, Hailei Liu, Xueyong Wang, Deyu Gong
2022, 43(5):  1059-1072.  doi:10.11743/ogg20220505
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The lower play of the western Central Depression in the Junggar Basin is of favorable conditions to generating large-scale oil/gas fields. Given its low exploration maturity, the zone is a key to strategic exploration. Hydrocarbon accumulation models are established and play fairways are selected by systematically studying the structures, source rocks and reservoirs and sorting out the play element combinations of the lower play. The study shows that there are three suites of source rocks developed in the zone: the Carboniferous, the Lower Permian Jiamuhe and Fengcheng Formations and the Middle Permian Xiawuerhe Formation. Among others, the Fengcheng Formation is of high-quality source rocks deposited in alkali lake environment with great hydrocarbon generation potential and is in direct contact with the structural trap of the lower play that reaches into the Mahu Sag, indicating a good source-reservoir configuration. Reservoirs of volcanic rock type, argillaceous dolomite type and clastic rock type are well developed in the lower play, and a favorable reservoir-caprock combination is formed under the shelter of regional dolomitic mudstone seal in the Fengcheng Formation. Overpressure is generally seen in the lower play, where gas condensates and natural gas dominate the hydrocarbon phase at a burial depth below 6 200 m, prone to be of high productivity. In terms of source-reservoir configuration, three hydrocarbon accumulation models are established, that is, of source-edge draping type, source-edge juxtaposition type and intra-source overlapping type. Among others, the structural traps of the last two types have large hydrocarbon supply window developed between target reservoir and source rocks, favorable for hydrocarbon accumulation. Five play fairways are selected and evaluated, namely, the northeast ring belt of Penyijingxi Sag, Mosuowan Uplift Mazhong structural belt, west ring belt of Shawan Sag, and Dabasong nose-like salient zone, for strategic breakthroughs in oil/gas exploration in the lower play.

Main factors controlling the medium-mature shale oil enrichment and exploration breakthrough in the Paleogene lower E3s1L in Qikou Sag, Bohai Bay Basin
Lihong Zhou, Haiqing He, Xujie Guo, Changwei Chen, Guomeng Han, Fei Yang, Jianying Ma, Suyan Zhou
2022, 43(5):  1073-1086.  doi:10.11743/ogg20220506
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Continental lacustrine basins are rich in shale oil resources of medium maturity, which features rapid lithofacies change, low diagenetic evolution, strong reservoir sensitivity, and high crude oil viscosity. Engineering development strategies in this regard are thereby still in the exploratory stage. The typical lacustrine source rocks of medium maturity in the lower sub-member of the first member of the Paleogene Shahejie Formation (E3s1L) at the southwestern margin of Qikou Sag, Bohai Bay Basin are analyzed to discover the major control factors of medium-mature shale oil enrichment and engineering countermeasures to them, and explore the development and production plan of medium-maturity shale oil. The research shows that the clay mineral content in the dolomitic limy shale intercalated by thin-bedded carbonate rocks is less than 30 %, and the anti-swelling fracturing fluid system of seawater-based slickwater can inhibit the sensitivity of such reservoirs and solve the problem of formation collapse caused by the water swelling of clay minerals; Factors including reservoir brittleness, hydrocarbon generation condition, oil content and reservoir water sensitivity are the key in controlling the distribution of medium-maturity shale oil sweet spots, and the thickness of these sweet spots generally ranges between 10 m and 12 m. Application of pre-stack migration data volumes to carry out reservoir prediction, accurately reading the depth of the landing point, accurately steering rotary drilling, accurately keeping lateral within the sweet spot interval, and improving the drilling encounter rate of sweet spots, all are the prerequisites for gaining high and stable production; and swelling resistance with seawater-based fracturing fluid, volume fracturing with energy accumulated at the toe end, and energy enhancement and viscosity reduction with CO2 as pad fluid, can help realize production enhancement by fracturing. These new understandings have been applied to optimize the completion of Well QY1H, resulting in a total production of 7 303.28 tonnes of oil in 437 days (a daily output of 16.75 tonnes on average), which marks a breakthrough made in the exploration of the medium-maturity shale oil.

Fractures in cores from the Lower Paleozoic Wufeng-Longmaxi shale in southern Sichuan Basin and their implications for shale gas exploration
Zhensheng Shi, Shengxian Zhao, Qun Zhao, Shasha Sun, Tianqi Zhou, Feng Cheng, Shaojun Shi, Jin Wu
2022, 43(5):  1087-1101.  doi:10.11743/ogg20220507
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Fractures are closely linked to the reservoir capacity of shale intervals, therefore also have a direct control over the test results of wells drilled into the intervals. A macro delineation based on cores and outcrops observation are combined with polarized microscopic and scanning electron microscopic images of samples to reveal the characteristics of fractures in the gas-bearing shale intervals of the Wufeng-Longmaxi Formations, southern Sichuan Basin. It shows that the shale intervals have highly-developed bedding-parallel and non-bedding-parallel macro and micro fractures, of which the bedding-parallel fractures take dominance and account for 75 % of the total macro fractures and 87 % of the total micro fractures. For macro fractures, the bedding-parallel fractures are generally foliation fractures and inter-layer sliding fractures, and the non-bedding-parallel fractures are oblique and vertical. For micro fractures, the bedding-parallel fractures are dominated by foliation fractures, while the non-bedding-parallel fractures are mostly fractures caused by abnormal pressure from hydrocarbon generation, diagenetic contraction fractures, and dissolution fractures. The density and distribution of the fractures are largely controlled by burial depth with the 3 500 m serving as a dividing line. For intervals below the line, fractures are mostly developed in the L1(1-3) layer; for intervals above the line, fractures are concentrated in the L1(1) layer. Density of both macro and micro fractures increases with increasing burial depth. There are wells showing that density of macro fractures in shale intervals below the line is ten times higher than those above the line. The fracture density is also controlled by TOC content and bedding types, which is well illustrated by shale intervals with high TOC content and bedding of striped siltstone type and graded (siltstone to claystone) type also having the highest fracture density. As fractures are essential to the reservoir capacity of shale intervals, the L1(1-3) and L1(1) layers contain the best quality shale intervals. However, with diagenetic contraction micro-fractures well developed, the bottom of the L1(2) also hosts some quality shale intervals.

Development of the Upper Permian Wujiaping shale in Hongxing area, eastern Sichuan Basin, and its significance to shale gas enrichment
Pengwei Wang, Zhongbao Liu, Xiong Li, Haotian Liu, Lin Zhou, Xiong Xiao, Ruyue Wang, Peng Li
2022, 43(5):  1102-1114.  doi:10.11743/ogg20220508
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The Wujiaping marine shale of the Upper Permian in the eastern Sichuan Basin is an important successor for shale gas exploration in the Sichuan Basin, where shale gas enrichment conditions and favorable exploration intervals are yet to be clarified. The study investigates source rocks and reservoir quality of the Wujiaping shale in Well HY 1 and their changes in vertical via petrology and geochemistry analysis, reservoir description and characterization as well as gas-bearing potential test, and the exploration prospects are thereby identified. Results show that the organic-rich shale is developed in the first sub-member of the first member of the Wujiaping Formation (Wu11) of tidal flat-lagoon facies and the second member (Wu2) of deep shelf facies. To be specific, the Wu11 is dominated by argillaceous shale, Wu21 bymixed siliceous shale and siliceous shale, and the Wu22 by siliceous shale and calcareous shale. Compared with the argillaceous shale in the Wu11, the mixed siliceous and siliceous shale in the Wu2 is characterized by high brittle mineral content (63.2 %), high organic matter abundance (TOC = 8.87 %), favorable organic matters (Type Ⅱ1), well-developed organic pores, and good physical properties (a porosity of 5.59 %). Among others, the source rock quality and reservoir capacity of the Wu21 are better than those of the Wu22. It can be concluded that the organic-rich shale of the Wu21 is favorable for shale gas exploration in the study area.

Microscopic pore characteristics and controlling factors of black shale in the Da’anzhai Member of Jurassic Ziliujing Formation, central Sichuan Basin
Haihua Zhu, Lin Chen, Zhenglin Cao, Minglei Wang, Haitao Hong, Yucong Li, Rui Zhang, Shaomin Zhang, Guangyi Zhu, Xu Zeng, Wei Yang
2022, 43(5):  1115-1126.  doi:10.11743/ogg20220509
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The reservoir space characteristics and controlling factors of shale oil reservoirs in the Da’anzhai Member of the Jurassic Ziliujing Formation, central Sichuan Basin, are analyzed based on the pore types, porosity and oil content data gathered from thin sections, SEM images, microfluorescence responses, porosity and permeability tests, N2 adsorption behaviour,and organic carbon measurements as well as Rock-Eval pyrolysis on samples from the member. The results show that:① The reservoir space consists of microfractures and pores. The microfractures are mainly lamellation fractures and shell calcite cleavage fractures, while the pores are mainly clay pores followed by shell calcite and authigenic quartz intercrystalline pores together with a small number of organic pores and pyrite intergranular pores. ② A negative correlation between pore size and pore volume is observed in the shale oil reservoirs in the member. The shale is high in porosity (averaged at 5.69 %) and dominated by clay pores that are small in size and tend to form horizontal pore-fracture system with laminated fractures; the intercalated shell limestone is low in porosity (averaged at 3.27 %) though it contains more shell calcite and authigenic quartz intercrystalline pores with large pore size that are likely to form pore-fracture networks with cleavage fractures. ③ The micro occurrence of oil in the reservoirs is highly heterogenous. The organic rich shale intervals have high oil content (S1 and OSI) but with oil widely dispersed in micro clay pores, they give out weak fluorescence and have low mobility. While the larger shell calcite / quartz intergranular pores, shell calcite cleavage and lamination fractures are hosting more oil and give out strong fluorescence. ④ Micro-fractures are essential to the accumulation of hydrocarbon. Hydrocarbons expelled from kerogen would preferentially migrate into lamellation fractures and accumulate in shell calcite / quartz intergranular pores around the lamellation fractures, resulting in low oil content in shell calcite pores that are far away from the lamination fractures. On that account, we suggest an emphasis on pore size and fracture development for assessing the quality of shale oil intervals in the Da’anzhai Member.

Lithofacies types and reservoir characteristics of Jurassic shale in the Sichuan Basin revealed by the Da’anzhai Member, Well Y2, Yuanba area
Qianwen Li, Zhongbao Liu, Feiran Chen, Guangxiang Liu, Dianwei Zhang, Peng Li, Pengwei Wang
2022, 43(5):  1127-1140.  doi:10.11743/ogg20220510
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The continental shale strata are generally lithologically complicated and highly heterogeneous, making it difficult to pinpoint sweet spots and deploy exploratory wells. To evaluate the reservoir characteristics and gas-bearing capacity of different lithofacies types of Jurassic shale sequences in the Sichuan Basin, the Da’anzhai Member (Well Y2) in the Yuanba area was taken as an example. Experimental methods including TOC content measurement, whole-rock mineral composition analysis, thin section observation, FIB-SEM, mercury injection-N2 adsorption measurements as well as tests for physical properties were performed to classify the lithofacies types of shale and interlayers in the second sub-member of the Da'anzhai Formation (hereinafter referred to as the J1da2) and then to single out the lithofacies or lithofacies assemblages with the highest hydrocarbon potential based on physical properties, pore structure, gas content and occurrence as well as fracability. Results show that the shale can be divided into three categories and six sub-categories of lithofacies types, and the interlayers into two categories and six sub-categories for lithofacies types, which can be further grouped into three lithofacies assemblage types from a macro perspective. The shale contains mostly inorganic pores such as interlayer pores within clay minerals and dissolved pores of calcite, providing storage space for gas. The total shale gas content is calculated to be 2.59-4.38 m3/t, in which the free gas accounts for an average of 67 %, indicating a good exploration potential. However, the brittle mineral content of shale lithofacies is barely 50 %, indicating a poor fracability. It is concluded that type AB-Ⅰ lithographic assemblages in the sub-member are the most promising exploration targets as they are observed to contain well developed cleavage and lamellation fractures, favorable hydrocarbon generation conditions, higher gas content and proportion of free gas as well as brittle interlayers, all indicating the most promising exploration targets of all types.

Sedimentary origins and reservoir characteristics of the Triassic Chang 73 tuffs in the southern Ordos Basin
Qing Li, Hao Lu, Shenghe Wu, Dongling Xia, Jiangshan Li, Fengqiang Qi, Yupu Fu, Yue Wu
2022, 43(5):  1141-1154.  doi:10.11743/ogg20220511
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A large number of tuff layers with plenty of oil and gas shows in the third sub-member of the seventh member of the Yanchang Formation (Chang 73), Ordos Basin, are deemed as targets for further exploration of unconventional oil and gas. The tuffs in this area are characterized by diverse types, complex origins, great differences in reservoir property, and a lack of systematic study. The study determines the lithofacies types of the highly tuffaceous rocks, clarify the sedimentary origins of different types of tuffs, and disclose the reservoir space characteristics of the tuff reservoirs in the Chang 73, Ordos Basin. The results show that there are five lithofacies types dominating the highly tuffaceous rocks, including vitric tuff, crystal-vitric tuff, crystal tuff, tuffite, and tuffaceous sandstone. In terms of origin, there are two types of tuffs in the study area, that is, the volcanic ash deposition type and the gravity flow deposition type. The tuffs of volcanic ash deposition origin are mainly distributed in the Zhangjiatan shale in the form of laminae, while tuffs of the gravity flow origin are mainly in the lower part of Chang 73. The latter consists of debris flow, turbidite and small-scale slumping deposits accumulated in channel systems, featuring large thickness and higher oil content. The reservoir spaces in tuffs can be divided into several types including intergranular pores/seams (including devitrification pores), intragranular pores/cracks, intercrystalline pores, marginal pores/seams of organic grains, and fractures, etc. Lithofacies plays a marked role in controlling the types and development of the reservoir spaces in tuffs, and the vitric tuffs, among others, are the best in terms of physical properties.

Application of backward elimination procedure in evaluation of microscopic reservoir heterogeneity in Chang 6 Member, Ordos Basin
Fanchi Nan, Liangbiao Lin, Yu Yu, Zhaobing Chen, Shangchao Mu, Hongbo Wang, Guanhua Ji, Junmin Ma
2022, 43(5):  1155-1166.  doi:10.11743/ogg20220512
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The sixth and eighth members of Yanchang Formation (Chang 6 Member in Huaqing area and Chang 8 Member in Heshui area), Ordos Basin, feature complex pore-throat structure and strong micro-heterogeneity under strong diagenetic transformation. The study re-defines evaluation parameters of the microscopic reservoir heterogeneity proposed by predecessors to better quantitatively characterize the microscopic heterogeneity of the reservoir. The backward elimination procedure is adopted to establish a regression equation for the Chang 6 reservoir in Huaqing area using relevant parameters proposed by the predecessors, with N, a parameter for comprehensive evaluation, proposed. Meanwhile, another regression equation is set for the Chang 8 reservoir in Heshui area using the parameters related to high-pressure mercury intrusion, with parameters Gk and Gs proposed, to make quantitative and comprehensive evaluation of the microscopic reservoir heterogeneity while verifying the feasibility of backward elimination in solving geological problems. The results show that parameter N is in good correlation with permeability, and the reservoir can be divided into three sections according to microscopic heterogeneity: strong, medium and weak sections, with correlation coefficients of 0.9812, 0.9496 and 0.7724, respectively. Besides, parameter Gk and porosity, as well as parameter Gs and permeability are also in good correlation. The correlation of the three comprehensive evaluation parameters is significantly higher than that of a single evaluation factor, indicating that backward elimination has achieved relatively ideal results in the quantitative evaluation of microscopic reservoir heterogeneity, and the mathematical method can provide new ideas for solving geological problems related.

Oil enrichment law of the Jurassic Yan’an Formation, Hongde block, Longdong area, Ordos Basin
Shuai Yin, Zhonghu Wu, Xiaoming Wu, Jianping Liu, Chengqian Tan, Ruyue Wang, Hui Yuan, Yawei Dai
2022, 43(5):  1167-1179.  doi:10.11743/ogg20220513
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The Jurassic in the Hongde block, Longdong area, Ordos Basin, has been less explored until some oil discoveries were made in the same system of nearby blocks. The Yan’an Formation in the Hongde block is taken as the subject of study to reveal conditions for possible oil accumulations in the formation based on relevant core, logging, seismic and testing data as well as analyses of sedimentary systems, tectonics and paleo geomorphology-the three most likely controlling factors for oil pooling. The results indicate that from the Yan’an Formation to the Yanchang Formation in the area there are three local unconformities respectively between the Fuxian Formation and the Yanchang Formation, the Yan 10 sub-member (Yan’an Formation) and Yanchang Formation, and the Yan 9 sub-member (Yan’an Formation)and Yanchang Formation. Five types of pre-Jurassic paleomorphologic units are also recognized:the paleovalley, inter-channel hill, ancient terrace, slope belt and low residual hill. Among them, the slope belt and low residual hill are conducive to hydrocarbon accumulation,and the ancient terrace and valley in the Yan 10 sub-member also have certain hydrocarbon accumulation potential. In addition, favorable conditions for oil accumulation also exist in the slope mouth where sandbodies have low shale content as several tributaries converged here and washed shale away. Oil caps and oil-water contacts are easily recognized. From a plan view, fault zones are intersecting sandbodies. The hanging walls of faults form perfect traps for oil, while the footwalls, without effective sealing, are usually barren in terms of oil pool. Normal faults are comparatively better oil pool holders than reverse ones. High-quality sedimentary reservoirs are necessary but insufficient conditions for the Yan’an Formation to contain oil pools. Based on the understanding, a method for identifying reservoirs in the Jurassic Yan’an Formation in the Hongde block is proposed.

Lithofacies architecture of lacustrine fine-grained mixed reservoirs and its control over sweet spot: A case study of Permian Lucaogou Formation shale oil reservoir in the Jimsar Sag, Juggar Basin
Senlin Yin, Gongyang Chen, Changfu Xu, Xianyue Xiong, Jun Zhao, Ke Hu
2022, 43(5):  1180-1193.  doi:10.11743/ogg20220514
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The lithofacies architecture of lacustrine fine-grained mixed silisiclastic-carbonate reservoirs is suggested to have certain control over the distribution of sweet spot of medium- to high-mature shale oil. But how exactly the mechanism works is still not clear. To tackle the issue, a case study is performed on the Permian Lucaogou Formation shale reservoir in the Jimsar Sag, Juggar Basin. Core and outcrop observation, logging data, lab test and production test are integrated to analyze the control of lithofacies architecture upon the distribution of sweet spots and its role in formulating development strategy. During the process, technologies including lithofacies architecture analysis, drone (or unmanned aerial vehicle) photography and 3D geologic modeling are used. The results show that combining the traditional outcrop observation measures with drone photography can be very useful in representing the lithofacies architecture features of the outcrops as the technology facilitates the well setting optimization, quantitative litho facies delineation with parameters, framework modeling and 3D lithofacies modeling. The lithofacies architecture patterns of this fine-grained mixed silisiclastic-carbonate reservoirs can be classified into three types, including the gradational, abrupt and special patterns. The gradational pattern, which usually indicates the occurrence of quality sweet spots, tends to be longitudinally symmetric and contains well-developed sandbodies mostly in a superimposition of multiple similar cycles with high quality source rocks and reservoirs with well-dissolved pores. The abrupt pattern hosts medium-developed sandbodies in an asymmetric architecture with a superimposition of several similar cycles. The source rocks are fine but reservoirs are thin. The special pattern is mostly thick clastic rocks with well-developed sandbodies in superimposed single cyrcles, which contain tight reservoirs and less fine sweet spots. The lithofacies architecture patterns control the distribution of sweet spots and shall be seriously considered during development planning for the formation. The gradational architecture pattern with well-developed sweet spots, is less heterogeneous and can be easily and economically tapped with pad drilling. The abrupt architecture pattern is the second-best choice due to their strong heterogeneity. The special architecture pattern features in scattered and discontinuous distribution of sweet spots as well as strong heterogeneity, thus is comparatively the worst development targets. A fine study of the spatial distribution of the lithofacies architecture may be needed before any development plan being drawn.

Pore throat characteristics of fine-grained mixed deposits in shale oil reservoirs and their control on reservoir physical properties: A case study of the Permian Lucaogou Formation, Jimsar Sag, Junggar Basin
Ke Ma, Jiagen Hou, Hu Dong, Guoqiang Wu, Lin Yan, Liwei Zhang
2022, 43(5):  1194-1205.  doi:10.11743/ogg20220515
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Major breakthroughs have been made in recent years in shale oil exploration in the Permian Lucaogou Formation of the Jimsar Sag, Junggar Basin, which has become an important target for the exploration and development of continental shale oil reservoirs. An integration of cast thin sections, trace elements, SEM (scanning electron microscopy), micron and nanometer CT imaging, PMI (pressure-controlled mercury injection) and RMI (rate-controlled mercury injection) was applied to study the pore throat structure of the fine-grained mixed (silisiclastic-carbonate) deposits in the Lucaogou Formation of saline lacustrine basin facies and its control on porosity and permeability of reservoirs. The results suggest that the dolomitic siltstone, silty dolomite and dolomicrite are of the best lithologic types in sweet spots. Micro-/nano-scale pore throat systems take a dominant position in reservoir space, with a pore throat radius ranging from hundreds of micrometers to tens of nanometers. Micro- to sub-micro-scale pore throats make the greatest contribution to permeability with the main throat radius ranging from 0.25 μm to 0.63 μm. The structure of micro- to nano-scale pore throats controls the permeability and oil content. The dominant lithologic types have larger peak values and wider distribution of pore throat radius. Besides, the distribution of throat radius, mainstream throat radius and volume of effective throat volume are the key factors controlling reservoir permeability. These understandings act as important geological supports for reservoir hydrocarbon charging prediction and well deployment in dominant sedimentary facies zones.

Wettability of the Permian Fengcheng Formation shale in the Mahu Sag, Junggar Basin, and its main control factors
Guowei Zheng, Zhiye Gao, Liliang Huang, Zhenxue Jiang, Wenjun He, Jiaqi Chang, Longfei Duan, Weihang Wei, Zhiwei Wang
2022, 43(5):  1206-1220.  doi:10.11743/ogg20220516
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Shale wettability is of great significance to relative permeability and capillary force of a reservoir, ultimately affecting the hydrocarbon accumulation process in shale as well as the shale oil/gas recovery rate. This study focuses on the wettability characteristics and main control factors of the Permian Fengcheng Formation shale in the Mahu Sag, Junggar Basin, by means of contact angle measurement and spontaneous imbibition plus micro-CT experiment. The results are shown as follows. First, the Fengcheng Formation shale in the Mahu Sag is of generally mixed wettability while prone to oil-wet, and the affinity of different lithofacies to water is listed out from the top, that is, felsic shale, limy felsic shale, dolomitic felsic shale, felsic dolomitic shale, and siltstone. Second, the shale wettability is jointly controlled by multiple factors including organic matter abundance, mineral composition, and pore structure. The shale hydrophobicity is positively correlated with the TOC, dolomite content, negatively with the quartz content, and in staged correlation with the calcite content; the larger the pore volume occupied by macro-pores, the more oil-wet is the shale sample. Third, the connectivity of oil-wet pores tends to grow better along the direction of crack and lamina development. A pore system of clustered type and certain connectivity is prone to form in dolomitic glomerates, which can form an unique shale oil storage space and migration pathway together with the interconnected small pores in the shale matrix. Fourth, the pores less than 1 μm have better connectivity and act as the main migration pathways for shale oil, while those larger than 1 μm with poor connectivity serve as the main storage space for shale oil. Fifth, it is preliminarily concluded that the high-quality shale oil reservoirs are of siltstone and dolomitic felsic shale with well-developed cracks, dolomitic glomerates and laminar structures in the study area according to the wettability characteristics of different lithofacies and sedimentary structure characteristics.

Methods and Technologies
Application of an integrated geology-reservoir engineering approach to fracturing in unconventional gas reservoirs, Sichuan Basin and some insights
Guangfu Wang, Fengxia Li, Haibo Wang, Jun Li, Hong Zhang, Tong Zhou, Xiaofei Shang, Linhua Pan, Yunqi Shen
2022, 43(5):  1221-1237.  doi:10.11743/ogg20220517
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The concept of engineering integrated with geology originated and prevailing in North America has also been accepted and put into practice during the exploration and development of unconventional oil and gas in the rest of the world. It runs through the whole process that starts with well design, drilling and completion, and ends with fracturing and production, only with focus shifting along the way in adaptation to different geological conditions and exploration or development stages. The introduction of the integrated geology-reservoir engineering approach to fracturing in the Fuling shale gas field in Sichuan Basin has been quite fruitful except for some restrictions frequently felt during exploration activities in some frontier blocks. Due to the diversity and complexity of sedimentary and tectonics in the Sichuan Basin, exploratory/appraisal wells after fracturing are tested with greatly varying results (mostly with low gas flow) even if they are located in the same block or arranged closely to one another. This might be explained by an insufficient accuracy of 3D quantitative characterization and models for key geological engineering parameters such as fractures and in-situ stress in sweet spots, and a lack of adaptive quantitative simulation technologies such as optimal fracturing stage/cluster division, fracture geometry optimization and fracturing process control. This paper systematically summarizes the experiences of an integrated geological-reservoir engineering team gained during fracturing practice in exploratory and appraisal wells for tight gas and shale gas in the Sichuan basin. Their work includes a 3D geo-engineering model of regional unconventional gas reservoirs built with key parameters of geology and engineering such as spatial reservoir distribution, physical properties, gas content, rock mechanics, in-situ stress field, initial formation pressure, mineral contents and natural fractures based on multi-discipline collaboration of geology, well-logging, seismic survey and engineering. Furthermore, the team carried out a fracture propagation simulation based on the model and formed a specific integrative technology series and workflow that could enhance SRV and single well production through tracing and optimization of perforation and diversion parameters, fracturing fluid and proppant volume as well as pumping rate. The implementation of the approach in developing tight gas from the Qianfoya Formation, Puguang Gas field and shale gas from Lintanchang Block in the basin has been proven successful, thus providing a reference for the development of similar unconventional reservoirs.

Application of an integrated geology-reservoir engineering approach to shale oil development in Ansai area, Ordos Basin
Chenglin Liu, Xinju Liu, Hongjun Zhang, Liyong Fan, Xiya Yang, Qibiao Zang, Bo Dai, Yue Meng, Hongliang Huo, Fang Wang
2022, 43(5):  1238-1248.  doi:10.11743/ogg20220518
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Shale oil is a hot spot for petroleum exploration and development over the world. Given its unique geological features, enrichment mechanisms, and development conditions, shale oil is desirable for an integration of geology and reservoir engineering. A variety of technical methods such as field geological survey, core observation and sampling, logging data interpretation, geological and engineering parameter analysis and testing, are applied to geological evaluation of shale oil with focuses on reservoir lithology, thickness, porosity, permeability, oil saturation, fracture development and selection of sweet spots; while in terms of reservoir engineering evaluation, we propose a development method and parameters appropriate to the shale oil in Ansai area regarding the geological characteristics of sweet spots. The main understandings obtained are as follows. First, the 7th member of the Triassic Yanchang Formation (Chang 7 Member) in Ansai area, Ordos Basin is typical of continental shale oil mainly of delta front and semi-deep lacustrine subfacies. The tight sandstones therein mainly belong to reservoirs with low to ultra-low porosity and ultra-low permeability or non-reservoirs. In light of geological and geomechanical parameters, the reservoirs can be classified into 4 types from good to poor, including Type Ⅰ, Type Ⅱ, Type Ⅲ and Type Ⅳ, with the first two types being targets of sweet spots. Second, the flow of shale oil in reservoirs of the Chang 7 Member are driven by reservoir fluid, rocks'elastic expansion and gas dissolution, while the potential of energy supplement via gas injection is limited. For Type Ⅰ reservoirs selected by geological evaluation, the nine-point, seven-point or staggered horizontal well patterns are used for quasi-natural depletion, while the seven-point or five-point horizontal well patterns are used for development of Type Ⅱ reservoirs. Third, the typical horizontal wells in the Chang 7 Member is of low initial production with a high decline rate at the initial stage, but the decline rate gradually reduces in the later stages. Moreover, it features a long production cycle with a relatively stable production.

Kinetics of high-quality lacustrine source rocks of Paleogene Wenchang Formation, Huizhou Sag, Pearl River Mouth Basin
Xiangtao Zhang, Yuling Shi, Jie Liu, Huahua Wen, Xingye Yang
2022, 43(5):  1249-1258.  doi:10.11743/ogg20220519
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The hydrocarbon yields of high-quality semi-deep lacustrine source rocks in the Wenchang Formation from the southwestern Huizhou Sag, Pearl River Mouth Basin, is simulated by the glass-tube pyrolysis experiment under two heating rates in which the thermal crack of hydrocarbon can be avoided to the maximum degree to establish for the first time the kinetic models of heavy hydrocarbon (C14+) generation and gas (C1-5) thermal degration. The theoretical relationship between the transformation ratio and the geological temperature of semi-deep lacusrine source rocks is reconstructed based on the geological extrapolation. It is suggested that the hydrocarbon generation of source rocks in the Huizhou-26 low exhibits the characteristics of “early oil and late gas”. The rapid generation of heavy hydrocarbon (oil) in the early stage corresponds to a geo-temperature from 95 ℃ to 160 ℃ and an age from 40 Ma to 15 Ma. While the large-scale generation of light hydrocarbons (gas) during the late stage corresponds to a temperature range of 119 ℃ to 205 ℃. The source rocks currently are at the end of its major gas generation stage.

Oil-source correlation based on multivariate statistical analysis: A case study of the Bodong Sag, Bohai Bay Basin
Xihao Guo, Fanghao Xu, Xiaobo Huang, Tao Jiang, Haoran Liang, Changzhi Li, Zhichao Li
2022, 43(5):  1259-1270.  doi:10.11743/ogg20220520
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Multivariate statistical analysis can comprehensively consider the interrelationship of multiple related parameters, and is of unique advantage in classification-based identification. It is thus an effective method for oil-source correlation with multiple suites of source rocks occurring simultaneously in a zone. However, no authors have conducted special research and analysis on the comparative application and applicability of diverse multivariate statistical methods to one case. There are multiple suites of source rocks developed in the Bodong Sag, Bohai Bay Basin, and the sources of oil and gas in different structures are extremely complex, which is the best case for oil-source correlation using multivariate statistical analysis. The comparison of principal component analysis (PCA), Fisher linear discriminant analysis (LDA) and Bayes LDA in terms of application shows that PCA cannot be applied to distinguish source rocks at different horizons, so it cannot be used for oil-source correlation in the Bodong Sag. However, the models established by Fisher and Bayes LDA can do this effectively, though neither of them could accurately identify oil of mixed sources. Based on the classification probability obtained from the Bayes LDA model, a discriminant model suitable for the Bodong Sag is established with the discriminant criteria of the Bayes model optimized, and four oil familys are identified in the Bodong Sag, with oil family A, B and C are derived from the source rocks of the third member of the Oligocene Dongying Formation (Ed3), the first to second member (Es1-2) and the third member (Es3) of the Eocene Shahejie Formation, respectively, and oil family D from the source rocks of Ed3and Es1-2.

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