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06 May 2022, Volume 43 Issue 3
Petroleum Geology
Factors controlling lacustrine shale oil adsorption in the Jiyang Depression, Bohai Bay Basin
Yongshi Wang, Zheng Li, Min Wang, Youshu Bao, Rifang Zhu, Jun Liu, Lianbo Wu, Limin Yu
2022, 43(3):  489-498.  doi:10.11743/ogg20220301
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It is of great significance to determining the amount of adsorbed oil and its control factors for the evaluation of shale oil resources and prediction of play fairway in shale oil exploration. The Shahejie shale in the Jiyang Depression is taken to determine the quantity of adsorbed shale oil by improved pyrolysis experiments. Meanwhile, the study reveals the factors influencing shale oil adsorption volume such as shale physical properties, shale compositions, shale oil components, as well as maturity of organic matters, temperature and pressure, etc., by an integration of molecular simulation technology, micro-adsorption mechanism and macro-experimental data. The results show that shale oil reservoirs featuring large pore volume, high TOC content, low saturated hydrocarbon content, low maturity, and low temperature, are of higher oil adsorption capacity, and TOC content and organic matter maturity are the main factors controlling oil adsorption of lacustrine shale oil reservoirs with low maturity.

Research of “synthem units of exploration” in mature exploration area and its application in the Dongying Sag, Bohai Bay Basin
Mingshui Song, Yongshi Wang, Xuejun Wang, Xuefeng Hao, Youqiang Li, Dongxia Chen, Fuwei Wang, Qiaochu Wang, Xuebin Shi, Yi Zou
2022, 43(3):  499-513.  doi:10.11743/ogg20220302
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The explored areas in eastern China have reached high exploration maturity as a whole. Selecting evaluation units suitable for fine exploration in these areas is of great significance to reducing exploration risks and achieving efficient exploration. As a hydrocarbon accumulation-geological evaluation method suitable for current geological understanding and fine exploration, research of “synthem units of exploration” mainly includes the definition and division of synthem units of exploration, the evaluation of remaining resource potential, the analysis of oil and gas accumulation patterns and accumulation modes, the selection and classification evaluation, and related exploration deployment. The concept of synthem unit of exploration inheriting the thought of system theory, is an extension and development of sedimentary facies-control theory and structural zone-control theory in hydrocarbon play study. It puts forward a comprehensive research system integrating petroleum system, play and oil and gas reservoir, featuring research methodology embodying the unity of the whole and individual, the unity of systematization and heterogeneity, the unity of systematic research and fine characterization, and the unity of predictability and effectiveness. According to the concept, the 193 synthem units of exploration are recognized in 9 exploration target zones in Dongying Sag and classified into 20 types, comprehensively considering the differences and similarities of reservoir-forming processes and characteristics. Thereby, fine, multi-dimensional geological understanding and exploration deployment have been achieved. Fine assessment of the remaining resource potentials and benefit evaluation of exploration targets are carried out based on the concept of exploration structural-layer unit, making it clear that there are 22 optimal exploration structural-layer units in 4 domains of interest in Dongying Sag, including the upper fourth member of Shahejie Formation-Dongying Formation of the Paleogene, the Neogene Guantao Formation-Minghuazhen Formation, the Paleozoic buried hill, and the Paleogene Kongdian Formation-the lower fourth member of Shahejie Formation. These are important targets for reserve growth and exploration breakthrough in the near and medium term, serving as a basis for efficient exploration.

Characteristics, quality-controlling factors and sweet spot model of the Mesozoic weathering crust reservoirs with complex lithologies in the Jiyang Depression, Bohai Bay Basin
Shifa Zhu, Ye Jia, Lichi Ma, Dian Cui, Anyu Jing, Huan Tong
2022, 43(3):  514-527.  doi:10.11743/ogg20220303
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The Mesozoic reservoir in the Jiyang Depression, Bohai Bay Basin, is taken to solve the following problems faced by petroleum exploration, including lithology, diagenetic types, reservoir space, quality-controlling factors of weathering crust reservoirs, through core and thin section observation, as well as other means of lab analysis. Eventually, a multivariate reserve-controlling model is established to provide geological evidence for weathering crust reservoir exploration and development in the Jiyang Depression together with other continental basins in China. The results show that there are 12 types of rock in 5 categories in the Mesozoic weathering crust reservoir in the Jiyang Depression, including terrigenous clastic rock, pyroclastic rock, volcanic lava, subvolcanic rock and plutonic intrusive rock, among which the clastic rock reservoir is of the best with higher initial porosity. The dissolution of meteoric fresh water and organic acids serves to significantly improve the clastic reservoir quality. Gas spillage, condensation shrinkage, high-temperature and acid water dissolution are of constructive value to igneous reservoir quality. Clastic reservoirs are largely affected by compositional maturity, compaction strength, early calcite cementation and organic acid reformation during secondary burial. Secondary dissolution pores serve as the main reservoir space. The quality of weathering crust reservoir in the study area is comprehensively affected by many factors, such as lithology and lithofacies, diagenetic fluid, tectonic evolution, and present burial depth, etc.; therefore a five-element reservoir-controlling model is proposed. The development model of the Mesozoic reservoir sweet spots is established, by which the favorable reservoirs can be grouped into five types, that is, fault and unconformity window dissolution zone, residual leaching zone after cementation, residual leaching zone following denudation, mixed dissolution zone and favorable lithofacies juxtaposition zone.

Study and practice of characterizing hydrocarbon charging capacity of different fault zones, Shunbei area, Tarim Basin
Wenge Hu
2022, 43(3):  528-541.  doi:10.11743/ogg20220304
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The Middle-Lower Ordovician carbonate reservoirs along strike-slip fault zones in the Shunbei area, Tarim Basin, are hydrocarbon bearing and have been the main targets of exploration and development for oil and gas. However, hydrocarbon charging efficiency at difference parts of the zones varies greatly, posing challenge for accurately evaluating and describing the dominant parts of charging, which in turn determines the efficiency and scale of exploration and development in the Shunbei area. To tackle the problem, this study is focused on the relationship between the faults and reservoirs in two of the fault zones (No.1 and 5) to discuss for the first time the main factors affecting the hydrocarbon charging efficiency in the zones that are reflected through the internal structure of the zones, the local strain intensity as well as the size and direction of the present in-situ stress. Based on the study, the concept of “charging body” is proposed and implemented to semi-quantitatively determine the vertical transport of hydrocarbons at different parts of the zones with results fitting well with the actual production data. This method not only provides strong theoretical support for segmental assessment of the zones and description of “high-speed channels” for vertical hydrocarbon migration, but also serves as a guidance to rapidly increasing reserves and promoting production with less wells in the Shunbei area.

Characteristics and distribution pattern of late hydrocarbon accumulation in northern Xinjiang, China
Dongming Zhi, Jian Cao, Jingkun Zhang, Menglin Zheng, Zhijun Qin
2022, 43(3):  542-552.  doi:10.11743/ogg20220305
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The late hydrocarbon accumulation serves as an important feature for the formation of large-scale oil and gas field. To improve the understanding of the research front in petroleum geology, we investigate the characteristics and general patterns of the late hydrocarbon accumulation in northern Xinjiang from the perspective of far-field response during the Cenozoic Qinghai-Tibet Plateau uplifting, while integrating current understanding and latest geological and geochemical data. The results indicate that the Qinghai-Tibet Plateau uplifting is likely to have a significant influence on the late hydrocarbon accumulation from far away in northern Xinjiang in terms of hydrocarbon generation evolution, primary-secondary reservoir adjustment, and oil reservoir destruction. As for the evolution of hydrocarbon generation, the southern margin of the Junggar Basin came into being with Tian Shan mountain range getting rapidly uplifted and strongly denuded, accompanied by thick piedmont deposits, which serves for necessary geothermal gradient for the hydrocarbon generation evolution of multiple underlying source rock sequences. A large amount of hydrocarbon got generated thereby. As for primary-secondary reservoir adjustment, the central Junggar Basin has undergone south-to-north structural uplifting during the Himalayan movement period; as a result, shallow faults are developed, multi-source oil and gas mixing and secondary adjustment occur frequently, and a large-scale complex petroleum system with primary and secondary reservoirs superimposed is formed stretching hundreds of kilometers from south to north. As for structural destruction of oil and gas reservoirs, the Junggar Basin margin has experienced strong structural deformation caused by far-field response of the Qinghai-Tibet Plateau uplifting, resulting in piedmont fold and fault development and widely exposure of structural oil and gas seeps, the “living fossils” for hydrocarbon accumulation. In all, it is suggested that the basins in northern Xinjiang including the Junggar and Turpan?Hami Basins, should be considered in a unified total petroleum system of super basin in the future, where a holistic view on the petroleum exploration is supposed to be promoted.

Theory on genesis of coaliferous petroleum in the China Sea
Gongcheng Zhang, Ying Chen, Zengxue Li, Youchuan Li, Lei Lan, Shixiang Liu, Rui Sun
2022, 43(3):  553-565.  doi:10.11743/ogg20220306
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The study systematically analyzes coaliferous petroleum in a giant coal-bearing basin belt consisting of 14 coal-bearing basins in the China Sea and its surroundings. The evolution of the giant coal-bearing basin belt spanned five periods of coal formation, that is the Paleocene, the Eocene, the Early Oligocene, the Late Oligocene and the Miocene-Pliocene, in coal-forming environments of deltas, fan deltas and tidal flat-lagoons. The coaliferous source rocks dominate in the giant basin belt, featuring a “dual structure” of coaliferous and marine source rocks development. Experimental analysis of source rock, gas condensate, and natural gas samples indicates that the coal-bearing basin belt is characterized by a “four-stage” hydrocarbon generation model, and the coal-generating limit of the coaliferous source rocks (reflectance of vitrinite Ro) can reach up to 4.38 %, greatly expanding the lower limit of natural gas generation. The coaliferous source rocks in the basin belt feature two modes of hydrocarbon accumulation, that is, accumulation of hydrocarbons from the coal measures on the margin of the sag and that from the terrigenous marine source rocks within the sag, under the guidance of which multiple large-scale coaliferous hydrocarbon enrichment zones are developed.

Characteristics and reservoiring patterns of “teeth-brush-shaped” oil pools in the Shulu Sag, Bohai Bay Basin
Chuanbing Lyu, Xiongqi Pang, Kuiyou Ma, Hong Pang, Xungang Huo, Liangliang Fu, Xingang Zhang, Xingru Liang, Song Wu
2022, 43(3):  566-581.  doi:10.11743/ogg20220307
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The Shulu Sag in Jizhong Depression possesses all the necessary factors for hydrocarbon to accumulate. However, many vertical wells drilled there failed to yield any industrial oil/gas flow. This paper takes the “teeth-brush-shaped” reservoirs in the Shahejie Formation, Checheng oilfield, Shulu Sag as study subject to crack the enigma. The characteristics, barrier bed features, cap rock features, oil and gas sources and migration paths for the reservoirs are investigated to analyze the conditions and process of hydrocarbon accumulation and summarize reservoiring patterns. The results shows that the “teeth-brush-shaped” oil pools occur with a high net-to-gross ratio, and adjacent oil pools are segmented into different pressure systems by cap rocks, while single oil pool is compartmentalized into a series of reservoir units of the same pressure system by relatively tight sandstone barriers. Faults were the major vertical and lateral migration pathways for hydrocarbon. Two stages of hydrocarbon charging occurred in the area with the first stage being the main contributor of hydrocarbons to the reservoirs. However, some of the hydrocarbons were dissipated while moving upward to the shallower layers along faults, resulting in a gradually thinning reservoirs with decreasing depth. With the migration direction roughly perpendicular to the distribution direction of reservoirs with dominant net-to-gross ratio, hydrocarbons tend to accumulate in fault traps along migration pathways, forming a series of “teeth-brush-shaped” oil pools. The genetic mechanism of the oil pools revealed by the study can be used as a reliable theoretical basis for subsequent exploration and development in the study area.

“Three-factor” driven microbial carbonate reservoirs and their distribution
Anjiang Shen, Anping Hu, Jie Zhang, Xiaofang Wang, Hui Wang
2022, 43(3):  582-596.  doi:10.11743/ogg20220308
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Microbial carbonates are very important hydrocarbon reservoirs. Stromatolite and thrombolite carbonates are commonly suggested to be better hydrocarbon reservoirs than other microbial and no-microbial carbonates. To check the truth of the suggestion, this study characterizes some modern microbial deposits and salt lake deposits, and performs simulated experiments of early degradation and buried pyrolysis of microbial organic matter, and genesis of dolomites by early precipitation and replacement. The results confirm the suggestion and reveal the uniqueness of microbial carbonate reservoirs with their special sediments and depositional setting. Three factors that control the formation of the reservoir are also proposed. (1) Compared with other microbial and no-microbial carbonates, stromatolite and thrombolite carbonates contain more original pores and microbial organic matter and therefore have a better material basis for becoming hydrocarbon reservoirs. (2) The production of organic acid by early degradation and later pyrolysis of microbial organic matter helps preserving and enhancing the original porosity, serving as a key factor for the development of stromatolite and thrombolite carbonate reservoirs. (3) Early dolomitization favors the preservation of pre-burial porosity and two types of low-temperature dolomitization (early deposition and replacement) tend to occur in carbonate-evaporite sedimentary sequences, which result in the development of stromatolite and thrombolite reservoirs in these sequences. The understanding is of great significance to the prediction of distribution of microbial carbonate reservoirs, based on which stromatolite and thrombolite zones are suggested to be promising hydrocarbon reservoirs in carbonate-evaporite sedimentary sequences.

Physical property and heterogeneity of tight sandstone reservoirs: A case of the Upper Triassic 6th member of Xujiahe Formation, Guang’an, central Sichuan Basin
Liang Yue, Qingqiang Meng, Ziliang Liu, Wei Yang, Hui Jin, Fang Shen, Junjian Zhang, Sibing Liu
2022, 43(3):  597-609.  doi:10.11743/ogg20220309
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Unconventional tight sandstone reservoirs with proven natural gas reserves up to trillion cubic meters are typical of the Upper Triassic Xujiahe Formation of clastic rocks in the Sichuan Basin of southwestern China. The 6th member of Xujiahe Formation (Xu 6 Member) in Guang’an area, central Sichuan Basin, contains gas reservoirs with great exploration and development potential. In this study, thin section observation, physical property measurements, mercury intrusion tests and fractal theory were integrated to analyze a suite of the Xu 6 Member tight gas sandstone samples in terms of pore structure, physical property and reservoir heterogeneity. The results show that the sandstone reservoirs studied can be classified into three types. That is, TypeⅠreservoir (with an average porosity of 12.27 % and average permeability ratio of 6.037 6 × 10-3 μm2) is dominated by macro- or meso-scale pores, and its fractal dimension varies between 2.42 and 2.59. TypeⅡreservoir (with an average porosity of 9.26 % and average permeability ratio of 1.152 3 × 10-3 μm2) is dominated by meso-scale pores, followed by micro-scale pores, with macro-scale pores poorly developed; and its fractal dimension ranges from 2.47 to 2.56. TypeⅢreservoir (with an average porosity of 5.20 % and average permeability ratio of 0.351 7 × 10-3 μm2) is dominated by micro- or meso-scale pores, together with poorly developed or undeveloped macro-scale pores; and its fractal dimension varies between 2.45 and 2.81. The different distribution of pore types leads to obvious changes in the heterogeneity of various types of reservoirs, which mainly shows that the heterogeneity of TypeⅢreservoir is stronger than that of TypeⅠreservoir. Differential distribution of pore types is directly related to reservoir heterogeneity, as manifested by stronger heterogeneity of TypeⅢreservoir compared with TypeⅡ. Correlation analysis reveals that differential pore types are coupled with reservoir heterogeneity, and there is a critical value. When the fractal dimension ranges between 2.45 and 2.60, the porosity and fractal dimension are in positive correlation, and the variation of permeability is irregular; when the parameter is greater than 2.60, there is a negative correlation between porosity and fractal dimension, and permeability is in linear relationship with fractal dimension with a slope close to 0. In all, the quantitative study on physical properties and fractal characteristics of tight sandstone reservoirs, is of great theoretical and practical significance to discussing the evaluation criteria of unconventional high-quality natural gas reservoirs, and guiding the exploration and development of unconventional reservoirs in China.

Reservoir exploration of the Permian Maokou Formation in the Sichuan Basin and enlightenment obtained
Yu Zhang, Qinggu Cao, Kaiping Luo, Longlong Li, Jinlian Liu
2022, 43(3):  610-620.  doi:10.11743/ogg20220310
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The Permian Maokou Formation among the first batch of exploration targets is one of the most actively explored formations in the Sichuan Basin. Gas reservoirs in the formation can be classified into five types in terms of reservoir characteristics, that is, reservoirs of fracture type, karst fractured-vuggy type, hydrothermal dolomite type, shoal dolomite type and marl type, and are dominated by lithologic reservoirs. These types of gas reservoirs show diverse characters in different stratigraphic intervals and different plays. The previous exploration results of Maokou Formation and characteristics of different gas reservoirs, demonstrate that favorable sedimentary facies zones such as marginal-platform shoals, intra-platform shoals and deep gentle slopes serve as basic conditions for reservoir development, and pseudocontemporaneous dolomitization, late hydrothermal dolomitization, epigenetic karstification, and tectonic transformations such as folds and faults are the keys to forming high-quality reservoirs therein. It is pointed out that karst fractured-vuggy reservoirs, dolomite reservoirs of shoal facies and marl reservoirs in the first member of Maokou Formation (Mao 1 Member) as controlled by facies (geomorphology), characterized by continuous distribution and high quality, are the main exploration targets in the future. The northeastern and southern-southeastern parts of Sichuan Basin are the most favorable play fairways for the exploration of Maokou Formation. The superposition of various reservoir types and exploration fields is of value to setting up a holistic pattern for exploration.

Hydrocarbon generation and expulsion characteristics and resource potential of source rocks in the Longtan Formation of Upper Permian, Sichuan Basin
Xingang Zhang, Hong Pang, Xiongqi Pang, Junqing Cheng, Song Wu, Kuiyou Ma, Siyu Zhang
2022, 43(3):  621-632.  doi:10.11743/ogg20220311
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The Upper Permian Longtan Formation provides most source rocks for many large- and medium-sized gas fields in the Sichuan Basin. Due to their advanced thermal evolution, the source rocks are beyond ordinary thermal simulation experiments or chemical kinetics methods that are commonly resorted to while defining the hydrocarbon generation and expulsion characteristics of less thermally evolved source rocks. This hinders the exploration and development of oil and gas generated from the formation. To deal with the problem, this study proposes a model suitable for analyzing the characteristics of hydrocarbon generation and expulsion, evaluating hydrocarbon potential and predicting most promising hydrocarbon accumulation for these source rocks by using the hydrocarbon potential method based on pyrolysis, TOC content analysis and maturity data of source rocks from major petroliferous basins in China. The model shows that the formation has great hydrocarbon generation potential. TypesⅡand Ⅲ organic matter in the formation initiate hydrocarbon generation process when the vitrinite reflectance (Ro) reaches 0.5 %, and start to expel hydrocarbon when Ro reaches 0.8 % and 0.9 %, respectively. The original hydrocarbon generation potential indexes for the two types are 570 mg/g and 200 mg/g respectively. The formation has up to the present the capacity of generating 746 billion tons of hydrocarbon, of which 384 billion tons of hydrocarbon are expelled and 362 billion tons of hydrocarbon are left behind. Shale gas resource volume is 24.6 TCM in the formation, of which 3.7 TCM of gas is recoverable. This predication indicates that exploration shall be focused on finding conventional gas in the north part of the basin, on both conventional and unconventional gases in central and northeast parts of the basin, and on shale gas in the east and south parts of the basin.

Prediction of shale reservoir sweet spots of the Upper Ordovician Wufeng-Longmaxi Formations in Lintanchang area, southeastern margin of Sichuan Basin
Xun Ge, Tonglou Guo, Yongsheng Ma, Guoli Wang, Maowen Li, Xiaoqun Yu, Peirong Zhao, Zhentao Wen, Peng Wang
2022, 43(3):  633-647.  doi:10.11743/ogg20220312
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Well LY 1, an exploration well drilled by Sinopec Southwest Oil & Gas Company in the southeastern margin of Sichuan Basin in 2017, has proven that the Lintanchang structure was in a zone of deep water shelf facies during the Late Ordovician to Early Silurian, favorable for shale gas generation. In order to further explore the shale gas potential in this area and realize commercial breakthrough in shale gas development in the Wufeng-Longmaxi Formations, the study analyzes the structure and sedimentary features of Lintanchang area. Based on an array of geological data for tests, we select the TOC content, effective porosity and gas content of high-quality shale as the key parameters for evaluating the geological sweet spots, brittleness index and horizontal principal stress difference for determining engineering sweet spots, and dip angle, fracture influence and formation pressure for assessing preservation conditions. Seismic elastic parameters (including transverse to longitudinal velocity ratio, density and Poisson’s ratio) are obtained by pre-stack inversion. The quantitative relationship between reservoir evaluation parameters and seismic elastic parameters is established by core sample tests, as well as petrophysical analysis and lithologic characters of reservoir rocks. In addition, the AVO characteristics of lithologic information in CRP gathers are applied to predict planar changes of the shale reservoir parameters related, and eventually the favorable shale gas target in the Lintanchang structure is determined. The accurate planar mapping of shale reservoir sweet spots in structurally complex areas in the southeastern margin of the Sichuan Basin by a combination of geological-logging-geophysical methods is of guiding significance to the integrated exploration and development of shale gas therein.

Sedimentary environment of lacustrine organic matter in the central Ordos Basin
Bojiang Fan, Liang Shi, Jie Yang, Shan Su, Rong Ma, Yaoli Yuan, Cengyu Zeng
2022, 43(3):  648-657.  doi:10.11743/ogg20220313
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The numbers of measurable elements in lacutrine shale sampels by energy dispersive X-ray fluorescence (ED-XRF) technology are often limited due to insufficient energy configuration. This makes it difficult to fully restore the paleosedimentary environment for samples concerned. Furthermore, some lacutrine organic matter can be of river input origin or indigenously produced in lake water, quantitatively distinguishing one from the another is challenging. In this study, energy configuration of ED-XRF is improved to increase the number of accurately measurable elements to 28. Shale samples from the seventh member of Yanchang Formation (Chang 7) in central Ordos Basin are tested for geochemical characterization of organic carbon, major elements, trace elements and carbon isotopes. The results are used to restore the paleosedimentary environment of organic matter from Chang 7, including the water column, redox and productivity. A mathematical model is established to analyze the organic matter. The results show that the organic matter of Chang 7 shale is formed in fresh water with oxygen deficiency or anoxia, high paleoproductivity but a moderate primary productivity against a warm and humid climate background. For shale in the central Ordos Basin, organic matter is rather lake water produced than terrestrial input.

Dissolved micro-pores in alkali feldspar and their contribution to improved properties of tight sandstone reservoirs: A case study from Triassic Chang-63 sub-member, Huaqing area, Ordos Basin
Guangyuan Sun, Zhelin Wang, Peigang Liu, Zhiqiang Zhang
2022, 43(3):  658-669.  doi:10.11743/ogg20220314
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Secondary dissolved pores in feldspar account for a major part of the micro-nano pore-throat system in tight sandstone reservoirs. To further clarify the characteristics of these pores and their contribution to improved physical properties of tight sandstone reservoirs, this paper takes the third sub-member of the sixth member of Yanchang Formation (Chang-63) in the Huaqing area, Ordos Basin as the research object. The energy-dispersive spectrometry is used to obtain the distribution and composition characteristics of sodium and potassium of all kinds of feldspar in the member and field emission environmental scanning electron microscope images of feldspar samples are processed binarily to extract parameters with Image J software. Two kinds of feldspar, wavy perthite and mottled perthite, with typical dissolution characteristics are imaged and analyzed to establish a method for evaluating the contribution of the dissolved pores to reservoir space based on dissolution intensity parameters and the impact of feldspar dissolution on pore development. Parameters for characterizing feldspar development, dissolution rate, porosity contribution are determined. The results show that the dissolved micropores in alkali feldspar in the study area are mostly in banded and honeycombed morphology as a result of weak dissolution. The porosity contribution rate is between 2.06 % and 35.20 %, with an average of 13.99 %. Moreover, the proportion of pore area formed by dissolution is negatively correlated with the potassium ion enrichment area size in alkali feldspar.

Sequence stratigraphy and sedimentary filling characteristics of a half-graben rift lake basin during the initial rifting period: A case study of the 2nd member of Lower Cretaceous Huoshiling Formation, Linan Sag, Songliao Basin
Zhongcheng Li, Zhidong Bao, Zhaodheng Wei, Guoyi Zhang, Yanqqing Shi, Mingyi Hu, Qingjie Deng
2022, 43(3):  670-681.  doi:10.11743/ogg20220315
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Due to the Jurassic-Cretaceous regional tectonic extension, a series of half-graben rift lake basins were formed in the Songliao Basin. The latest exploration and development shows good source-reservoir-cap rock combinations and hydrocarbon potential in these basins. The second member of Huoshiling Formation in Linan trough of southern Songliao Basin is chosen as the research object to reveal the sequence stratigraphy, sedimentary system and basin filling evolution characteristics in these basins based on the integration of core, logging and 3D seismic data. The member can be divided into three medium-term base level cycles of fan delta and lake facies. The scales of fan deltas in steep slope zone are different from that in gentle slope zone during the early rifting stage of the half graben rift basins. With the expansion of the basin, the sedimentary facies belts in different structural belts take on obvious differentiation characteristics. The steep slope belt is often dominated by long and narrow coarse-grained isolated thick fan delta deposits, while the gentle slope belt is dominated by large superimposed fine-grained thin fan delta deposits. Based on these understanding, this study discusses the main controlling factors of sedimentary difference between steep slope zone and gentle slope zone of the member and brings forward the sedimentary model for the early stage of half-graben rift lake basins.

Characteristics and geochemical indication of over-mature source rocks in the Paleozoic, Yingen-Ejinaqi Basin
Zhijun Chen, Chunming Zhang, Yonghong He, Zhigang Wen, Fangxia Ma, Wei Li, Yiwen Gao, Yiguo Chen, Huiyuan Zhang, Dongtao Wei
2022, 43(3):  682-695.  doi:10.11743/ogg20220316
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The Paleozoic source rocks encountered by some wells and present on outcrops in Yingen-Ejinaqi Basin are believed to carry with them important geochemical information that will help understand the oil and gas accumulation conditions in the Paleozoic system of the basin. Geochemical characterization and hydrocarbon generation potential assessment of 25 such rock samples indicate over-mature (Ro=4.20 %) source rocks with low total organic carbon (TOC) content (1.31 %) and mixed kerogen types (typesⅡ1-Ⅱ2). They are mainly products of thermal degradation with biomarkers showing a bimodal N-alkanes, abnormally high relative abundance of pregnane (including L pregnane) in steroids, high relative abundance of tricyclic terpene in terpenoids, a predominance of low-carbon ring number compounds in aromatic compounds and other characteristics. A concept of “organic carbon composition” is presented and a method for calculating its value based on rock pyrolysis data is established. The results show that although the pyrolytically effective carbon only accounts for a fraction of organic carbon composition of the over-mature source rock, the original average TOC content of source rock is high at 2.07 %. In addition, the duration of hydrocarbon generation is long and its yield is high. Therefore, the Paleozoic over-mature source rocks in Yingen-Ejinaqi Basin are suggested to have a high original hydrocarbon generation potential.

Methods and Technologies
Simulation of dynamic fracture network in fractured horizontal well for unconventional reservoirs: Theory and application
Chuanxi Liu, Wenchao Fang, Xuejie Qin
2022, 43(3):  696-702.  doi:10.11743/ogg20220317
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When a fractured horizontal well in unconventional reservoirs is brought online, change in reservoir pressure may lead to fracture deformation, resulting in the formation of dynamic fracture network that greatly affects well productivity. To simulate the effect of dynamic fracture network on the development of unconventional reservoirs and improve the designation level of development strategy, the study builds a mathematical model to characterize the time variant and the heterogeneous distribution of fracture width, which can simultaneously simulate the changes of fracture width and fracture length in a reservoir. Then, the conductivity modification technique is adopted to couple the fracture deformation model with discrete fracture simulation model. A reservoir numerical simulator that can model dynamic fracture network is then developed. Finally, the simulator is applied to the simulation of a typical reservoir model and an actual fractured horizontal well model, respectively. Dynamic fracture network models for different production periods are thereby obtained, and the effect of dynamic fracture network on the well productivity is clarified. In all, the simulation results indicate that the simulated performance is in better agreement with practical well performance when the fracture deformation is considered in the simulator.

Characteristics and influencing factors of adhesive force between hydrate particles and mineral surface
Ying Teng, Pengfei Wang, Zachary Aman
2022, 43(3):  703-710.  doi:10.11743/ogg20220318
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In the process of hydrate mining, the droplets and bubbles in pipeline can be turned into hydrate particles under the environmental conditions. The occurrence of hydrate particle aggregation can cause blockage, leading to potential safety problems. Adhesive force is the key to exploring the flow safety of mining pipeline and the accumulation/deposition mechanism of hydrate particles. At present, the relevant researches focus on the measurement and characterization of the adhesive force between hydrate particles or between particles and surface in transport process. However, documents on the effects of mineral wettability and surface roughness on the adhesion of hydrate deposits are rarely seen. In this study, the adhesive force between hydrate particle and carbon steel surface treated by different methods as well as mineral surface, such as quartz, malachite, calcite and kaolinite were measured and analyzed using the adhesive force measurement experimental system. The results show that the adhesive force of hydrate particles to mineral surface is 3-6 times that of hydrate particles to carbon steel surface, due to the wettability difference. In addition, the adhesive force between hydrate particles and mineral surfaces is also affected by contact time, as shown by the strengthened adhesive force of hydrate particles with calcite and kaolinite surfaces under prolonged contact. The effects of ionic surfactants on hydrate growth rate and adhesion were also investigated, and it was pointed out that the change of contact area between particles and water film on mineral surface serves to change the morphology of hydrate particles from layered structure to needle-like structure. Finally, in comparison of the experimental results with theoretical calculation, we concluded that the strong hydrophilicity of the mineral surface is key to the differences.

A new method for porosity prediction based on variable matrix parameters
Bo Shen, Gang Wang, Haitang Fan, Jinfeng Zhang, Yanpu Li
2022, 43(3):  711-716.  doi:10.11743/ogg20220319
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Porosity prediction from logging data is a common practice of quantitative evaluation of hydrocarbon reservoirs. The accuracy of reservoir porosity calculation directly affects the reliability of subsequent reservoir evaluation. For evaluating reservoirs with complex mineral components, the main challenge is how to quickly and accurately predict the porosity with logging data. One such example is the tuffaceous sandstone reservoir developed in L Formation of A Sag. Its complex and changeable sandy composition and late diagenesis result in logging responses varying significantly from one layer to another, making the applicability of the single-porosity model built based core-calibrated logging data and multivariate statistical method to porosity calculation of these layers rather questionable. To tackle the issue, this study proposes a porosity prediction method based on variable matrix parameters by using a volume physical model and through principal component analysis. Porosity prediction by using the method with actual logging data fits well with core analysis results and the workflow for separate-layer porosity interpretation is simplified. The method may serve as a certain reference for the prediction of porosity of rocks with complex components.

Progress and research direction of EOR technology in eastern mature oilfields of Sinopec
Li Zhang
2022, 43(3):  717-723.  doi:10.11743/ogg20220320
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In view of the characteristics and challenges in developing the eastern mature oilfields of Sinopec, the study focuses on major progress made in research of enhancing oil recovery (EOR) technologies such as water flooding, chemical flooding, thermal recovery of heavy oil and CO2 flooding, and introduces the application effects. For water flooding, focus is put on improvement of local injection-recovery relation, being combined with separate intelligent separate-layer injection and production technology, and pressurized water injection experiment is carried out in ultra-low permeability reservoirs. For chemical flooding, the binary surfactant/polymer flooding and the heterogeneous phase combination flooding (HPCF) have been developed and applied, with calcium/magnesium-tolerant agents developed for oil reservoirs with high temperature and high salinity. In terms of heavy oil reservoirs, compound viscosity-reducing flooding has been implemented in reservoirs with low displacement efficiency, H(horizontal well)+D(viscosity reducer)+C(CO2)+S(steam) technology in deep ultra-heavy oil reservoirs, H (horizontal well)+D(viscosity reducer)+N(N2)+S(steam) technology in shallow ultra-heavy oil reservoirs, chemical steam flooding in deep heavy oil reservoirs. As for CO2 flooding, long-effect soaking and large slug injection technology has been put into effect in high-permeability reservoirs with high water cut; high-pressure and low-rate injection and alternate injection of water and gas from different wells in low permeability/ultra-low permeability reservoirs; and asynchronous cyclic injection and production in tight reservoirs. Analysis of the progress mentioned above serves for indicating the development direction of EOR promotion in different types of reservoirs.

Comparison of oil and gas resources/reserves classification systems and characteristics of the latest classification system of China
Hongjin Hu, Wenli Jiang, Denghua Li, Kai Zhao, Xuan Gao, Jun Jia, Xin Zan
2022, 43(3):  724-732.  doi:10.11743/ogg20220321
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A full understanding of China’s latest oil and gas reserves classification system and its differences with foreign classification systems serves to promote the reform of China’s resources/reserves management and provides the premise for international cooperation and exchange. The study focuses on introducing the latest framework of China’s oil and gas resource/reserve classification system in 2020 and the key points of revision, and expounding the characteristics of China’s system by comparing with the representative foreign classification systems, following the comprehensive illustration of China’s classification system evolution. China’s classification system has undergone five revisions, with the latest one simplifying the division of exploration and development stages, types of oil and gas reserves and economic significance. The representative classification systems in the world can be grouped into three levels: government, oil companies and international organizations. China and foreign countries have similarities in the overall structural framework and basic units, but they are different in the classification of resources and reserves and the definition of terms. The Chinese classification system taking oil and gas reservoirs and traps as basic units of gradation, and geological reserves as the basis of classification, has been designed along the line of “discovery, geological understanding, recoverability, and economic value. This meets the needs of Chinese government in management and development planning, as well as the complex geological conditions in reality. In view of the implementation of the new system, it is suggested that the data of relevant resources and reserves should be conversed and the evaluation system should be adjusted as soon as possible, and a cooperation and exchange mode that takes into account both domestic and international needs should be explored energetically.

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