石油实验地质

2023, 45(2): .

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New progress and prospect of oil and gas accumulation research in deep to ultra-deep strata

GUAN Xiaodong, GUO Lei

2023, 45(2): 203-209. doi: 10.11781/sysydz202302203

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There are abundant deep to ultra-deep oil and gas resources in China, but the degree of exploration is low. The deep to ultra-deep reservoirs have higher temperature and pressure than the shallow ones, and the fluid from the deep strata plays a stronger role in the deep formations. Therefore, the deep to ultra-deep reservoirs have different accumulation mechanism from the shallow ones. Through extensive research at home and abroad, the particularity and main control factors of hydrocarbon generation, reservoir-forming and hydrocarbon accumulation of deep to ultra-deep oil and gas reservoirs were systematically summarized. It was suggested that there are materials and energy in the deep to ultra-deep environment that can promote the continuous hydrocarbon generation of hydrocarbon kitchens, which can further promote oil and gas accumulation. Fluids originating from deep to ultra-deep reservoirs can play a constructive role in forming high-quality large-scale reservoirs in deep strata. The deep CO₂-rich fluid flowing through source rocks and reservoirs can promote the hydrocarbon expulsion in deep strata and the hydrocarbon accumulation in shallow strata. Although these theories summarized by predecessors are still controversial in guiding the discovery of large-scale oil and gas reservoirs, it should be seen that under the unique geological conditions of deep to ultra-deep strata, when summarizing the law of oil and gas accumulation, comprehensive and full consideration of various factors will help to improve the success rate of deep to ultra-deep exploration wells and reduce exploration costs.

Evaluation and optimization of "engineering sweet spot" in deep shale reservoir: case study on Yongchuan and Dingshan areas in southern Sichuan

GE Xun, GUO Tonglou, LI Maowen, ZHAO Peirong, DENG Hucheng, LI Wangpeng, ZHONG Cheng

2023, 45(2): 210-221. doi: 10.11781/sysydz202302210

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In the Sichuan Basin, geologic conditions are complex in deep layers. Burial depth of shale reservoirs is high. Gas well productivity is widespread low, and hydraulic fracturing confronts with extremely large challenges. In order to search for "engineering sweet spots" of deep shale gas layers, Yongchuan and Dingshan areas were taken as the examples. In-depth discussion was conducted on affecting factors of hydraulic fracturing results in deep shale reservoirs by drilling, core, imaging logging and seismic data, based on measured rock mechanical parameters, development degree of natural fractures, natural fracture effectiveness, horizontal stress difference and included angle between horizontal well track and the minimum main horizontal stress. Study results demonstrated that No.3¹ sublayer of the first member of Longmaxi Formation in Yongchuan and Dingshan areas, southern Sichuan had the best brittleness as the first-choice target horizon for hydraulic fracturing. Shorter distance from the faults, higher pump pressure and operational pressure, and smaller reconstructed volume. Good extension and turning capacity of the fractures could beguaranteed when included angle between hydraulic and natural fractures was roughly 55°. In silicon-rich shale and faulting zones, lower minimum main horizontal stress, easier-diffused hydraulic fractures. Higher included angle between horizontal well track and maximum main horizontal stress azimuth, better fracturing results. Finally, evaluation index system of "engineering sweet spots" for marine shale gas in complex structural areas of Yongchuan and Dingshan was constructed. Class Ⅰ geologic and engineering "sweet spots" were optimized in Yongchuan and Dingshan areas.

Relationship among oil and gas resources, reserves and production in China and suggestions for the development direction of national oil and gas resource assessment

HU Hongjin, LI Denghua, ZHAO Kai, JIANG Wenli, ZHENG Zhihong, GAO Yang, ZAN Xin, JIANG Hang, JIA Jun, GAO Xuan

2023, 45(2): 222-228. doi: 10.11781/sysydz202302222

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In order to understand the connotation and substantive relationship among oil and gas resources, reserves and production on a deeper level, this paper made an in-depth analysis and elaboration on the relationship among the three from the perspectives of theoretical logic and practical transformation, and presented suggestions for the development direction of resource assessment based on the analysis of application status. Resources, reserves and production are estimated quantities of oil and gas mineral resources under different levels of understanding. In China's classification system, reserves and resources are in a parallel relationship, and reserves and production are in an inclusive relationship. Both resources and reserves are geological meanings. The transformation from resources to reserves is affected by factors such as national spatial planning, mining right setting, theoretical technical and economic conditions, and the transformation from reserves to production is affected by factors such as reserves upgrading and transformation, whether to convert to mining, recovery factor, etc. China still has some deficiencies in resource assessment and release, as well as quantitative prediction of resources, reserves and production, which is also an important reason for the cognitive confusion of "China has rich resource potential but it is difficult to increase reserves and production". The enrichment and development of resource assessment contents and results application analysis will help to form a comprehensive and in-depth understanding of the current situation of oil and gas resources. It should be further explored and developed in the future from the following respects: mobility classification of assessment results, assessment of (undiscovered) recoverable resources and application analysis of results.

Strategic area selection and key exploration fields in central and western large basins

CHENG Jian, ZHOU Xiaojin, LIU Chaoying, DUAN Tiejun, YU Qixiang, CAO Qinggu

2023, 45(2): 229-237. doi: 10.11781/sysydz202302229

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The reform and new policies of national oil and gas mining rights system have led to the increasing number of mining rights blank areas. In order to tap and enhance the strategic value of oil and gas in blank areas, promote the discovery and full use of oil and gas resources, and ensure the national energy security, this paper, by analyzing the structure and distribution characteristics of China's oil and gas resources, focusing on the main mining rights blank areas on land, and studying on oil and gas geological conditions and key reservoir-forming factors, carries out the optimization of strategic area selection and key exploration fields according to the process of stepwise classification. It is pointed out that the natural gas in the deep depression of the north and south platform basins of the Tarim Basin, the Carboniferous natural gas and the Jurassic coalbed gas in the northeast of the Junggar Basin, the Neopaleozoic tight gas and coalbed gas in the eastern and northern margins of the Ordos Basin, and the marine shale gas in the extrabasinal gentle structural area of the Middle and Upper Yangtze are the key directions for future strategic area selection. Combined with the characteristics and key problems of oil and gas geology in various fields, suggestions are put forward for deepening research, technical research direction and evaluation of selected areas.

Petroleum allocation method for exploration strategic planning

XU Huaming, CHENG Zhe, HONG Taiyuan, SHI Lei, ZHANG Peng

2023, 45(2): 238-242. doi: 10.11781/sysydz202302238

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During petroleum exploration strategic planning and study, it is necessary to comprehensively analyze on exploration status, resource basis and engineering technology conditions of various plays, in order to determine resource replacement sequence, petroleum exploration focus and development targets in the future. According to quantitative calculation on exploration status and potential of different plays, a resource allocation method in exploration strategic planning was established. In this method, an exploration status index formula for three key parameters, namely, resource proven degree, exploration well density and 3D seismic coverage density was firstly constructed for quantitative calculation on current exploration status. Secondly, a quantitative evaluation formula for exploration potential was established and incorporated comprehensively with some parameters like interim proven speed that reflected exploration status, resource potential and engineering technologic adaptability. Finally, exploration status index and potential index were put in unified evaluation platform in order to construct a cross plot for these two indexes, and then eight petroleum resource allocation areas were formed. According to the thinking of the strategy defined by the comparison on exploration status and the target by quantitative potential evaluation, petroleum exploration and development focus and exploration allocation sequence in certain period were determined to meet resource allocation requirement on clear status, quantitative potential and feasible technology for various plays. This evaluation method has the characteristics of universality and unique evaluation result.

Co-evolution characteristics of organic matter and reservoir in continental shale: a case study of Shahezi Formation in Changling Faulted Depression, Songliao Basin

ZHOU Zhuoming

2023, 45(2): 243-251. doi: 10.11781/sysydz202302243

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In the Songnan Faulted Depression Group of the Songliao Basin, continental shale gas has huge exploration potential. However, evolution regularities of both organic geochemistry and pore characteristics in organic shale containing type Ⅲ kerogen in this area are not made clear. According to thermal evolution simulation experiments on hydrocarbon generation and expulsion and the tests and analysis on a series of reservoir characteristics in organic shale samples containing type Ⅲ kerogen from the Lower Cretaceous Shahezi Formation, Changling Faulted Depression, it was found that there were two key nodes of thermal evolution in the shale, namely, equivalent vitrinite reflectance (R_o) of 1.5% and 2.0%, respectively. In the stage of equivalent R_o from 0.7% to 1.5%, it was the stage of fast oil generation and expulsion, in which pore shapes were mainly ink bottle and partially plate, few pores and caverns were developed gradually in organic matters, and mineral dissolution pores occurred. In the stage of equivalent R_o from 1.5% to 2.0%, it was the stage of rapid gas generation, in which a large amount of organic pores and inter-mineral pores were developed with gradually increasing pore size by organic matter decomposition, and the pores were mainly platy or wedge-like and a few pores were ink bottle shaped. In the stage of equivalent R_o higher than 2.0%, only platy or wedge-like pores were developed with further increasing size. The stage with R_o higher than 1.5% is favorable for co-evolution of source rocks and reservoir of shale gas accumulation in the Songnan Faulted Depression Group.

Evolution of sedimentary environment of the Lower Cambrian Xishanbulake-Xidashan formations in the Tarim Basin

GUO Tingting, ZHU Bi, YANG Tao, CHEN Yongquan

2023, 45(2): 252-265. doi: 10.11781/sysydz202302252

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The Cambrian is a critical period in the evolution of life and environment. Reconstructing the marine sedimentary environment, notably redox conditions during this period is key to explore the spatial and temporal evolutionary characteristics of seawater chemistry and the relationship between biological evolution and environmental changes. In this study, the major elements, trace elements and organic carbon content (TOC) of the Lower Cambrian Xishanbulake-Xidashan formations in well Tadong 2 in the Tarim Basin were analyzed, and the sedimentary environment of the formations was reconstructed. The results show that the water body was in weak restriction condition when the formation was deposited. The redox state of bottom waters was dominated by anoxia, but the degree of anoxia varied obviously. Specifically, the middle part of the Xishanbulake Formation showed elevated anoxia (euxinia) in bottom waters compared to the lower and the upper parts, while the Xidashan Formation showed a decrease in anoxia from the middle and the lower parts to the upper part and an expansion of bottom water oxidability. The results are consistent with the observation in previous studies that the Early Cambrian seawaters was characterized by dynamic changes in the redox state. The analysis of organic matter enrichment mechanism of Xishanbulake-Xidashan formations in well Tadong 2 shows that organic matter enrichments in the formations is not controlled by a single factor. The anoxic/euxinic environment in the middle and the lower parts of the Xishanbulake Formation is more conducive to the preservation of organic matter, while the higher level of primary productivity played a key role in the sedimentation of the upper part of the Xishanbulake Formation and the Xidashan Formation.

Depositional model and petroleum significance of the Cretaceous Yageliemu Formation in Xinhe area on the southern slope of Kuqa Depression, Tarim Basin

ZHOU Xuewen, LIN Huixi, GUO Jingxiang, YAO Wei, ZHANG Tan, ZHOU Xuehui, LUO Liang

2023, 45(2): 266-279. doi: 10.11781/sysydz202302266

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In recent years, drilling in Xinhe area on the southern slope of Kuqa Depression, Tarim Basin revealed that the Cretaceous Yageliemu Formation has active hydrocarbon display and high exploration potential in the field of stratigraphic-lithologic traps, which is an important direction for the next strategic breakthrough. Based on the core, well logs, drill cuttings and 3D seismic data, this paper integrated the analytical methods of core facies, electrofacies, wavelet transform, "impression method" paleogeomorphic restoring, seismic facies, seismic attributes, and coherence cube slicing to study the depositional system of the Cretaceous Yageliemu Formation in Xinhe area under the sequence stratigraphic and paleogeomorphic controls. The study shows that there are two types of paleogeomorphology developed in Xinhe area: (ⅰ) western multi-stage slope breaks, and (ⅱ) eastern thrust cliff paleogeomorphology. The Cretaceous Yageliemu Formation can be classified as a third-order sequence SQ1, which includes the lowstand systems tract (LST), transgressive systems tract (TST) and highstand systems tract (HST), corresponding to the three-phase seismic reflections of this formation overlying the western slope, respectively. The LST slope fans and HST fan deltas of the Yageliemu Formation were deposited above the western multi-stage slope breaks, and TST-HST subaqueous nearshore fans were deposited below the eastern thrust cliff, with erosional valleys on the cliff serving as the feeder channels and providing sources for the subaqueous fans. In situ mature hydrocarbon source rocks of the Triassic Karamay Formation are developed in Xinhe area, which is the migration direction of oil and gas far away from hydrocarbon source, indicating excellent source rocks and reservoir forming conditions. The stratigraphic-lithologic traps of the LST slope fan and TST-HST subaqueous nearshore fan located in the northern part of the thrust cliff are lying on the Triassic source rocks and have great exploration potential, which can serve as the main targets for the next subtle oil and gas exploration in this area.

Evolution and resource potential of Permian source rocks in the southern depression of the South Yellow Sea Basin

CAO Qian, LI Haihua, SHAN Shuaiqiang, QI Jiazhen, LI Fengxun, WANG Bin, HAN Yu

2023, 45(2): 280-287. doi: 10.11781/sysydz202302280

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The Permian source rocks in the southern depression of the South Yellow Sea Basin have been proved to be fair-good source rocks. Based on the analysis of recent Paleozoic oil and gas exploration and research results, the thermal evolution and hydrocarbon generation process of Permian source rocks was analyzed by selecting appropriate geological parameters and using TSM basin simulation method, and then the potential of oil and gas resources was evaluated. The research shows that the Permian source rocks distributed in Cenozoic fault depression in the southern depression experienced a two-stage hydrocarbon generation and expulsion process. The first hydrocarbon generation and expulsion occurred from the Late Triassic to the Early-Middle Jurassic and the secondary hydrocarbon generation and expulsion mainly occurred in Paleogene. However, the Permian source rocks distributed in the uplift area only experienced the first hydrocarbon generation and expulsion process. The Permian source rocks have a total resource scale of 20.76×10⁸ t and a resource abundance of 12.2×10⁴ t/km², reaching a medium abundance level, which can provide a certain material basis for the formation of oil and gas fields. The secondary hydrocarbon generation resources of the Permian source rocks in the whole southern depression account for 30.4% of the total resources, but it can reach 55% in the Cenozoic fault depression. The Cenozoic fault depression with Permian source rocks and Cenozoic source rocks developed is a favorable exploration area.

New understanding on development conditions of lacustrine source rocks and characterization of high-quality source rocks in Songxi Sag, Qiongdongnan Basin

FU Dawei, HU Desheng, SUN Wenzhao, GONG Liyuan, HE Xiaohu, CHEN Kui, LEI Xin, WAN Yang

2023, 45(2): 288-295. doi: 10.11781/sysydz202302288

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With respect to the problems of unclear development mechanisms of source rocks and distribution of high-quality source rocks in the Songxi Sag, Qiongdongnan Basin, medium-deep lacustrine source rocks in eastern and western subsags of the Songxi Sag were predicted, according to geochemical characteristics of crude oil from drilled wells, sedimentary characteristics of these subsags, the characteristics of seismic facies and attributes, and comparison and analysis of seismic velocity spectrum. In this study, it was defined that the following three factors, namely, subsidence rate of the Songxi Sag, injection points in the provenance, and sag structure, coupled and controlled the development and distribution of lacustrine source rocks. Western subsag of the Songxi Sag was adjacent to large-scale injection points in the provenance as sand-rich sediments, and then led to low paleo-productivity level, high depositional rate, and open and oxidized water body setting. It had no fundamental conditions for the formation of high-quality source rocks. Eastern subsag was far from these large-scale injection points with the development of only small sand bodies, which was favorable for the development of medium-deep lacustrine source rocks. Geochemical indexes of crude oil from drilled wells indicated that the crude oil in the study area came from medium-deep lacustrine source rocks, which confirmed the occurrence of good source rocks in the Songxi Sag. Eocene medium-deep lacustrine source rocks had seismic characteristics of low frequency, continuous and strong reflectance in addition with low velocity anomalies that were mainly distributed in the center of eastern subsag and gentle slope zone. Above-mentioned understandings could guide well allocation in eastern subsag. Eocene oil shale was firstly encountered in the wells, Qiongdongnan Basin. These source rocks had good indexes and enhanced oil exploration prospect in the Songxi Sag, Qiongdongnan Basin.

Gas-bearing characteristics and major controlling factors of shallow marine shale: a case study of the Lower Silurian Longmaxi Formation in Taiyang block of Zhaotong area

LI Juan, CHEN Lei, JI Yubing, CHENG Qingsong

2023, 45(2): 296-306. doi: 10.11781/sysydz202302296

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The exploration and development of shallow shale gas has the advantages of low cost and fast production reduction, so it has attracted wide attention. The shale gas in the Lower Silurian Longmaxi Formation (S₁l) in Taiyang block of Zhaotong National Shale Gas Demonstration Zone has the characteristics of shallow burial depth and high gas content, showing good exploration potential. However, the gas-bearing distribution and major controlling factors of shallow shale gas in this research area are rarely studied previously. In order to enrich the research in this field, this paper analyzes the gas-bearing characteristics and the major controlling factors of the shale samples of Long-1 subsection (S₁l₁) in the Taiyang block. The results show that the total gas content of S₁l₁^1-1 is the highest in the study area, the gas content gradually decreases from bottom to top, and areas with high shale gas content in S₁l₁ appear in the north and south of the study area on the plane. Organic matter content, mineral components, reservoir properties and external preservation conditions have a certain role in controlling the gas properties of shallow shale in S₁l₁¹ subsection of the study area. Organic matter is the primitivesubstance that generates shale gas, at the same time, the organic matter hole formed by hydrocarbon cracking sprovides a lot of storage pace for shale gas, while the content of organic matter is the major controlling factor of the total gas content. The high content of silicoide is conducive to later fracturing development. The adjacent rock strata of S₁l₁¹ subsection in the study area are compact in lithology, with good sealing property, showing the characteristics of slight overpressure and overpressure, which effectively prevents the escape of shale gas, and has broad exploration prospects.

Characteristics and controlling factors of volcanic clastic rock reservoirs in Wujiaping Formation of Upper Permian in northern Sichuan Basin

XIA Wenqian, ZHU Xiang, JIN Mindong, ZHANG Lei, LIU Yanting, ZHANG Siyao

2023, 45(2): 307-316. doi: 10.11781/sysydz202302307

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Under the influence of the Emei Taphrogenesis, a set of volcanic clastic rocks were developed in the Wujiapingian age of Permian in the northern Sichuan Basin. The lithology is highly heterogeneous, and the pore structure of reservoir is complex. In order to further clarify the characteristics and formation mechanism of volcanic clastic rock reservoirs in the Wujiaping Formation and confirm the exploration potential, this paper analyzes the reservoir characteristics and formation mechanism of volcanic clastic rock reservoirs in the Wujiaping Formation in the northern Sichuan Basin based on outcrop and actual drilling data and combining with core section observation, geochemistry analysis, argon ion buffing scanning electron microscope, combined determination of cap micro pores and other methods. It is concluded that the volcanic clastic rocks can be divided into three rock types: tuffaceous mudstone, argillaceous tuff and sedimentary tuff. The reservoir space is dominated by nano-scale clay mineral shrinkage pore and organic matter pore and micron-scale organic acid dissolution pore and micro fracture. The reservoir physical property is characterized by medium to high porosity and ultra-low permeability, and the tuffaceous mudstone has the best physical property among the three rock types. The analysis of the controlling factors of the reservoir shows that the near-crater slope and shelf environment is favorable for the development of volcanic clastic rock reservoirs, and the devitrification of volcanic ash and secondary organic acid dissolution are the main causes for the formation of pores in the reservoir.

Characteristics and controlling factors of heavy oil distribution in Liaohe Depression, Bohai Bay Basin

CHEN Xingzhou, SHAO Jianxin, SUN Zhuan, HAN Hongwei, GUO Qiang, YIN Yipeng, SUN Xinyu

2023, 45(2): 317-326. doi: 10.11781/sysydz202302317

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There are various types of crude oil in the Liaohe Depression, Bohai Bay Basin, among which heavy oil accounts for more than 50% of the oil and gas reserves and production. Therefore, it is of great significance for future exploration and development to study the main controlling factors of oil densification in the Liaohe Depression and to understand the distribution pattern of heavy oil. Based on the systematic understanding of heavy oil reservoir exploration and development in Liaohe oilfield for decades of years, and by aiming at the geological characteristics of heavy oil reservoirs such as "edge, shallow, unconsolidated, early, falling, loss, capping", this paper discusses the distribution and densification reasons of heavy oil from six aspects, including structural pattern, reservoir characteristics, reservoir burial depth, underground fluid, capping conditions, oxidation and biodegradation. The densification mechanism and controlling factors of heavy oil reservoirs in the Liaohe Depression are basically clarified. Through the study of heavy oil densification factors, it is found that structural pattern is the main factor controlling heavy oil distribution, geological conditions such as reservoir, burial depth, underground fluid and cap rocks are important factors controlling heavy oil densification, while biodegradation and oxidation are the keys for heavy oil densification. Oil and gas reservoirs accumulate around sags in the Liaohe Depression. There are three rings around the oil-generating sags from the inside to the outside. The inner zone is dominated by natural gas and condensate gas, and the buried depth is generally more than 3 500 m. The middle zone is dominated by light oil, partially enriched with natural gas and heavy oil, with a buried depth of 1 700-3 500 m. The outer zone is dominated by heavy oil, and the buried depth is generally less than 1 700 m.

GC×GC-TOFMS analysis of ethanodiamondoids in Ordovician oil from well SN1, Tarim Basin

YU Xiao, MA Anlai, LI Xianqing, ZHU Xiuxiang, FEI Jianwei

2023, 45(2): 327-337. doi: 10.11781/sysydz202302327

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By 2D gas chromatography and time-of-flight mass spectrometry (GC×GC-TOFMS), 81 ethanodiamondoids with 1-3 cages were detected quantitatively in Ordovician crude oil from well SN1, Shuntuoguole area, Tarim Basin, including 47 ethanoadamantanes with total content of 27 594.0 μg/g, 32 ethanodiamantanes with total content of 4 415.1 μg/g, and 2 ethanotriamantanes with total content of 16.8 μg/g. The template of 2D chromatogram retention index of diamondoids-ethanodiamondoids was constructed. The results demonstrated that the position of retention time of diamondoids and ethanodiamondoids has the following relationship, namely, adamantanes < ethanoadamantanes < diamantanes < ethanodiamantanes < triadamantanes < ethanotriamantanes < tetradamantanes. Quantitative analytical result of ethanoadamantanes as saturated hydrocarbons with the highest thermal stability in crude oil, is expected to provide new indexes for secondary reservoir reconstruction, i.e. crude oil cracking and thermochemical sulfate reduction (TSR).

Geochemical characteristics and hydrocarbon generation potential of Lucaogou Formation source rocks in Fukang Sag, Junggar Basin

LIU Chaowei, YOU Xincai, LI Hui, LI Shubo, CHEN Hong, WANG Zesheng, CHEN Mengna, LI Zonghao

2023, 45(2): 338-346. doi: 10.11781/sysydz202302338

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As the largest hydrocarbon-rich sag in the Junggar Basin, the limited research on geochemical characteristics, sedimentary environment and bio-precursors of source rocks in the Fukang Sag has seriously restricted the understanding of the poor proven oil and gas resources in the sag. A combined geological and geochemical analysis on the Lucaogou Formation source rocks in the Fukang Sag was conducted based on the experimental analysis results of the newly drilled source rock samples on the Fudong Slope. The results show that the Lucaogou Formation source rocks in the Fukang Sag generally belong to source rocks of medium to very good quality, with organic matter of type Ⅱ to type Ⅲ, and mainly in the thermal maturity of early mature to mature stage. Biomarker characteristics show that the organic matter of Lucaogou Formation source rocks in the Fukang Sag is a mixture of terrestrial higher plants and aquatic algae and bacteria and maybe some Dunaliella-like green algae, which is generally deposited in the anoxic to oxic, fresh to brackish, intermittent stratified water environment, showing obvious mudstone sedimentary characteristics. The geochemical characteristics of the Lucaogou Formation source rocks in the Fukang Sag vary greatly on the plane, among which, the central and northern subsags show great hydrocarbon generation potential.

Reason for abnormally high drying coefficient of natural gas in Cainan area, Junggar Basin

LI Erting, JIN Ruihan, LIU Xiangjun, LI Ji, ZHANG Yu, HU Wenxuan, WANG Haijing

2023, 45(2): 347-355. doi: 10.11781/sysydz202302347

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In order to clarify the reason for generally high drying coefficient of natural gas in Cainan area of the Junggar Basin and find out the law of natural gas migration and accumulation, the analysis of natural gas components and carbon isotopes, rock mineral composition in reservoir, bulk carbon and oxygen isotope of calcite, and laboratory hydrocarbon oxidation simulation experiments were carried out. The Jurassic natural gas in Cainan area is dominated by methane, with drying coefficient of generally greater than 0.95, and δ¹³C₁ value of basically greater than -32‰. Among C₇ light hydrocarbons, methylcyclohexane is dominant, with methylcyclohexane index of greater than 50%, indicating that natural gas in the study area came from high-over mature Carboniferous source rocks. Judging from natural gas migration identification index of ln(C₁/C₂) with δ¹³C₁-δ¹³C₂, from well block Cai-47 to well block Cai-31, and then to well block Cai-003, ln(C₁/C₂) values gradually increased, but δ¹³C₁-δ¹³C₂ values did not show a trend of decreasing or increasing, indicating that migration or maturity is not the main controlling factor for the changes of natural gas composition and carbon isotope in the study area. Hydrocarbon thermal oxidation simulation experiments showed that alcohols in oil and gas were oxidized by MnO₂ to generate methane and carbon dioxide at 125℃, and methane could only be oxidized to generate CO₂ when the temperature reached 200℃, thus changing the composition of oil and gas and increasing methane content in natural gas. Using backscattered electron probe technology, it was found that there are two types of calcite in Jurassic dry gas interval in the study area. One type of calcite has a high Mn content, which can be as high as 3%. It appears bright orange and orange under cathodoluminescence. In addition, the bulk carbon isotope of calcite is negatively biased, which is 5‰-10‰ more negative than that of normal calcite. The higher content of Mn in calcite, the more negative of bulk carbon isotope, confirming that there is widespread weak oxidation of hydrocarbons in the Jurassic dry gas interval in Cainan area. Comprehensive analysis suggested that the reason for abnormally high drying coefficient of Jurassic natural gas in Cainan area is that the humic source rocks have undergone high-over mature evolution to generate natural gas that accumulated in Jurassic reservoirs rich in oxidizing minerals. Then the oxidation of hydrocarbons caused methane content in natural gas to increase further, resulting in a generally high drying coefficient of natural gas.

Basic characteristics and genesis analysis of shale oil in the second member of Paleogene Funing Formation in Qintong Sag, Subei Basin

ZAN Ling, BAI Luanxi, YIN Yanling, ZHANG Wanlu

2023, 45(2): 356-365. doi: 10.11781/sysydz202302356

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The continental shale oil in the second member of Paleogene Funing Formation in the Qintong Sag, Subei Basin has the characteristics of high and stable production. Based on the analysis of the formation conditions of shale oil, the genesis of shale oil is revealed according to crude oil geochemical characteristics and physical properties. Results show that the mud shale in the second member of Funing Formation in the Qintong Sag is widely distributed and thick, with medium organic matter abundance and moderate thermal evolution, which provides a good material basis for the formation of shale oil. The shale oil in the second member of Funing Formation has the characteristics of low gas/oil ratio and high pressure, which belongs to low sulfur and light to medium oil, with high content of saturated hydrocarbon and light hydrocarbon, and the associated gas of crude oil is mainly methane. The shale oil has high content of β-daucane and gammacerane, and its sterane content of ααα-C₂₉ (20R) is higher than that of ααα-C₂₇ (20R), indicating that it is formed in the reduction environment of salt water. Thermal evolution is the key factor controlling the quality of continental shale oil. The second member of Funing Formation in the deep sag zone is at the peak of hydrocarbon generation, and the shale oil has good fluidity. The mud shale in the Ⅰ and Ⅱ submembers of the second member of Funing Formation generally contains suberinite (6.6%) and amorphous body of benthic algal (11.5%), and can generate oil and gas at the mature stage, which is conducive to the formation of light components. The mud shale in the second member of Funing Formation has the characteristics of moderate organic matter abundance and mineral composition, and its oil adsorption capacity is poor, making the remaining oil mainly free oil (69%-96%).

On-line microscopic imaging investigation on oil charging characteristics in tight reservoirs

JIANG Wenbin, LIN Mian, JI Lili, CAO Gaohui, ZHANG Likuan, DOU Wenchao, ZHENG Siping, CHEN Zhuo, QIU Xin

2023, 45(2): 366-377. doi: 10.11781/sysydz202302366

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Abstract:
The reservoir space of tight reservoir with low-permeability is controlled by micro and nano scale pores, making the influence of capillary force significantly enhanced. Therefore, understanding the microscopic charging characteristics of oil and gas is the basis for analyzing the migration and accumulation of reservoirs. In this paper, the self-developed on-line three-dimensional microscopic imaging system for core fluid displacement is used to observe the oil charging process of two tight reservoir samples, and core-level and pore-level quantitative analysis methods for oil content characteristics are proposed. Taking the on-line nuclear magnetic resonance testing with the same process of displacement as a contrast, it is revealed that the average difference of on-line 2D DR (Digital Radiography) images at different times can be used to evaluate the overall oil content change of the sample. The calculation method of pore level fluid saturation based on high-precision pore network extraction algorithm realizes the quantitative evaluation of oil charging degree of the CT resolved pores and pore throats. The combination of multi-level data and different methods can meet the different needs of different researches on dynamic feature capture, pore resolution and imaging field of vision. The analysis results show that the oil saturation of two rock samples from different tight reservoirs in the Ordos Basin increases rapidly at the beginning and slows down later with oil injection increasing. At the same injection flow rate, the oil saturation of the sample with higher permeability increases faster at the initial oil charging stage, making its final oil saturation higher. With the increase of oil injection, the oil saturation of macropores in the sample with higher permeability continuously increases, while that of macropores in the sample with lower permeability shows a U-shaped change, showing the characte-ristics of repeated occupation of pores by oil and water.

Effects of magnetic field intensity and gradient on measurement results of core nuclear magnetic resonance T₂ spectrum

LIU Yang, ZHANG Gong, QIN Yingyao, ZHANG Jiacheng, LI Sen

2023, 45(2): 378-384. doi: 10.11781/sysydz202302378

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Abstract:
Nuclear magnetic resonance(NMR) logging instruments generally measure the NMR signal of reservoir fluid in a uniform field with a resonance frequency of 2 MHz or a gradient field with a resonance frequency of less than 1 MHz. For laboratory NMR core analyzers, in addition to the commonly used 2 MHz, the resonance frequency of 12 MHz or 21 MHz is often used for experimental measurement of tight reservoirs such as shale. In order to determine the effects of magnetic field intensity and gradient on the NMR measurement results, the sensitivity of the magnetic field intensity and gradient of the core experiments of glutenite and shale cores under water-saturatedstate was systematically studied, and the relationship between the T₂ spectrum shape, position, nuclear magnetic porosity, T₂ geometric mean and the magnetic field intensity and gradient of different rock samples was analyzed. The experimental results show that under the uniform field, glutenite samples are very sensitive to the change of magnetic field intensity, while shale samples are less sensitive. The existence of external gradient field will make the short relaxation information of glutenite and shale missing, resulting in that the NMR signal cannot be measured completely. When the NMR core experiment is used to calibrate the interpretation parameters of NMR logging, the experimental measurement results must be corrected if there is a large difference in the magnetic field intensity or gradient between the laboratory NMR core analyzer and the NMR logging instrument.

Stress sensitivity characteristics and influencing factors of different types of sandstone reservoirs in gas storage

LI Meng, ZHENG Dewen, QIU Xiaosong, LIU Mancang

2023, 45(2): 385-392. doi: 10.11781/sysydz202302385

Abstract(223) HTML (83) PDF-CN(39)

Abstract:
The storage capacity and productivity of gas storage are affected by many factors, among which porosity and permeability are the main factors. To reveal the variation characteristics of porosity and permeability of different types of sandstone reservoirs in gas storage, five different types of sandstone samples were collected from the S gas storage in the Liaohe Depression, Bohai Bay Basin, and stress sensibility comparative experiments were carried out. The results show that the porosity and permeability decrease with the increase of effective stress, which affects the storage capacity and productivity of the gas storage. The porosity and permeability damage rate can be used to characterize the damage degree of the storage capacity and productivity. A damage factor characterizing storage capacity and productivity was proposed for the first time, which can be used to quantitatively evaluate storage capacity and productivity of gas storage under alternating load conditions. The porosity and permeability damage rate of the S gas storage increase linearly with the increase of effective stress. The porosity stress damage rate of the argillaceous siltstone reservoir is the largest, and the porosity stress damage rate of the medium-grained sandstone reservoir is the lowest. The argillaceous siltstone reservoir also has the largest permeability damage rate and its own permeability is too low, so it contributes less during emergency supply assurance. The medium-grained sandstone reservoir has the smallest permeability damage rate, and it has little influence on the capacity of peak regulating and supply assurance. Through this study, the quantified damage degree of stress to gas storage capacity was determined. According to the damage rate of different types of reservoirs, the storage capacity and productivity of gas storage can be maximized by optimizing the operation condition of gas storage reasonably.

Femtosecond Laser Microetching Technology

2023, 45(2): 393-393.

Abstract(287) PDF-CN(57)

Abstract:

2023 Vol. 45, No. 2

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