石油实验地质

2022, 44(3)

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2022, 44(3)

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Evolution of prototype basin and change of tectonic-sedimentary pattern in Paleozoic, Ordos Basin

HE Faqi, WANG Fubin, GUO Liguo, AN Chuan

2022, 44(3): 373-384. doi: 10.11781/sysydz202203373

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Abstract:
The Ordos Basin is a craton basin developed based on the Precambrian rift system during Permian. It has experienced two stages of evolution including the Early Paleozoic craton marginal depression and Late Paleozoic intra-craton depression. The former can be further divided into four periods, including the Zhangxia period of Early to Middle Cambrian, the Sanshanzi period of Late Cambrian to the Liangjiashan period of Early Ordovician, the Majiagou period of Middle Ordovician, and the Pingliang-Beiguoshan period of Late Ordovician. The latter can be further divided into two periods, including Late Carboniferous to Early Permian and Middle to Late Permian. Controlled by the transformation of surrounding tectonic system of North-China blocks, the Paleozoic Ordos Basin evolved from passive continental margin extension in the Early Cambrian and Middle Ordovician to active continental convergence in Late Ordovician and Permian. The internal tectonic sedimentary pattern gradually evolved from a "L-shaped" extensional rift in the southwest of the basin during Cambrian, to "depression-slope-uplift-plateau" druing the Middle-Late Cambrian, to "depression-slope-uplift-depression" during Ordovician, and to "one uplift and two depressions" during[JP] Carboniferous to Permian. During the evolution of Paleozoic prototype basin and the change of tectonic-sedimentary pattern, several sets of source rocks and reservoirs were derived. In the continuously subsiding region on the western and southern margin of the basin, coal-measure source rocks deposited in Cambrian Suyukou-Xinji formations, Xuzhuang Formation, [JP]Ordovician Pingliang Formation, Carboniferous and Permian. In the subsiding region inside the basin, coal-measure source rocks deposited in Ordovician Xuzhuang Formation, Majigou Formation, Carboniferous and Permian. Effective reservoirs, such as high-energy oolitic beach of Zhangxia Formation, bioreef-granule beach of Krimori Formation and granulate beach of Majiagou Formation, were developed in the western and eastern paleoslope areas of the central paleouplift of the Paleozoic Ordos Basin.

Characteristics and evolution of microbial carbonate reservoirs in the Pingliang Formation on the southern margin of Ordos Basin

ZHANG Juntao, JIN Xiaohui, SUN Dongsheng, YANG Jiaqi, DING Qian

2022, 44(3): 385-393. doi: 10.11781/sysydz202203385

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As an important type of hydrocarbon reservoir, microbial carbonate contains great amount of hydrocarbon resources. The recent discovery of industrial gas in the Majiagou Formation in the east-central part of the Ordos Basin indicates that this type of reservoir also has very extensive exploration prospects in the Ordos Basin. Similar reservoirs were also found on the southern margin of the basin, but no systematic research has been achieved. In this paper, petrological and geochemical anatomy of microbial carbonate reservoirs of the Pingliang Formation on the southern margin of the Ordos Basin were carried out. Results show that the study area developed a larger-scale microbial mound and beach complex than that in the basin, and the thickness can be more than 100 m. The microbial rock is mainly agglomerate limestone with dissolution pores. The microbial rock reservoir of the Pingliang Formation has experienced two stages of karsting, quasi-syngenesis and epigenesis, which are the key for the formation of reservoir, and there is oil and gas charging during the burial period. Among them, the quasi-syngenetic stage experienced a brief exposure, forming a large amount of irregular-shaped solution pores, and radial fibrous calcite pores filled in the early pores, with similar oxygen/carbon isotope and trace element contents such as Sr and Mg as the matrix calcite, it is then indicated that they were originated from seawater in the same period. During the epigenetic stage, the early solution pores expanded, forming a large number of fractures and cavities, accompanied by the filling of coarse-crystal calcite and mud, and some of the coarse crystalline calcite shows the characteristics of atmospheric precipitation. There is oil and gas charging during the burial period. There are bituminous residues in the present-day solution holes and fractures, and a large amount of liquid hydrocarbon-bearing inclusions can be observed in the calcite filling during the burial period. In summary, the microbial carbonate of the Pingliang Formation at the southern edge of the Ordos Basin, where both reservoir formation and hydrocarbon filling processes exist, is a potential exploration field.

Controls of strike-slip fault activities on hydrocarbon accumulation in Tahe Oilfield, Tarim Basin: a case study of TP 39 fault zone

WANG Yang, ZHANG Shaonan, LIU Yongli

2022, 44(3): 394-401. doi: 10.11781/sysydz202203394

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Strike-slip faults were well-developed in the Tahe Oilfield, Tarim Basin, which had an important control to the accumulation and distribution of hydrocarbon. In this study, the TP 39 strike-slip fault in the Tahe Oilfield was taken as an example, fluid inclusions were used to determine the time of hydrocarbon charge based on the analysis of strike-slip fault activity history, and the relationship between hydrocarbon charging events and fault activity history was studied. Results show that the TP 39 strike-slip fault zone is rich in oil and gas resources and has the characteristics of multi-stage activities, namely the Middle-Late Caledonian to Early Hercynian, Indosinian and Yanshanian-Himalayan, respectively. The results of fluid inclusion analysis indicated that there were four episodes of oil charge to the Ordovician reservoir of TP 39 strike-slip fault, and oil inclusions with golden-yellowish, yellow, yellow-green and blue fluorescing colors were trapped. According to the minimum homogenization temperature of aqueous inclusions co-existed with oil-bearing inclusions, combined with the burial and thermal histories of reservoirs, there were four periods of oil filling events occurred at around 440, 324, 220 and 110 Ma, corresponding to Caledonian, Hercynian, Indosinian and Yanshanian, respectively, which showed a good response relationship with the fault activity time. It was then concluded that the activities of strike-slip fault constrained the time of hydrocarbon charge, thus controlled the whole process of hydrocarbon accumulation.

Influence of geothermal field on ultra-deep Ordovician oil and gas in Shunbei field, Tarim Basin: a case study of Shunbei No. 5 strike-slip fault

WU Xian, LI Dan, ZHU Xiuxiang, WANG Jianfeng

2022, 44(3): 402-412. doi: 10.11781/sysydz202203402

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The types and properties of the ultra-deep Ordovician oil and gas resources in the strike-slip fault zone of the Shunbei oil and gas field in the Tarim Basin show regular changes. The main controlling factors of oil and gas distributional characters are unclear, which restricts the evaluation of resource types and exploration sequences. To study the consequences of the thermal field on oil and gas, the statistical analyses of the current measured temperature of the Shunbei No. 5 fault zone, the restoration of the thermal history[JP] of the key accumulation period, the thermal evolution of the source rocks, and the Ordovician oil and gas properties and maturity are carried out. Results show that present geothermal field in the Shunbei area has a vertical gradient gradually decreasing from shallow to deep, and the present geothermal temperature as well as the paleotemperature of key accumulation period of the source rocks of the Cambrian Yuertus Formation gradually increase from north to south. With the background of ultra-deep low geothermal gradient, the temperature of the Shunbei Ordovician ultra-deep reservoir did not reach the cracking temperature point for large amounts of crude oil, which provided favorable conditions for the preservation of liquid oil. In the critical accumulation period, the late Hercynian, the thermal evolution degree of the local Cambrian Yuertus Formation source rocks gradually increases from north to south. Since high pressure inhibited the evolution of hydrocarbon generation, the northern and middle segments of the Shunbei No. 5 fault zone are mainly oil-generating, while the southern segment is condensate-gas-generating. The evolution products are in good consistency with the current Ordovician ultra-deep oil and gas reservoir types, crude oil density, natural gas drying coefficient, production gas/oil ratio, and the plane distribution characteristics of oil and gas thermal evolution degree, indicating that the thermal evolution difference of source rocks under the control of geothermal field is the main factor affecting the distribution of resource types and oil and gas properties.

Genesis and pore development characteristics of Permian Lucaogou migmatites, Jimsar Sag, Junggar Basin

WANG Jian, YUAN Bo, LIU Jin, LI Yong, LI Erting, MA Cong, ZHANG Baozhen

2022, 44(3): 413-424. doi: 10.11781/sysydz202203413

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Taking the influence of lithological characteristics differences on the genesis of pores in migmatite reservoirs as the main line, the genesis and characteristics of pores were analyzed from the perspective of the relationship between rock fabric and grain deposition. (1) Based on hydrodynamic conditions, mineral assemblage and rock structure characteristics, and taking three endmembers of clastic particles, clay minerals and carbonate minerals for classification, the "sweet spots" in the Lucaogou Formation are mainly composed of tuffaceous sandy dolomite, tuffaceous feldspar detrital siltstone, tuffaceous dolomitic sandstone and tuffaceous dolomitic siltstone. The migmatites have mixed sources, and various types of migmatite layers can be superimposed on each other vertically to form facies marginal mixing. (2) Based on the spherical accumulation model, when the content of debris particles is lower than 52.4%, the porosity decreases with the increase of particle content. When the content of clastic particles reaches 52.4%, if the content of clastic particles[JP]continues to increase, large number of ntergranular pores will appear, and the porosity will increase rapidly, gradually evolving into a conventional sandstone reservoir. (3) According to the relationship between the three end components of clastic particles, clay minerals and carbonate minerals and the pore types, the pores of the migmatites in the Lucaogou Formation can be classified into three types including intergranular pore, intragranular dissolved pore and intercrystalline pore. The pores in tuffaceous sandy dolomites and tuffaceous feldspar detrital siltstones are mainly intergranular and intragranular dissolved ones supported by particles, while those in tuffaceous dolomitic sandstones and tuffaceous dolomitic siltstones are mainly intercrystalline ones. The development of high-quality migmatite reservoirs is mainly controlled by the content of clastic particle components, and the content of soluble clastic components and dissolution effect can improve the physical properties of the reservoirs.

Geological characteristics and exploration practice of deep carbonate buried hills in Nanpu Sag, Bohai Bay Basin

YANG Xiaoli, WANG Zhengjun, GAO Wenzhong, ZHAO Zhongxin, MA Qian, WANG Qi, ZHANG Hongchen

2022, 44(3): 425-432. doi: 10.11781/sysydz202203425

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The deep carbonate buried hills are widely distributed in the Nanpu Sag of Bohai Bay Basin. They have experienced multiple stages of tectonic movement and show a tectonic pattern of alternating sub-sags and uplifts. Exploration discoveries have been achieved in multiple structural belts and layers. In recent years, with the deepening of exploration, many exploratory wells deployed on the slope belt and the downdip area were failed, and the exploration of deep buried hills has stagnated. To clarify the potential and exploration direction of buried hills, on the basis of improvement of seismic data, starting from the three major accumulation elements of hydrocarbon supply window and supply mode, high-quality reservoir type and trap type, the re-evaluation of accumulation conditions and the re-understanding of main controlling factors of accumulation were carried out. The deep Paleozoic carbonate buried hills in the Nanpu Sag develop multiple sets of reservoir-caprock assemblages, [JP] with good physical properties, high-quality source rocks, and favorable hydrocarbon accumulation conditions. Oil and gas enrichment is controlled by multiple factors such as hydrocarbon supply mode, window, type of trap and reservoir, and there are various types of oil and gas reservoirs. The hydrocarbon supply window controls the formation of hydrocarbon accumulation, the type of trap determines the type of oil and gas reservoir, and the dominant reservoirs formed by fault fracture-supergenic karst and fracture-burial karst generated by multi-stage karstification control the enrichment and high yield. The comprehensive evaluation of the accumulation conditions shows that the slope area and the trough area of the Nanpu No. 1 and No. 2 structural belts as well as the inner Cambrian buried hills of the Nanpu No. 2 and Nanpu No. 3 structural belts are important directions for the next exploration. Through the integrated application of geological research and exploration technology, several favorable[JP] exploration targets have been classified and evaluated. Overall breakthroughs have been made in the exploration of carbonate buried hills in the Nanpu Sag, and good exploration results have been achieved.

Characteristics and exploration prospect of dolograinstone beach reservoir in Xixiangchi Formation, Cambrian, Sichuan Basin

SHI Shuyuan, HU Suyun, WANG Zecheng, WEN Long, XU Zuxin, LIU Wei, XIE Wuren, JIANG Hua, BIAN Congsheng, LIU Jingjiang, LU Bin, SU Wang, FENG Qingfu, ZHOU Gang, HAO Tao

2022, 44(3): 433-447. doi: 10.11781/sysydz202203433

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The Cambrian Xixiangchi Formation is an important succeeding stratum for gas exploration[JP] in the Lower Paleozoic of the Sichuan Basin, for which, the exploration degree is low, and the main controlling factors of large-scale[JP] reservoirs have not been well understood. Based on core, logging, seismic data and combined with regional tectonic evolution analysis, the original sedimentary structure of the shoal dolograinstone reservoir in the Cambrian[JP] Xixiangchi Formation were restored in this paper, and the characteristics, controlling factors and development patterns were further discussed. Results show that: (1) The rock types of shoal dolograinstone karst reservoirs mainly include dolograinstone, dolopackstone, granulated dolostone, residual granular dolomite and grain dolomite. It appeared to be developed in a variety of sedimentary micro-facies assemblages, such as shoal-shoal superimposition, flat-shoal-flat, lagoon-beach-lagoon and flat-beach-lagoon. The main reservoir space are intergranular (dissolution) holes, intergranular (dissolution) pores and vugs, and a small number[JP] of (dissolution)fractures were also developed. The reservoir porosity ranges from 3% to 6% with a highest value greater than 10%. (2) The dolograinstone shoal facies dolostone of the Xixiangchi Formation was controlled by the sea level changes and synsedimentary paleouplift. A large number of dissolution pores were formed after multiple exposure in the contemporaneous and quasi contemporaneous periods. Among them, the Yunan Movement at the end of the Xixiangchi period has a wide range of distribution and extensive exposure in the whole basin, which may further transform the middle and upper reservoirs of the Xixiangchi Formation. The Duyun and Guangxi movements might transform the central Sichuan paleouplift and its surrounding reservoirs. (3) The favorable reservoir distribution areas of Xixiangchi Formation in the Sichuan Basin were predicted to develop mainly type Ⅱ and Ⅲ reservoirs. Among them, the scale reservoir distribution area of the slope belt around the paleo-uplift is more than 10 000 km²[JP], which is expected to be the most favorable exploration area at present.

Fracture distribution and prediction of weathering crust reservoirs on the top of Leikoupo Formation, Xinchang area, western Sichuan Basin

ZHU Lan, LI Suhua, YU Yang, LI Rong, SHI Guoshan, XU Guoming

2022, 44(3): 448-455. doi: 10.11781/sysydz202203448

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The Leikoupo Formation in the western Sichuan Basin is an important potential stratum for the increasing of reserves and production. Recently, industrial grade gas flow has been discovered for the first time from weathering crust reservoirs on the top of Leikoupo Formation in the Xinchang area. Dissolution pores as well as fractures are the main storage spaces and seepage channels. Then, how to accurately predict the distribution characteristics of these cracks in weathered crust reservoirs has become a key factor of advance exploration process. Core observation and imaging logging data were applied to clarify the geological characteristics of fracture development. Some seismic prediction methods such as coherence, dip, azimuth and curvature attributes were used to identify the plane distribution characteristics of fracture development zones. Results show that the fractures in weathering crust reservoirs on the top of Leikoupo Formation are mainly in the middle and low angles, and the strikes are NE-SW, NW-SE and near EW. Structure fractures are mainly distributed in the central and eastern area and near the big fault. In the southwest of the study area, there are NE-SW and NW-SE "net-like" interlayer fractures, of which the scale of the NE fractures is larger. Based on the analysis results of multiple methods, the fracture characteristics and distribution rules of weathered crust reservoirs on the top of Leikoupo Formation were determined. The prediction results are consistent with newly drilled wells and provide a reliable basis for the evaluation in future.

Paleo-environment of the first member of Niutitang Formation on the southern margin of Huangling anticline, western Hubei province: a case study of well ZD-1

XU Lulu, WEN Yaru, ZHOU Xianghui, JU Zilong, CHEN Wei, YANG Jie, REN Zhijun, WEN Jianhang

2022, 44(3): 456-465. doi: 10.11781/sysydz202203456

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The Lower Cambrian Niutitang Formation is one of the important targets for shale gas exploration in the western Hubei province. In view of the relatively weak research on the paleo-sedimentary environment of the 1st member of Niutitang Formation (Ꞓ₁n¹), the paleo-climate, paleo-redox conditions and paleo-productivity of the Ꞓ₁n¹ were comprehensively studied based on data collected from well ZD-1 on the southern margin of the Huangling anticline in Yichang area, and a model of the evolution of sedimentary environment was proposed. Compared with the organic-poor strata, the organic-rich strata have higher TOC content, much higher SiO₂, U, V, Ni, Zn and Cu contents, and lower CaO content. The chemical index of alteration (CIA) shows that the Ꞓ₁n¹ has experienced weathering effect of moderate intensity. The climate was warm and humid when the organic-rich layers deposited, and became hot and wet when the upper layers deposited. The redox conditions of Ꞓ₁n¹ have experienced 3 stages, including euxinic in the lower part of organic-rich strata, dysoxic in the upper part of organic-rich strata, and oxic in organic-poor strata. In general, the study area developed in a weak-medium retention basin. The ratio of Mo_EF/U_EF is usually 1-3 times than that of normal seawater due to the high content of Mo, showing much higher retention rate. The organic-rich strata have higher paleo-productivity, and there are 2 periods of rapid decline of paleo-productivity, which is directly related to the change of water from anaerobic to oxidized due to the decrease of sea level. According to paleo-climate, paleo-redox conditions and paleo-productivity characteristics, the Ꞓ₁n¹ can be divided into 3 evolution stages.

Control of Paleocene volcanic edifice on favorable reservoirs: a case study of the southwestern Huizhou Sag, Pearl River Mouth Basin

WANG Xucheng, CHEN Weitao, HE Ye, LIU Hanqing, WANG Wenyong

2022, 44(3): 466-475. doi: 10.11781/sysydz202203466

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Thickly developed Paleocene volcanic rocks in the southwestern Huizhou Sag of the Pearl River Mouth Basin are continuously filled with lacustrine sedimentary rocks of the overlying Eocene Wenchang Formation in the sag, forming the main rock formations in the early and middle stages of the rift basin. Absence of obvious sedimentary hiatus leads to poor supergene exposure as well as weathering and leaching effects of volcanic rocks. Therefore, the distribution of favorable reservoirs in volcanic rocks is mainly affected by volcanic edifices. Based on the latest drilling, logging and 3D seismic data, through the observation of cores and thin sections, the research on the volcanic edifice and lithofacies characteristics in the southwestern Huizhou Sag was carried out, and the controls of volcanic edifice on favorable reservoirs was analyzed. Results show that the study area has the progressive characteristics of "the eruption mode controls the type of volcanic edifice, the volcanic edifice controls the distribution of favorable facies, and the favorable facies controls the distribution of favorable reservoirs". (1) Controlled by the period and mode of volcanic eruption, stratovolcano and dome-like volcanic edifices developed in the study area, and the crater shape is of two types: subsidence caldera and dome. (2) The dominant lithofacies of volcanic rocks are distributed in the center of the volcanic edifice. The volcanic conduit facies, explosive facies and effusive facies in or near the crater are favorable combination of lithofacies. (3) The distribution of favorable reservoirs is affected by lithofacies assemblages and fault activities during the rifting period. The reservoir space is divided into primary and secondary pores and fractures. The lithofacies controls the enrichment degree of primary pores and fractures, while the later fault activities reform and connect the protogenetic pores and fractures, forming large-scale oil and gas reservoirs of pore-fracture type in volcanic rocks. (4) Extrusive facies, which is on the top of dome volcanic edifice, shows a poor oil-bearing property with hardly pores and fractures due to low volatiles content during the magma extruding.

Flume simulation of response of deltaic sedimentary process to Paleogene flexural gentle slope belt in Baiyun Sag, Pearl River Mouth Basin, northern South China Sea

WU Yuxiang, LIU Baojun, ZHANG Chunsheng, DING Lin, XIE Shiwen, LI Xiaoping, LONG Gengsheng

2022, 44(3): 476-486. doi: 10.11781/sysydz202203476

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Hundreds of millions of tons of oil and gas geological reserves have been discovered through exploration in the deep-water area of the northern continental margin of the South China Sea, and the distribution of large-scale effective reservoirs during the Paleogene rifting period is the key for continuous discovery in deep-water zone. Large braided fluvio-delta can be observed on seismic data from the Wenchang to Enping formations in the rifting period of the flexural gentle slope belt in the deep-water zone. Due to the absence of wells or few wells, for further understanding of the composition and evolution of large braided fluvio-deltaic sedimentary units in the process of tectonic subsidence in this region, a simulation experiment on the response of the deltaic sedimentation process in the flexural gentle slope zone to flume was carried out. Three phases of tectonic subsidence were designed for the simulation experiment, corresponding to the three phases of Paleogene regional tectonic-sedimentary evolution including balanced rifting period, detached rifting period, and faulted depression rifting period. The hydrodynamic mechanism of tractive flow was simulated by means of flood, flat water, and dry water. Simulation results showed that during the three stages of tectonic subsidence, the flexural gentle slope zone deve-loped distributary channels, natural dikes, floodplains, underwater distributary channels, estuaries, inter-tributary bays and other delta sedimentary microfacies units. The braided river delta was dominated by lateral proliferation, supplemented by vertical accretion, and the sedimentary center gradually migrated to the center of the depression, with almost no degeneration. The delta shape and sand body distribution in the flexural gentle slope zone during the rifting period were mainly controlled by factors such as structural geomorphology, river discharge amount into lake, lake level changes, and water depth.

Origin and exploration prospect of Upper Paleozoic crude oil from Hangjinqi area, Ordos Basin

ZHAO Yongqiang, XU Jin, NI Chunhua, ZHANG Yi, MA Zhongliang, WANG Fubin, JIA Huichong, ZHANG Wei, QI Rong, AN Chuan

2022, 44(3): 487-496. doi: 10.11781/sysydz202203487

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A set of favorable source-reservoir-caprock assemblage was found in the Upper Paleozoic transitional facies in the Hangjinqi area of the Ordos Basin, and the large-scale Dongsheng gas field has been discovered. A small amount of crude oil has been produced from the Lower Shihezi Formation reservoir in the area; however, the origin and exploration prospect of the oil are unclear. Based on the studies including physical property, n-alkanes distributions, biomarker compositions, and carbon isotopic ratios, the origin and exploration prospect of the crude oil has been discussed. Results indicate that the Upper Paleozoic crude oil from the Hangjinqi area is mature oil with a low sulfur content and a high wax content, and the steranes are dominated by terrestrial plants and assisted by a small amount of aquatic organism. The Pr/Ph ratio is 2.16-2.26, and the diasteranes and rearranged hopanes are in high contents with a relative low content of gammacerane. The carbon isotopic ratios of the oil range from -27.7‰ to -26.8‰, [JP+1]and the carbon isotopic[JP]compositions of individual n-alkanes are heavy at first and then become light with the increasing of carbon numbers. These characteristics suggest that the oil was generated by the dark mudstone in terrigenous coal measures under weak oxidation environment. The comprehensive analysis combining organic petrography, hydrocarbon generation simulation, and pyrolysis of source rocks in the Shanxi Formation indicates that, the dark mudstone in the Permian Shanxi Formation in the Hangjinqi area displays a certain amount of oil generation potential. In combination with the location of oil producing wells and the spatial distribution of source rocks, the dark mudstone in the Shanxi Formation might be the potential source rocks for the produced oil. The oil exploration of the Upper Paleozoic strata in the Hangjinqi area deserves attention as suggested by the favorable conditions of source-reservoir-caprock assemblage, and the adjacent areas of the Borjianghaizi fault are potential favorable areas for oil accumulation.

Evaluation of evaporative loss of hydrocarbon in shale samples and its geological implications

QIAN Menhui, LI Maowen, JIANG Qigui, LI Zhiming, TAO Guoliang, BAO Yunjie

2022, 44(3): 497-504. doi: 10.11781/sysydz202203497

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Free hydrocarbon content is one of the key parameters for resource assessment of shale oil.However, due to the evaporative loss of hydrocarbon, the measured results from lab analysis differ greatly from real value, which leads to the "distortion" of shale oil resource assessment.To investigate the process of the evaporative loss and obtain the hydrocarbon loss amount and correction coefficient, a time-series of analysis has been carried out on fresh oil-bearing shale samples with similar maturity and different lithofacies for both the amount and the composition of the hydrocarbons retained in shale.Results suggest that there are two processes of early rapid and later slow evaporative loss. The volatile components are mainly light hydrocarbons with carbon numbers smaller than C₁₃-C₁₅, while the medium to heavy components are less affected. Both the amount and processes of hydrocarbon evaporative loss are controlled by the physical properties and original oil content of shale samples.The oil-bearing shale samples with higher original oil content and better physical properties appear to have more evaporative loss of hydrocarbons.Results also indicate that more attention should be exercised on original hydrocarbon content when conducting the shale oil resource assessment of "sweet spot" section with high oil content and good physical properties.

Inorganic geochemical characteristics and evaluation of Sinian-Cambrian post-mature source rocks in Sichuan Basin

SHI Chunhua, SHAN Shujiao, HAO Jing, LUO Bing, CAO Jian

2022, 44(3): 505-514. doi: 10.11781/sysydz202203505

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The evaluation of post-mature source rocks gradually invalidates by traditional organic geochemical parameters due to the decreasing of organic matter abundance. However, the inorganic elements in source rocks are not easily affected by thermal evolution. In order to explore the application of inorganic geochemical methods in the evaluation of post-mature source rocks, this paper takes the Sinian-Cambrian source rocks in the Sichuan Basin as an example to conduct an evaluation based on the inorganic (major, trace and rare earth elements) geochemistry method with three controlling factors of source rock development (paleoproductivity, sedimentary environment and sedimentary rate). The concentration of Ba and Ni, Mo_EF and U_EF and Ce anomaly, and TiO₂/Al₂O₃ ratio could be correlated with paleoproducitvity, sedimentary environment and sedimentary rate, respectively. The results showed that the enrichment of organic matter was mainly controlled by sedimentary environment and paleoproductivity, but are barely effected by sedimentary rate. Source rocks in the Lower Cambrian Qiongzhusi Formation in the central Sichuan and Weiyuan-Ziyang areas have high paleoproductivity and the most reduced environment and thus are one of the best quality source rocks. Therefore, inorganic geochemistry method is an effective way to evaluate postmature source rocks.

Analysis of polar compounds in biodegraded heavy oil by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry

LI Erting, SHI Quan, MA Cong, LEI Haiyan, WU Jianxun, ROUZI Dilidaer, GAO Xiuwei, WANG Ming

2022, 44(3): 515-521. doi: 10.11781/sysydz202203515

Abstract(442) HTML (149) PDF-CN(26)

Abstract:
Taking the influence of lithological characteristics differences on the genesis of pores in migmatite reservoirs as the main line, the genesis and characteristics of pores were analyzed from the perspective of the relationship between rock fabric and grain deposition. (1) Based on hydrodynamic conditions, mineral assemblage and rock structure characteristics, and taking three endmembers of clastic particles, clay minerals and carbonate minerals for classification, the "sweet spots" in the Lucaogou Formation are mainly composed of tuffaceous sandy dolomite, tuffaceous feldspar detrital siltstone, tuffaceous dolomitic sandstone and tuffaceous dolomitic siltstone. The migmatites have mixed sources, and various types of migmatite layers can be superimposed on each other vertically to form facies marginal mixing. (2) Based on the spherical accumulation model, when the content of debris particles is lower than 52.4%, the porosity decreases with the increase of particle content. When the content of clastic particles reaches 52.4%, if the content of clastic particles[JP]continues to increase, large number of ntergranular pores will appear, and the porosity will increase rapidly, gradually evolving into a conventional sandstone reservoir. (3) According to the relationship between the three end components of clastic particles, clay minerals and carbonate minerals and the pore types, the pores of the migmatites in the Lucaogou Formation can be classified into three types including intergranular pore, intragranular dissolved pore and intercrystalline pore. The pores in tuffaceous sandy dolomites and tuffaceous feldspar detrital siltstones are mainly intergranular and intragranular dissolved ones supported by particles, while those in tuffaceous dolomitic sandstones and tuffaceous dolomitic siltstones are mainly intercrystalline ones. The development of high-quality migmatite reservoirs is mainly controlled by the content of clastic particle components, and the content of soluble clastic components and dissolution effect can improve the physical properties of the reservoirs.

Distribution and significance of maleimides in organic matter-enriched shales and derived crude oils

WANG Guangli, CHANG Rui, LI Jingyi, ZHU Lei

2022, 44(3): 522-529. doi: 10.11781/sysydz202203522

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Abstract:
The maleimides, or 1H-pyrrole-2, 5-diones, are one of the novel types of biological markers existed in both sedimentary organic matter and petroleum, which could be products derived from the degradation of chlorophylls and bacteriochlorophylls. In this study, the maleimides composition was investigated in detail for several crude oils and petroleum source rocks with varying depositional settings, and abundant 2-methyl-3-n-propyl (Me, n-Pr) and 2-methyl-3-isobutyl (Me, i-Bu) maleimide, together with aryl isoprenoids were detected. These pigments-derived products were prominently generated by Chlorobiaceae and are especially abundant in the saline and marine black shales and related oils, indicating that PZE (photic zone euxinia) occurred in the brackish water environment of the Nenjiang Formation in the Songliao Basin, the saline to semi-saline water environment of the Shahejie Formation in the Bohai Bay Basin, and the marine environment of the Napo Formation in the Oriente Basin, showing strong reduction preservation conditions. It can be concluded that PZE is a favorable factor for the enrichment of organic matter and the development of high-quality source layers. In addition, the ratios of Me, i-Bu/Me, Et and Me, n-Pr/Me, when coupled together, could be used to diagnose the depositional conditions such as marine, saline, semi-saline or brackish water column.

Evaluation methods for oil reserve upgrading

HU Yan

2022, 44(3): 530-534. doi: 10.11781/sysydz202203530

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Abstract:
Whether the predicted and controlled reserves can be upgraded is highly related to factors such as geological understanding of reservoir, technical support, and economic benefits. Especially at low-oil-price periods, the block economy has become a key factor restricting the upgrade.In order to quickly screen out economical and effective blocks from a large number of candidates, combined with the characteristics of reserve blocks, the method of calculating the economic limit of a single well is studied, and a cash flow model and calculation process of the economic limit of a single well are proposed.This method is generally applicable for the economic judgment of retained reserves and newly added reserves, moreover, it can also be applied to the evaluation of exploration and development projects at different stages, providing technical support for efficient exploration and profitable development as well.Taking an ultra-low permeability oil reservoir in a certain area of SINOPEC as an example, it is estimated that the potential reserves related to benefits are (50, 80, 120, 160, 170)×10⁶ t at oil prices of 40, 50, 60, 70, 80 USD/bbl.

Application of "retention coefficiency" method in shale gas resource evaluation: a case study of Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation, southeastern Sichuan Basin

SONG Zhenxiang, WANG Baohua, WEI Xiangfeng, MA Zhongliang

2022, 44(3): 535-544. doi: 10.11781/sysydz202203535

Abstract(426) HTML (171) PDF-CN(44)

Abstract:
Besides providing resulting data, the evaluation of resource abundance is more important to provide evidence for the optimization of favorable areas and the deployment of exploration. In view of the drawbacks of traditional methods, combined with the newly developed thermal-pressure simulation method of formation porosity and basin simulation method, this paper puts forward the process of shale gas resource evaluation using the "retention coefficiency" method. It is pointed out that the model of hydrocarbon generation-expulsion-retention and the "retention coefficiency"are the two most critical parameters in this new method. The Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation, which are the most highly explored shale in China, are taken as targets to illustrate the application process of the new method. Results show that the new method has good applicability and feasibility in shale gas resource evaluation. Compared with traditional methods, the "retention coefficiency" method not only considers the dynamic evolution process of shale gas, but also considers the influence of late preservation conditions on shale gas enrichment. In addition, the new method can describe the spatial distribution characteristics of shale gas resources, with a broad application prospect in shale gas resource evaluation and favorable area optimization.

An estimation method for hydrocarbon expected value of exploration target group in mature exploration area

LI Jun, YAN Xiangbin

2022, 44(3): 545-551. doi: 10.11781/sysydz202203545

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Abstract:
The transfer system reform of mining rights in China is promoted in a comprehensive way. The rational evaluation of the expected value of blocks is crucial to the decision-making of bidding and exiting blocks for oil companies. As to the mature exploration area containing multiple exploration targets, adding up each expected value of each target simply usually over-estimates the expected value of blocks. Two factors should be comprehensively considered. The first one is geological conditions, including relative spatial relation, drilling success rate, resource scale and geological relation, etc. The second one is exploration strategy of oil company, including the bearing capacity of dry well, the number of commitment well and the drilling sorting of each exploration target, etc. Therefore, a set of estimation methods for exploration target group based on probabilistic theory was proposed from the oil company's practical decision-making of bidding and exiting blocks. The effect of the above factors on the expected value of blocks was also discussed using a specific block as an example.

Identification method for the origin of natural hydrogen gas in geological bodies

MENG Qingqiang

2022, 44(3): 552-558. doi: 10.11781/sysydz202203552

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Abstract:
As one of the clean energies, hydrogen has attracted more and more global research interest. With the acceleration of hydrogen energy economic construction in China, higher requirements have been put forward for the production and storage of hydrogen. At present, the main way to obtain hydrogen is to produce hydrogen artificially. Whether there is a high content of hydrogen in nature and its origin are the premise for the discovery and utilization of natural hydrogen, but the related research is weak. The discrimination of hydrogen origin, especially deep and shallow source origin, depends mainly on the isotopic composition characteristics of accompanied rare gases, which is difficult to be determined in some cases, the origin of hydrogen is then difficult to be identified. Based on the analysis and analytical results of hydrogen and associated gas content and isotopic composition in different structural parts, this paper analyzes the duration and content variation characteristics of high content hydrogen in the Kansas Basin, USA. A discrimination method for hydrogen genesis based on the relationship between methane and hydrogen content and hydrogen isotopic composition has been proposed, access of hydrogen genesis identification is easier to be achieved. Based on the above research, it is believed that there is a hydrogen supplement mechanism with underground conditions, which can continuously produce high content of natural hydrogen. It is considered that it is a favorable distribution area for high content of hydrogen around the plate collision zone. The hydrogen source can be classified by the hydrogen isotopic composition of -700‰ (VSMOW) and ln(CH₄/H₂). Shell source hydrogen has a δD value generally greater than -700‰ and a ln(CH₄/H₂) value lower than -8. Mantle derived hydrogen has a δD value generally lower than -700‰ and a ln(CH₄/H₂) value greater than -4. Hydrogen remaining after CO₂ rich fluid is oxidized on the surface has a δD value greater than -700‰ and a ln(CH₄/H₂) value greater than -8. After the deep source hydrogen rich fluid is oxidized on the surface, the residual hydrogen has a δD value lower than -700 ‰ and a ln(CH₄/H₂) value under -4. This proposed method can quickly classify the origin of hydrogen without determining the composition and isotopic composition of rare gases.

2022 Vol. 44, No. 3

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