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01 December 2022, Volume 43 Issue 6 Previous Issue    Next Issue

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OIL AND GAS EXPLORATION

Shale Oil Enrichment Mechanism and Sweet Spot Selection of Fengcheng Formation in Mahu Sag，Junggar Basin JIN Zhijun, LIANG Xinping, WANG Xiaojun, ZHU Rukai, ZHANG Yuanyuan, LIU Guoping, GAO Jiahong 2022, 43 (6):  631-639.  doi: 10.7657/XJPG20220601 Abstract ( 355 )   HTML ( 27 )   PDF (5508KB) ( 197 )   Save The Fengcheng formation in the Mahu sag is an alkaline lake sediment，and is divided into Feng 1 member，Feng 2 member and Feng 3 member from bottom to top. In the Fengcheng formation，the lithology vertically changes rapidly，the mineral composition is complex，and the organic-rich shale source is integrated with the shale reservoir. The formation bears oil universally，but the sweet spots are scattered. The results of formation testing for single layers are not satisfied，showing an unclear production potential. According to core slices and geochemical analyses，the Fengcheng formation in the Mahu sag is dominated by lamellar silty shale intercalated with dolomite，which are mainly composed of terrigenous clastic minerals and carbonate minerals. With the variation of burial depth，the pore volume changes consistently with the variation of surface area of pores，and the pore volume is mainly contributed by macropores (pore diameter > 50 nm). The source rock is dominated by Type Ⅱ organic matter，and the vitrinite reflectance ranges from 0.85% to 1.40%，indicating a peak oil generation period. There are many shear fractures with middle to high angles in the Feng 2 member，and shear fractures with middle to high angles and structural fractures with low angles in the Feng 3 member，whose formation and development degree are controlled by lithology，mineral composition，rock mechanical properties，etc. Based on the characteristics of lithologic assemblage，reservoir property and oil-bearing property，four relatively concentrated sweet spots have been identified. When performing multi-interval formation testing and production testing in vertical wells，it is necessary to select sweet spots with good oil content and more fractures to conduct geological research and geology-engineering integration technology research，and to perform production improvement tests in horizontal wells，so as to realize comprehensive breakthrough for shale oil exploration and development in the Fengcheng formation in the study area Figures and Tables | References | Related Articles | Metrics

Lithofacies Paleogeography and Petroleum Exploration of Fengcheng Formation in Western Central Depression of Junggar Basin HE Haiqing, TANG Yong, ZOU Zhiwen, GUO Huajun, XU Yang, LI Yazhe 2022, 43 (6):  640-653.  doi: 10.7657/XJPG20220602 Abstract ( 296 )   HTML ( 16 )   PDF (18669KB) ( 153 )   Save In order to evaluate the petroleum exploration prospects of the Fengcheng formation in the western Central depression of the Junggar basin, the lithology assemblage and sedimentary facies of the Fengcheng formation were studied by means of analyzing the trace element, core, rock thin section, paleomorphology, logging facies and seismic facies of the formation. The results show that the Fengcheng formation in the study area is fan delta-alkaline lake deposition which formed in an arid-semi-arid climate, foreland tectonic setting and geological environment with periodic changes in water salinity and water depth. From the edge to the center of the Central depression, basin-marginal fan-delta clastic rocks, outer-slope front dolomites, slope-highland volcanic rocks, low-uplift dolomitic calcareous beach-bar peperites, and the central lake-basin alkali rocks are found in sequence. According to the sedimentary characteristics, an evolution model for the fan delta-alkali lake deposition was established for the Fengcheng formation. Based on the sedimentary facies, lithofacies and petroleum exploration status, the Fengcheng formation in the study area can be divided into 5 areas such as conventional hydrocarbon area in basin-margin ultra-denudation belt, tight hydrocarbon area in outer-slope front, shale oil/gas area in lake basin, conventional hydrocarbon area in low-uplift dolomitic calcareous beach-bar and conventional hydrocarbon area in highland volcanic rocks. The favorable exploration area with the depth less than 7 000 m reaches 1.2×104 km2, with hydrocarbon reserves of more than one billion tons, which demonstrates huge exploration potential of the total petroleum system of the Fengcheng formation in the study area Figures and Tables | References | Related Articles | Metrics

Total Petroleum System and Inner-Source Natural Gas Exploration in Permian Strata of Junggar Basin TANG Yong, LEI Dewen, CAO Jian, LIU Yin, HUANG Liliang, LI Hui 2022, 43 (6):  654-662.  doi: 10.7657/XJPG20220603 Abstract ( 263 )   HTML ( 8 )   PDF (15608KB) ( 134 )   Save In order to further expand the petroleum exploration and enrich the basic theory and understanding on petroleum geology in Junggar basin，based on the exploration practices in recent years，the characteristics of present petroleum exploration and its trend are summarized，and the total petroleum system in Permian strata and the conditions for large-scale accumulation of unconventional natural gas in inner-source deep strata are described. The results show that the middle and lower reservoir assemblages have become the major exploration targets，deep strata have the conditions for oil and gas accumulation，the source rock controls the distribution of oil and gas，and a distribution pattern with orderly coexistence of conventional and unconventional reservoirs is established. The continuous hydrocarbon supply from high-quality Permian source rocks is the premise for the formation of the total petroleum system，and two sets of oil and gas accumulation systems are found inside source and outside source. Various inner-source reservoirs are developed in the Permian strata，which have great potential for natural gas exploration，especially unconventional natural gas. The understanding not only broadens the petroleum exploration area in Junggar basin，but provides theoretical and practical support for construction of a large oil and gas field. Moreover，it is of great significance for further improving the theory of total petroleum system. Deep inner-source unconventional oil and gas are becoming an important frontier for future exploration in hydrocarbon-rich sags. Figures and Tables | References | Related Articles | Metrics

Hydrocarbon Accumulation Mechanism of Total Petroleum System in Permian Fengcheng Formation，Mahu Sag HE Wenjun, SONG Yong, TANG Shiqi, YOU Xincai, BAI Yu, ZHAO Yi 2022, 43 (6):  663-673.  doi: 10.7657/XJPG20220604 Abstract ( 260 )   HTML ( 9 )   PDF (10902KB) ( 116 )   Save The exploration practice in the Fengcheng formation in the Mahu sag，western Junggar basin，has revealed the orderly coexistence of conventional and unconventional hydrocarbons. By finely dissecting the characteristics of the total petroleum system in the Fengcheng formation in the Mahu sag，and combining with the macro-and micro-analysis and production data of the reservoirs，the hydrocarbon accumulation mechanism of the total petroleum system was analyzed. The results show that conventional oil，tight oil and shale oil accumulate in an orderly pattern in the total petroleum system，which essentially complies with the hydrocarbon accumulation pattern of “source-reservoir coupling” and “dynamic sealing” of oil and gas. In the early diagenesis stage，the present tight reservoirs and shale reservoirs were conventional reservoirs with medium-large pore throats，where hydrocarbons accumulated because of buoyancy. In the middle-later diagenesis stage，the medium-large pore throats gradually evolved to micro-to nano-pore throats，the buoyancy was weakened，and the capillary force was strengthened. The hydrocarbon in the reservoir adjacent to or integrated with the source rocks underwent primary or micro migration continuously until the source-reservoir pressure difference resulted from hydrocarbon generation and expulsion and the capillary force originated from micro-to nano-pore throats reached a dynamic equilibrium，forming a “self-sealing” continuous unconventional oil and gas accumulation. The capillary force in the present conventional reservoirs is far less than the buoyancy，so “external sealing”is needed for hydrocarbon accumulation in traps. Generally，the hydrocarbon accumulation in the Fengcheng formation is represented by the space-time evolution of the pore throat structure in the reservoir and the dynamic coupling of source rock and reservoir on hydrocarbon generation-expulsion-migration-accumulation. Figures and Tables | References | Related Articles | Metrics

Gas Generation Potential of Fengcheng Formation Source Rocks and Exploration Fields in Junggar Basin GONG Deyu, LIU Hailei, YANG Haibo, LI Zonghao, WANG Ruiju, WU Weian 2022, 43 (6):  674-683.  doi: 10.7657/XJPG20220605 Abstract ( 234 )   HTML ( 7 )   PDF (14672KB) ( 108 )   Save In the Junggar basin，where the percent of proved gas reserves is low，it is urgent to find strategically successive areas. The Lower Permian Fengcheng formation is the most important source rocks in the basin，but its gas generation potential has rarely been investigated. Based on the latest drilling and seismic data，the gas generation potential of the Fengcheng formation was systematically analyzed by means of hydrocarbon generation kinetics，isotope kinetics，organic petrology and basin simulation. The results show that the Fengcheng formation source rocks are complex in lithology，including dark shale，as well as sedimentary tuff，tuff-bearing dolomitic mudstone，calcareous mudstone，silicified dolomitic shale and silico-boronic sodalite，and contain the organic matters of typeⅠ-Ⅱ1，indicating moderate-good source rocks. The methane yield of the Fengcheng formation source rocks is high with a maximum of 650 mL·g TOC. In the Shawan sag and West Pen-1 sag，the Fengcheng formation source rocks reached the peak of gas generation in the Early and Late Cretaceous respectively，and covered an area with the gas generation intensity higher than 20×108 m3/km2 of 13 000 km2，which suggests its resource potential of forming a medium-large gas field. A large amount of high-mature oil-type natural gas from the Fengcheng formation has been discovered in the periphery of the two sags，and favorable exploration targets are also found in near-source or inner-source traps，which are expected to become new fields for gas exploration in Junggar basin. Figures and Tables | References | Related Articles | Metrics

Changes and Significance of Biomarkers in Thermal Evolution of Fengcheng Formation Source Rocks in Northwestern Margin of Junggar Basin JIANG Wenlong, Ablimit·YIMING , BIAN Baoli, WANG Tao, REN Haijiao, HAN Yang 2022, 43 (6):  684-692.  doi: 10.7657/XJPG20220606 Abstract ( 232 )   HTML ( 6 )   PDF (1167KB) ( 142 )   Save A thermal simulation experiment in a closed system was conducted on the cores of argillaceous source rocks in the Permian Fengcheng formation of the Mahu sag in the northwestern margin of the Junggar basin. Taking the thermal simulation temperature as a scale，the evolution characteristics and applicable ranges of the biomarkers in different stages of maturity were analyzed in light of the parameters for identifying typical biomarkers in the Fengcheng formation source rocks，which includes Pr/Ph，Pr/nC17，absolute content of β-carotane，distribution patterns of C20，C21 and C23 tricyclic terpenes，and ratio of pregnane to regular sterane. On this basis，it is expected to more accurately identify the high-matured oil and gas from the Fengcheng formation in the northwestern margin of the Junggar basin by comparing with the crude oil in the West Pen-1 sag. The results show that，in addition to the distribution patterns of C20，C21 and C23 tricyclic terpenes，the typical oil-source correlation parameters of the Fengcheng formation such as Pr/Ph，Pr/nC17，and absolute content of β-carotane are greatly affected by maturity and cannot be used to identify the source of high-matured condensate oil and gas. By considering the maturity in the process of oil-source correlation and combined with the newly established carbon isotope chart of normal alkane monomers for the Fengcheng formation source rocks，the controversial high-mature condensate oil from Well Pen-5 in the West Pen-1 sag was analyzed. It is speculated that the condensate oil from Well Pen-5 mainly came from the Fengcheng formation source rocks. Figures and Tables | References | Related Articles | Metrics

Oil-Bearing Properties and Hydrocarbon Occurrence States of Fengcheng Formation Shale in Well Maye-1，Mahu Sag QIAN Menhui, WANG Xulong, LI Maowen, LI Zhiming, LENG Junying, SUN Zhongliang 2022, 43 (6):  693-703.  doi: 10.7657/XJPG20220607 Abstract ( 234 )   HTML ( 2 )   PDF (4169KB) ( 159 )   Save The oil-bearing properties and hydrocarbon occurrence states of shale are crucial to evaluating and selecting shale oil sweet spots. Through the analysis such as rock pyrolysis, multi-temperature-gradient pyrolysis and X-ray diffraction, the oil-bearing properties and hydrocarbon occurrence states of the shale in the Lower Permian Fengcheng formation in Well Maye-1, Mahu Sag, were investigated, and then the optimal sweet spot intervals for shale oil in the Fengcheng formation were defined. The results show that the Fengcheng formation shale in Mahu sag is mainly composed of three lithofacies associations. The quality of source rocks is the best in the Feng 2 member, partially moderate in the Feng 1 member, and poor in the Feng 3 member. The macerals of organic matter in the rocks are mainly vitrinite and inertinite, indicative of a mature stage, showing a good material basis for shale oil accumulation. Vertically, the Fengcheng formation shale can be divided into 6 sweet spot intervals with good oil-bearing properties. The lamellar felsic shale intervals at the top and in the middle of the Feng 2 member have the best oil-bearing property, and contain hydrocarbons mainly in free state, where free oil accounts for more than 80% of the total oil content and mainly occurs in intergranular pores and bedding fractures continuously, suggesting a good oil-bearing foundation and a good prospect of movable resource. The research results provide a theoretical basis and technical support for subsequent exploration and development of the shale oil in the Fengcheng formation Figures and Tables | References | Related Articles | Metrics

Characteristics and Controlling Factors of Tight Oil Reservoirs of Fengcheng Formation in Southern Slope of Mahu Sag SHAN Xiang, DOU Yang, YAN Qi, CHEN Xiguang, PENG Bo, YI Junfeng 2022, 43 (6):  704-713.  doi: 10.7657/XJPG20220608 Abstract ( 199 )   HTML ( 6 )   PDF (14443KB) ( 105 )   Save By means of thin section identification，X-ray diffraction analysis，physical property analysis，and high-pressure mercury intrusion testing，the depositional environment，lithofacies，pore type and pore structure of the tight oil reservoirs in the second member of the Permian Fengcheng formation in the southern slope of Mahu sag in Junggar basin were studied. The results show that the Fengcheng formation in the study area was deposited in a closed saline lake basin，and the reservoirs of the Fengcheng formation are composed of terrigenous clasts，pyroclastics and authigenic minerals in different proportions. According to sedimentary structure，mineral composition and particle size，the reservoirs of the Fengcheng formation are divided into four types of dominant lithofacies，that is，massive calcareous medium-fine sandstone facies，massive dolomitic fine sandstone facies，lamellar dolomitic siltstone facies，and lamellar dolomitic mudstone facies. The pore types of the reservoirs include residual intergranular pores，pyroclastic dissolved pores，mineral intercrystalline pores and fractures，which are different in pore diameters and throat radii. The type of lithofacies is the basis for the formation of high-quality reservoirs，the dissolution determines the formation of secondary pores，the felsic cements resulted from devitrification of volcanic ash aggravate reservoir densification，and developed fractures are necessary for high oil and gas production. The massive calcareous medium-fine sandstone facies and massive dolomitic fine sandstone facies show relatively weak compaction，relatively low contents of fine-grained volcanic ash，and relatively better physical properties and pore structures，indicating high-quality reservoirs. Figures and Tables | References | Related Articles | Metrics

Geological Characteristics and Sweet Spot Evaluation of Shale Oil Reservoir in Fengcheng Formation in Well Maye-1 ZOU Yang, QI Yanping, SONG Dong, CHEN Wenshun, WEI Panyun 2022, 43 (6):  714-723.  doi: 10.7657/XJPG20220609 Abstract ( 223 )   HTML ( 7 )   PDF (9201KB) ( 112 )   Save The Fengcheng formation in the Mahu sag is a set of high-quality alkaline lake source rocks dominated by lacustrine dark shale and dolomitic rocks, which contains a large quantity of shale oil resources and is a field with great potential for unconventional hydrocarbon exploration in the Junggar basin. It is necessary to clarify the geological characteristics and evaluate the sweet spots in the Fengcheng formation. Based on the continuous coring data from the Fengcheng formation in Well Maye-1, and by means of core observation, thin section identification, whole rock analysis, rock pyrolysis, kerogen microscopy, and brittleness evaluation, the sedimentary environment, dominant lithology, source rock quality, accumulation performance, natural fractures, brittleness and mobility of each submember of the Fengcheng formation were compared and analyzed. The results show that the fine-grained sedimentary rocks in the Fengcheng formation can be divided into twelve rock types according to mineral composition and bedding structure, and into five kinds of sedimentary assemblage according to sedimentary environment, that is, freshwater, salinity, evaporite, weak salinization and low salinity assemblage, which correspond to five sedimentary submembers of the Fengcheng formation. A set of mature source rocks with moderate abundance of organic matters dominated by type Ⅱ1 and type Ⅰ kerogens is found in the Feng 33 submember. Furthermore, the Feng 33 submember contains lamellar diamictite reservoirs with abundant fractures and dissolved pores, favorable physical and oil-bearing properties, high brittleness index, and strong oil and gas mobility. Thus, the Feng 33 submember has the favorable conditions of hydrocarbon generation and accumulation for shale oil and should be considered as the focus for subsequent shale oil exploration and development. Figures and Tables | References | Related Articles | Metrics

Genesis and Petroleum Significance of Silica-Rich Shale in Fengcheng Formation of Well Maye-1，Mahu Sag LEI Haiyan, QI Jing, ZHOU Ni, CHEN Jun, MENG Ying, ZHANG Xixin, CHEN Ruibing 2022, 43 (6):  724-732.  doi: 10.7657/XJPG20220610 Abstract ( 230 )   HTML ( 4 )   PDF (10888KB) ( 102 )   Save Silica-rich shale is developed in the Fengcheng formation of Well Maye-1 in the Mabei slope of the Mahu sag, Junggar basin, and it is characterized by enrichment of authigenic silica. The silica-rich shale interval is one of the intervals with shale oil widely distributed. Based on the data from core description, rock thin section, scanning electron microscope, paleontological analysis, and geochemical test, the Fengcheng formation silica-rich shale in the Mabei slope was investigated from the aspects of longitudinal distribution, authigenic silica genesis, algae types and hydrocarbon generation and silica-related pore structure by combining petrology and geochemistry in multiple scales. The results show that the development of the authigenic silica in the Fengcheng formation of Well Maye-1 is related to the high pH of the alkaline lake. Volcanic materials and terrigenous debris are likely to release dissolved silica under high pH conditions, which will form silica-rich shale after recrystallizing because of continuous evaporation or pH reduction. A large number of well-preserved spherical biological fossils developed in the flint strips and agglomerates of the silica-rich shale are mainly composed of dormant spores of paleosalina, which have a remarkable hydrocarbon generation capability. Fractures, intercrystalline pores, and intercrystalline dissolved pores are found in the silica-rich shale which has a high brittleness. The authigenic silica in the Fengcheng formation of Well Maye-1 played an important role in the process of hydrocarbon generation and reservoir formation. This recognition can provide references for the further exploration of shale oil in the Fengcheng formation in the Mabei slope. Figures and Tables | References | Related Articles | Metrics

Factors Influencing Shale Oil Production and Sweet Spot Evaluation of Fengcheng Formation，Mahu Sag LIU Caiguang, JI Ruixue, WANG Wei, ZHANG Rong 2022, 43 (6):  733-742.  doi: 10.7657/XJPG20220611 Abstract ( 217 )   HTML ( 8 )   PDF (2254KB) ( 175 )   Save The Fengcheng formation in the Mahu sag is of great potential in shale oil resources. However，its characteristics such as complex lithology，diverse mineral species，thin single layer，scattered sweet spots，and variable fracture occurrence make sweet spot selection challenging，which seriously affects the exploration process. Through the studies on the qualities of source rock，reservoir and engineering，and fracturing scale，the influences of TOC，chloroform bitumen “A” content，porosity，oil saturation，free oil content，brittleness index，pore pressure，two-directional stress difference and fracture development degree on production were systematically analyzed. The results show that the shale oil production from the Fengcheng formation are mainly controlled by four factors such as free oil content，brittleness index，fracture development degree，and large-scale volume fracturing. By using the logging data from 12 wells in the study area，the correlation between production and the controlling factors was analyzed，and a sweet spot classification standard suitable for the Fengcheng formation was established. The classification standard was successfully applied to the horizontal well Maye-1H for dividing sections and clusters，and it may provide a reference for subsequent exploration deployment. Figures and Tables | References | Related Articles | Metrics

APPLICATION OF TECHNOLOGY

Lithology Identification for Diamictite Based on Lithology Scan Logging: A Case Study on Fengcheng Formation, Mahu Sag MAO Rui, SHEN Ziming, ZHANG Hao, CHEN Shanhe, FAN Haitao 2022, 43 (6):  743-749.  doi: 10.7657/XJPG20220612 Abstract ( 227 )   HTML ( 3 )   PDF (1100KB) ( 130 )   Save Hydrocarbon-rich continental diamictite developed in the Permian Fengcheng formation of Mahu sag, Junggar basin is characterized by varieties of minerals and rapid variation of lithology. Conventional logging cannot be used to effectively identify the lithology of diamictite. By using lithology scan logging, the mass fractions of the main elements in the formation are obtained, and the mineral mass fraction is then calculated according to the relationship between mineral-sensitive elements and minerals. The diamictite index is constructed by using the ratio of felsic mineral content to carbonate mineral content, and the shale index is constructed by using the difference between neutron porosity and nuclear magnetic resonance total porosity. Then the lithology identification chart for the diamictite in the Fengcheng formation is formed by intersecting the diamictite index and the shale index. The results show that the calculation of mineral content is accurate with the average relative error of only 6.5%, and the lithology interpretation exhibits a coincidence rate of 90.9%, which meets the needs of logging evaluation. The proposed method provides a reference for logging lithology identification of diamictite shale reservoirs. Figures and Tables | References | Related Articles | Metrics

Simulation on Fracture Propagation During Hydraulic Fracturing in Horizontal Wells in Shale Reservoirs of Fengcheng Formation，Mahu Sag YU Peirong, ZHENG Guoqing, SUN Futai, WANG Zhenlin 2022, 43 (6):  750-756.  doi: 10.7657/XJPG20220613 Abstract ( 253 )   HTML ( 5 )   PDF (6827KB) ( 186 )   Save Hydraulic fracturing is an effective method for developing the shale reservoirs in the Permian Fengcheng formation in the Mahu sag，but the propagation characteristics of hydraulic fractures are unclear. Maye-1H，a typical horizontal well in this area，suffered from difficulties in fracturing initiation and sand addition. Thus，it is urgent to carry out hydraulic fracturing simulation to clarify the impacts of natural fractures，rock mechanical properties，and operation parameters on fracturing effect. According to the actual operation parameters such as pump pressure，fracturing fluid displacement and added sand volume in Well Maye-1H，a 2D hydraulic fracture propagation model and a 3D hydraulic fracturing model were established by using Abaqus software and Petrel software，and then numerical simulation on hydraulic fracture propagation was performed. The results show that the fracturing effect is closely related to natural fractures. The lower the tensile strength of the rock where natural fractures are developed，the easier the hydraulic fractures tend to be captured by the natural fractures. When the Young’s modulus in the fractured interval is relatively large，the hydraulic fractures formed are small in width，and most of them propagate and slip along the natural fracture trend，making it difficult to add sand. When the Young’s modulus in the fractured interval is relatively small，the hydraulic fractures formed are large in width，and they can directly pass through the natural fractures，making sand adding relatively easy. Figures and Tables | References | Related Articles | Metrics

Seismic Prediction Method of Geological and Engineering Shale Oil Sweet Spots and Its Application in Fengcheng Formation of Mahu Sag YU Jianglong, CHEN Gang, WU Junjun, LI Wei, YANG Sen, TANG Tingming 2022, 43 (6):  757-766.  doi: 10.7657/XJPG20220614 Abstract ( 247 )   HTML ( 5 )   PDF (3687KB) ( 164 )   Save In order to further accelerate the exploration and development for shale oil in the Lower Permian Fengcheng formation in the Mahu sag，Junggar basin，sweet spots of shale oil should be identified. Considering that lithology is the main factor controlling geological sweet spots，and brittleness index and horizontal principal stress difference are the main factors controlling engineering sweet spots，seismic methods of predicting geological and engineering sweet spots were established on the basis of prestack simultaneous inversion. In terms of geological sweet spots，by using core，experiment，drilling and logging data，the dominant lithology for shale oil sweet spots was identified to be dolomitic siltstone，the elastic parameters sensitive to the dominant lithology were selected，and the distribution of dolomitic siltstone was predicted by using prestack isimultaneous nversion and lithofacies probability analysis techniques. In terms of engineering sweet spots，using the Young’s modulus and Poisson’s ratio obtained from prestack simultaneous inversion，cubes of brittleness index and in-situ stress for the study area were obtained through the Rickman brittleness index method and a combined spring model. The predicted results are consistent with the actual drilling results，which confirms the accuracy of the prediction of geological and engineering sweet spots. The proposed methods can provide references for shale oil exploration and development in other areas. Figures and Tables | References | Related Articles | Metrics