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2023 Vol. 45, No. 1

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2023, 45(1): .
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Differential mechanism of tight oil enrichment and reservoir controlling model of source and reservoir combinations: a case study of Qingxi Sag, Jiuquan Basin
LUO Qun, HONG Lan, GAO Yang, DENG Yuan, LI Yingyan, WANG Shichen, MA Wenyu, XU Qian
2023, 45(1): 1-10. doi: 10.11781/sysydz202301001
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The quality of source and reservoir rocks as well as their spatial assemblage types are crucial factors that controlling the charging, accumulation, and enrichment of tight oil resource. However, there are still deficiencies in the understanding about these factors, especially regarding differences in the mechanism of charging, accumulation, and enrichment of tight oil between different source-reservoir assemblage types, which restricts the formation of theory as well as the process of exploration and development. Taking the Qingxi Sag of the Jiuquan Basin as an example, we have drawn conclusions by field observations, core descriptions, and drilling analyses that there may be eight types of source-reservoir assemblages. Among these, there are three basic types, namely, the lower-generating and upper-reserving type, upper-generating and lower-reserving type, and sandwich-like type. In addition, there are two special types, the self-generating and self-reserving type and source-reservoir interbedding type. Moreover, the other three types are composite types, which are the lowergenerating and upper-reserving type Ⅱ, upper-generating and lower-reserving type Ⅱ, and sandwich-like type Ⅱ. Among the above mentioned eight types, the first five types are common. The results of statistical drilling and nuclear magnetic resonance experiments mutually verify the corresponding relation between source-reservoir assemblage type and tight oil enrichment. The assemblage types can be ranked according to their positive correlation with enrichment in the following order, from the highest to the lowest: self-generating and self-reservoiring type, sandwich-like type, upper-generating and lower-reservoiring type, lower-generating and upper-reservoiring type, and source-reservoir interbedding type. Differences in the mechanism of charging, accumulation, and enrichment of tight oil among the five common source-reservoir assemblage types are revealed and explained. In addition, their accumulation and enrichment patterns are determined and principal controlling factors are proposed, namely, accumulation dynamics, charging patterns, and storage conditions, which are in fact closely related to the source-reservoir assemblage types.
Formation mechanism of high-quality reservoirs in deep strata of Paleogene, Bonan Subsag, Zhanhua Sag, Bohai Bay Basin
WANG Yongshi, ZHANG Shun
2023, 45(1): 11-19. doi: 10.11781/sysydz202301011
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With the continuous improvement of exploration degree, the main object for oil and gas exploration in the Bonan Subsag of Bohai Bay Basin gradually changes from medium and shallow layers to medium and deep layers. The key factor for deep exploration is the development of high-quality reservoir, which depends on the interaction between sedimentary filling and formation fluid. During the deposition of the fourth member of Paleogene Shahejie Formation, lacustrine clastic rocks and gypsum salt deposits developed in this area under a saline environment. Based on the comprehensive study of drilling data and experimental results, the sedimentary environment, formation water characteristics, reservoir petrographic characteristics and diagenetic types of the Paleogene reservoirs in the Bonan Subsag were discussed, the development and evolution mechanism of the deep and high-quality reservoirs in Paleogene was also indicated. Results show that: (1) Under the sealing of gypsum salt layer with wide distribution area and large sedimentary thickness, the clastic rocks of Es4 member in salty environment experienced alkaline diagenetic environment in the early stage. The carbonate cements formed in this period filled primary pores and effectively prevented the occurrence of compaction. (2) With the increase of the maturity of organic matters in source rocks, significant organic acid migrated to stratum driven by abnormal high pressure during the middle diagenesis stage. The early carbonate cements, feldspar and debris were generally dissolved and secondary pores were subsequently formed. Acidic and alkaline reworks are the main mechanism for the development of deep high-quality reservoirs. (3) The development of high-quality reservoirs was controlled by the distribution of gypsum salt layer, the development of source rock, fault system and the distribution of clastic sedimentary body. The front sand bodies of gentle-slope fan delta and steep-slope nearshore subaqueous fan strongly influenced by acid and alkali fluid are the high-quality reservoir development area of Es4 member in Bonan Subsag.
Differences in marine hydrocarbon accumulation between the eastern and western parts of Shunbei area, Tarim Basin
QI Lixin, DING Yong
2023, 45(1): 20-28. doi: 10.11781/sysydz202301020
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From the eastern to the western part of Shunbei area in the Tarim Basin, the accumulations of marine oil and gas obviously decrease with the exploration risks significantly increasing. To find out the reason for the differences, the oil and gas accumulations in the eastern Shunbei area on the western margin of Manjiaer Depression and the western Shunbei area on the eastern margin of Awati Depression were analyzed, and the differences of accumulation conditions were discussed. In recent years, the eastern part of Shunbei oil and gas field, which is represented by the northeastern section of Shunbei No.1, 4 and 8 belts, has continuously achieved high yield with multiple stages of oil and gas accumulations and obvious oil and gas enrichment characteristics. While in the western part of Shunbei area, only early-stage accumulations with small scale were discovered. There are differences in the distribution conditions of source beds, hydrocarbon generation evolution history, source resource scale, reservoir-forming period and reservoir-forming mode between the eastern and western parts of Shunbei area, which leads to multi-source and multi-stage accumulation in the east and single-source and single-stage accumulation in the west. Adjacent to the Manjiaer hydrocarbon-generating depression, the eastern part of Shunbei area has favorable accumulation conditions such as pathways for vertical migration, targets for hydrocarbon migration of various sources, qualified cap rocks and lateral sealing, and multiple stages of hydrocarbon generation with strong intensity, resulting in multi-source and multi-stage accumulation. While in the western part of Shunbei area, there is single-source and single-stage accumulation sourced from either local Cambrian Yurtus Formation or Ordovician in the Awati Depression. Breakthroughs in Ordovician in the Yingmaili Oilfield on the eastern margin of Awati Depression and well Yingxi 1 in recent years indicate that the accumulation of marine oil and gas mainly took place in the late Hercynian period, and partially in Himalayan period. Generally speaking, the development of source rocks around the Awati Depression is relatively poorer, and the distribution of marine oil and gas as well as the scale of hydrocarbon resources are limited.
Discussion on prospecting shale oil potential of Mesoproterozoic Hongshuizhuang Formation in the Jibei Depression
LI Zhiming, SUN Zhongliang, BAO Yunjie, XU Ershe, ZHOU Yongshui, ZHANG Yunxian
2023, 45(1): 29-40. doi: 10.11781/sysydz202301029
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In order to evaluate the shale oil prospecting potential of the Mesoproterozoic Hongshuizhuang Formation in the Jibei Depression, the analyses of routine Rock-Eval pyrolysis, matrix porosity and permeability, mercury intrusion and nitrogen adsorption, argon ion polishing SEM and X-ray diffraction of bulk rock of typical core samples were carried out in this study. The shale oil forming conditions of the Hongshuizhuang Formation were studied, the shale oil resource was assessed, and the prospecting potential was discussed based on previous results. The results show that the lagoon facies black shale in the middle section of the Hongshuizhuang Formation is mainly 35-55 m thick with organic matter types of Ⅱ1 and Ⅱ2, and is a set of good- to excellent-quality source rock. The equivalent vitrinite reflectance of organic-rich black shale is 0.80%-1.25%. The matrix porosity is mainly 1.0%-8.0%, and the matrix permeability is (2-100)×10-6 μm2. The pores are dominated by inorganic pores and fissures with the diameter of pore larger than 50 nm. The values of S1 after recovery is mainly 1.60-6.50 mg/g, and the content of residual hydrocarbon gas distributes 0.13-0.24 m3/t, indicating good hydrocarbon potential. The average content of brittle minerals is 61.9%-72.3%, with good frangibility. The shale oil geological resource is about 12×108 t, and the shale oil recoverable resource is about 0.72×108 t for organic matter rich shale of the Hongshuizhuang Formation. These suggested that the organic matter rich shale of the Hongshuizhuang Formation in the Jibei Depression has a resource basis for shale oil exploration with good prospecting potential.
Classified characteristics of helium gas resources and controlling factors for the enrichment
CHEN Xinjun, DING Yi, YI Jingjing, CHEN Gang
2023, 45(1): 41-48. doi: 10.11781/sysydz202301041
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The lack of helium resources accounts for the limited helium supply in China. It is of great significance to classify of helium-enriched natural gas according to the characteristics of helium reservoirs and the main controlling factors of helium enrichment, thus the exploration and development of helium can be promoted and the safety of helium use in China may be ensured. Based on the origin of helium, this paper classifies the helium reservoirs by the consideration of the abundance of helium resources, the spatial horizon of uranium and thorium radioactive elements, the types of carrier gas reservoirs, and the main controlling factors of helium accumulation. The helium gas resources can be characterized into two types: crust-mantle mixed type and crust-source type. Crust-source type can be further classified into crust-source far-source type (including hydrocarbon associated type and water-soluble gas type) and crust-source near-source type (shale gas type) with different characteristics. In terms of helium reservoirs discovered in China, only crust-source helium which was formed together with hydrocarbon gas reservoirs has industrial utilization value due to the great resource potential, although its resource grade is relatively low. The main controlling factors of helium accumulation in the crust-source far-source type and crust-source near-source type are roughly the same, but there are differences in helium sources and migration pathways.
Lithofacies palaeogeography of Middle Permian in the Sichuan Basin and its petroleum geological significance
PENG Jinning, DU Chongjiao, LI Longlong, JIANG Xiaoqiong, PAN Wenlei, LU Yongde, LUO Kaiping
2023, 45(1): 49-59. doi: 10.11781/sysydz202301049
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Exploration discoveries have been made in the Middle Permian of the Sichuan Basin, indicating great exploration potential. However, the Middle Permian has obvious tectonic-sedimentary variation, resulting in complexity of hydrocarbon accumulation mechanism. The reconstruction of sequence lithofacies paleogeography is of great significance to understand the tectonic-sedimentary evolution, sedimentary formation and hydrocarbon exploration. In this paper, the lithofacies palaeogeography during the Middle Permian Qixia-Maokou period in the Sichuan Basin was reconstructed with the third order sequence as the minimum longitudinal mapping unit based on oil and gas exploration results as well as typical drilling and outcropping data. Five third-order sequences for the Middle Permian in the Sichuan Basin were supposed in this study including: SQq1 (from the Liangshan Formation to the section B of the first member of the Qixia Formation), SQq2 (from the section A of the first member to the second member of the Qixia Formation), SQm1 (from the first member to the section C of the second member of the Maokou Formation), SQm2 (from the section B to the section A of the second member of the Maokou Formation), and SQm3 (from the third to the fourth member of Maokou Formation). During the deposition of SQq1 and SQm1, transgression and filling were dominant, and spot and banded beach bodies in some palaeo-highlands of the basin were discovered. During the deposition of SQq2, SQm2 and SQm3, continuous regression and geomorphic differentiation were dominant, the carbonate platforms depositional system with edge were developed in the margin parts of the basin, and large scale multi-stage and multi-zone beach bodies could be formed in the platform. SQq2 was the main shoal-forming stage in the Middle Permian. The relatively long regressive process [JP+1]and local block uplift promoted the formation of large scale multi-stage and multi-zone shoals in the platform margin of the western Sichuan Basin and the southern part of the central Sichuan Basin. During SQm3 period, the sedimentary differentiation in the northern Sichuan Basin was obviously intensified, and it was transformed into an alternate uplift and sag pattern with platform margin, slope and shelf. The platform margin and intra-platform shoal of the HST during the deposition of SQq2 and SQm3 provided material basis for dolomitization, dissolution and other later constructive diagenetic transformation. Therefore, the delineation of slope break zone geomorphology and the prediction of platform margin and intra-platform shoal should be regarded as the focus of further exploration.
Controlling effect of Late Paleozoic tectonic and sedimentary differentiation on multi-type gas reservoirs in Permian, Sichuan Basin
LI Longlong, LUO Kaiping, LIU Xu, ZHANG Changjiang, CAO Qinggu, LU Yongde, PENG Jinning
2023, 45(1): 60-71. doi: 10.11781/sysydz202301060
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Based on the analysis of the Late Paleozoic global tectonic background and the tectonic environment of the Upper Yangtze area, the basin prototype and the Permian lithofacies paleogeography of the Sichuan Basin, combined with the understanding of the Permian gas reservoir characteristics and accumulation conditions, the controlling effect of the Late Paleozoic tectonic and sedimentary differentiation on the Permian multi-type gas reservoirs in the Sichuan Basin is discussed. Results showed that the tectonic differential uplift or subsidence under the extensional background formed the palaeogeomorphology of alternating uplift and depression, which led to the evolution of the sedimentary pattern from the Middle Permian to the Late Permian: platform alternating with platform and depression in the early stage, and platform alternating with rift trough (shelf) in the late stage. Multiple sets and types of source rocks formed in the platform sag, tidal plane-lagoon, slope, shelf-basin and other facies, sub-facies or micro-facies, and multiple sets and types of reservoirs controlled by beaches, platform edge sand and later diagenesis transformation constituted a variety of source-reservoir combinations such as lower generation and upper storage, side generation and side storage, self-generation and self-storage etc. These source-reservoir combinations constitute the basic conditions for hydrocarbon accumulations in Permian. Multiple sets and types of reservoirs are the fundamental reasons for the wide distribution of Permian gas-bearing strata and various gas reservoir types. The Permian strata has favorable conditions for comprehensive and three-dimensional exploration owing to multi-domain, multi-type and multi-layer.
Correlation between formation pressure and hydrocarbon enrichment in Triassic Xujiahe Formation, Western Sichuan Depression
DUAN Zhengxin, LIU Yifeng, LOU Zhanghua, LIU Zhongqun
2023, 45(1): 72-79. doi: 10.11781/sysydz202301072
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The Upper Triassic Xujiahe Formation in the Xinchang structural belt of the Western Sichuan Depression is an important exploration target for tight sandstone hydrocarbon in the Sichuan Basin. Several studies are carried out in this paper to improve the effectiveness of exploration and development including the characteristics of oil and gas movement with a view of dynamic point combining with fluid dynamics, determine the distribution characteristics of oil and gas in combination with static research results, and study the relationship between hydrocarbon enrichment and formation pressure. Overpressure is developed in the study area, and the degree of overpressure in the third member of Xujiahe Formation is the highest. Whilst the formation pressure of the main gas producing layer, the second member of Xujiahe Formation, is generally between 60-80 MPa with the maximum residual pressure reaches 40 MPa. The residual pressure gradients in most areas distribute between 0-3 MPa/km and up to 10 MPa/km as the highest. According to dynamic hydrocarbon enrichment theory and the relevant research on residual pressure and residual pressure gradient, it is considered that it has an important relationship with faults, fluids, hydrocarbon distribution and production performance. The area with higher residual pressure gradient (1-2 MPa/km) both improves fluid migration capacity and provides favorable reservoir, which is conducive to long-term high-yield and stable production.
Coupled relationship between algal assemblages and water, surface sediments in modern shallow lakes of the Songnen Plain
WANG Xue, ZHANG Xinrong, LIU Jiakang, FU Xiuli, SU Yangxin
2023, 45(1): 80-88. doi: 10.11781/sysydz202301080
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In order to study the influencing factors for the enrichment of organic matter in Mesozoic and Cenozoic shale in lacustrine basins, the Pearson analyses of algal assemblages, physical and chemical properties of water body, sedimentary grain size, magnetic susceptibility and chromaticity were carried out from the aspect of modern lacustrine deposition. The coupled relationship between modern algal assemblages and water body, surface sediments in the Zhongnei Lake, which is a semi-enclosed shallow lake in the Songnen Plain, showed that topography and wind field affected the enrichment of organic matter in semi-enclosed brackish lakes with acidic and mixed water bodies. In different sites of the Zhongnei Lake, the abundance of cyanobacteria, green algae and diatoms are affected by water temperature and has a certain relationship with the degree of sediment sorting. Water temperature and sediment sorting are significantly controlled by the interference of local wind field.
Oil-bearing capacity of shale in the first member of Upper Cretaceous Qingshankou Formation, Sanzhao Sag, Central Depression, Songliao Basin
KANG Shujuan, YANG Yunfeng, WANG Huajian, JIANG Wenbin, HE Kun, LIU Ranran
2023, 45(1): 89-98. doi: 10.11781/sysydz202301089
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The lacustrine dark shale deposits of the Upper Cretaceous Qingshankou Formation in the Central Depression of Songliao Basin contain a considerable amount of shale oil resources, among which the shale oil exploration of the Qingshankou Formation in the Qijia-Gulong Sag has made a major breakthrough. In this paper, methods of sealed thermal release, combined with organic solvent extract and Rock-Eval, has been used for the comprehensive evaluation of shale oil-bearing capacity of the Qingshankou Formation. The TOC of shales from the Qingshankou Formation in the study area range from 1.48% to 6.97% with an average value of 3.40%. The values of Tmax range from 440 ℃ to 453 ℃ with an average of 448 ℃ for the samples after extraction, and the production index ranges from 0.28 to 0.67, indicating a middle stage of oil generation window. The shale oil contents of the studied samples range from 4.00 mg/g to 19.49 mg/g with an average value of 13.74 mg/g. The contents of free oil range from 3.41 mg/g to 13.63 mg/g with an average value of 8.70 mg/g; while the content of adsorbed oil ranges from 0.60 mg/g to 9.43 mg/g with an average value of 5.04 mg/g. Free oil is made up of restricted oil and movable oil in the proportion of 1∶1. The content of restricted oil of the studied samples ranges from 1.81 mg/g to 8.49 mg/g with an average of 4.15 mg/g; while the content of movable oil ranges from 1.36 mg/g to 11.05 mg/g with an average of 4.55 mg/g. The movable oil resource is 12 kg/m3 based on the average content of mobile oil, which reveals good resource potential of movable shale oil in the first member of Qing-shankou Formation of the Sanzhao Sag.
Logging identification and distribution characteristics of high-gamma sandstones in the 7th member of Triassic Yanchang Formation, Ordos Basin
YOU Fuliang, LIU Guangdi, SUN Mingliang, LI Chaozheng, LIU Xiangbai, AN Cheng, WANG Zixin, LI Yishu
2023, 45(1): 99-108. doi: 10.11781/sysydz202301099
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A large number of sandstone interlayers were developed in the shale strata of the 7th member of the Triassic Yanchang Formation (Chang 7 member) in the Ordos Basin. These sandstone interlayers are frequently accompanied by abnormally high natural gamma values as high as 330.5 API. As a result, they are undistingui shable from shale on well profile. To characterize the high-gamma sandstones, the clastic rocks of the Chang 7 member were classified into fine sandstone, siltstone, argillaceous siltstone, silty mudstone, mudstone and shale according to grain size and structural characteristics. Conventional logging data were recalculated and combined into two new parameters—A and B. The new intersection chart composed of parameters A and B and natural gamma logging data can effectively distinguish and identify normal sandstone and high-gamma sandstone in the Chang 7 member. The plane distribution of cumulative thickness of high-gamma sandstone in the Chang 7 member shows that the high-gamma sandstone is mainly located in the southwest of the study area. The finger-like distribution of high-gamma sandstone from the edge to the center of the lake basin indicates that its formation may be related to the volcanic eruption during sedimentary period.
Fracture characteristics and hydrocarbon significance in the Chang 7 to Chang 9 members of Triassic Yanchang Formation of Gufengzhuang area, western Ordos Basin
GUO Hui, ZHAO Hongge, LI Ying, LEI Linlin, WANG Jian, LI Jun, SHAO Xiaozhou
2023, 45(1): 109-121. doi: 10.11781/sysydz202301109
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Natural fractures play an important role for the exploration and development of low-permeability reservoirs in the Triassic Yanchang Formation of the Ordos Basin. Abundant hydrocarbon resources have been found in the Yanchang Formation of the Gufengzhuang area, showing good exploration prospects. However, only limited research results on fractures have been obtained, and the distribution law and active period are both unclear. Based on the analyses of core, imaging logging, carbon-oxygen isotope, homogenization temperature of fluid inclusion, acoustic emission, tectonic stress and fracture activity, the characteristics, formation stages and causes of natural fractures in the Chang 7 to Chang 9 members of the Yanchang Formation in the Gufengzhuang area were studied, and corresponding indications for petroleum accumulation were discussed. Natural fractures are well-developed in the Chang 7 to Chang 9 members of the Yanchang Formation, especially in the Chang 7 member. High-angle and vertical fractures are ubiquitously developed, mainly of NEE orientation, followed by NE, and finally NW and NWW. The fractures are generally unfilled or semi-filled by calcites, providing effective pathways for hydrocarbon migration. The fracture surface is flat, and obvious traces of oil migration such as oil invasion, oil stain and oil spot can be observed. The average values of oxygen isotopes of the Chang 7 to Chang 9 members are -15.8‰, -17.5‰ and -18.7‰, indicating that the factures were formed at depth of 1 675, 2 338 and 2 785 m. According to the homogenization temperature peaks of fluid inclusions of 90-100 ℃ and 120-130 ℃, the formation depth of fractures is 1 750-2 000 m and 2 500-2 750 m. The fractures in the Chang 7 to Chang 9 members were active during multiple stages, including the Indosinian, Yanshanian and Himalayan stages, with the Yanshanian and Himalayan as the main active stages. Effective fractures developed in multiple stages have improved the physical properties of low-permeability reservoirs in the study area. They provided important migration channels and storage spaces for the formation of multi-layer and deep-layer reservoirs such as Chang 7, Chang 8 and Chang 9, and affected the productivity of oil wells.
Provenance analysis and geological significance of Paleocene in Lishui Sag, East China Sea Shelf Basin
WU Zhekun, LI Qi, ZHANG Yingzhao, LI Fengxun, QIN Jun, SHAN Shuaiqiang, ZHU Zhenjun, GENG Hui, CHEN Hehe
2023, 45(1): 122-134. doi: 10.11781/sysydz202301122
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The provenance evolution of Paleocene is a main factor controlling the sedimentary filling characteristics of the East China Sea Shelf Basin, and is of great significance in the exploration and development of petro-bearing basins. Based on the core samples of the Paleocene of the Lishui Sag, East China Sea Shelf Basin, the systematic analysis of petrological and mineralogical characteristics was carried out. Through the study of the spatial and temporal distribution of sandstone clastic components, light and heavy mineral assemblages and related parameters, the evolution of the Paleocene provenance system in the study area and its significance for the sedimentary filling of the basin were discussed. The Paleocene clastic rocks in the Lishui Sag are mainly lithic feldspar sandstone, feldspar lithic sandstone and lithic sandstone. The sandstone is mainly moderately well-sorted, and the grains are mostly subangular or subrounded, with poor overall rounding, indicating that the structural maturity and compositional maturity are relatively lower. The light mineral assemblages of the Guifeng and Lingfeng formations of Peleocene are obviously different and show zoning features in different parts of the basin. In the central and northern parts of the western subsag, there are mainly parent rock types of cycle orogenic belts, while in the south of the western subsag, there are mainly parent rock types of cycle orogenic belts and magma arc. In the eastern subsag, there are mainly parent rock types of cycle orogenic belts. The characteristics of heavy mineral assemblages in the main horizons of the Paleocene in the study area change little. The Zhejiang-Fujian uplift belt in the west and the Yandang uplift in the east are the most important provenance areas of the Lishui Sag. The Lingfeng low uplift in the center of the basin suffered serious denudation before the deposition of the lower member of the Mingyuefeng Formation, providing local provenance for the basin. The provenance system analysis showed that when the Yueguifeng and Lingfeng formations were deposited, they were in the early and peak stages of fault depression, and the intra-basin provenance area was formed by the intra-basin fault uplift. As the upper member of the Lingfeng Formation was deposited, the basin entered the late stage of fault depression, local intra-basin provenance was no longer developed, and the main provenance system was extra-basin provenance.
Constraints on carbonate diagenetic fluid properties by microzone in situ analysis of carbon and oxygen isotopes: a case study of Cambrian-Ordovician, Tarim Basin
HAN Jun, YOU Donghua, QIAN Yixiong, DONG Shaofeng, PENG Shoutao
2023, 45(1): 135-144. doi: 10.11781/sysydz202301135
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As an important technological measure to uncover complex sedimentary-diagenetic process, microzone in situ geochemical analysis has drawn more and more attention along with improving analytical and test levels. According to the analysis of microzone in situ carbon and oxygen isotopes in combination with the data about carbon and oxygen isotopes acquired by conventional sample splitting methods, seawater cementation in syngenetic-burial stages, dissolution-infill by meteoric water in hypergenetic stage, and dolomitization fluid alternation-cementation infill in multiple stages were compared and discussed. The results demonstrated that the analysis of carbon and oxygen isotopes by microzone in situ sampling was favorable for revealing multi-stage and multi-period fluid property evolution in the process of cement formation. Carbon and oxygen isotopes of this cement type acquired by conventional sample splitting analysis might be mix values of multi-stage products. With respect to relatively single product (i.e. matrix components in carbonate rocks and calcite with meteoric water genesis), the analysis of carbon and oxygen isotopes by conventional sample splitting could basically meet study requirements. Microzone in situ sampling analysis on carbon and oxygen isotopes could provide a relatively "low cost and high accuracy" analytical measures, which has important meanings for precisely tracing diagenetic evolution of carbonate rocks.
Abnormal pressure genesis and its relationship with continental shale oil accumulation in Paleogene, Jiyang Depression, Bohai Bay Basin
MA Xiaoxiao, LI Maowen, HU Kezhen, TAO Guoliang, QIAN Menhui, CAO Tingting, LIU Peng, LI Zhiming, JIANG Qigui, LI Zheng
2023, 45(1): 145-156. doi: 10.11781/sysydz202301145
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In order to explore the effect of abnormal pressure on shale oil accumulation in the Jiyang Depression, Bohai Bay Basin, the formation pressure distribution, muddy shale lithofacies, hydrocarbon generation evolution of organic matters, clay mineral conversion, hydrocarbon retention, and the distribution of secondary pores and microfractures in each of four exploration wells specially for Paleogene shale oil were studied by total rock X-ray diffraction analysis, conventional pyrolysis analysis on hydrocarbon generation potential, systematic analysis on organic geochemistry, and other methods. In the study area, the lithofacies association in middle and lower submembers of the third member (Es3middle and Eslower)3, and upper submember of the fourth member (Es4upper) of Paleogene Shahejie Formation adjacent to subsag center is mainly carbonate rocks, while near sag margin, there are primarily mixed sediments of clastic rocks/chemical rocks. Based on the data from sonic, density and drilling loggings, pressure distribution and abnormal pressure characteristics in single well could be identified effectively; in addition, the factors that affect formation pore pressure in organic shale containing major association of carbonate and clay minerals could be uncovered. Comprehensive application of geochemical evidences confirmed that Paleogene overpressure in the Jiyang Depression was mainly originated from hydrocarbon generation pressurization in mature muddy shale. Self-source overpressure intervals in muddy shale are in correspondence with the zones of secondary pore development and peak hydrocarbon retention.
Evolution of environmental oxidation and reduction of sea water in Three Gorges area in Early Cambrian: evidence from decoupled carbon isotopes in Luojiacun section
WANG Jin, LIN Yibo, YANG Tao
2023, 45(1): 157-167. doi: 10.11781/sysydz202301157
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The Late Neoproterozoic to Early Cambrian is a remarkable time-interval that witnessed significant biolo-gical and environmental evolutions, during which, the deep ocean was extensively anoxic, whereas the surface ocean was characterized by considerable fluctuations in redox conditions. The Lower Cambrian Yanjiahe and Shuijingtuo formations in the Luojiacun section from the Three Gorges area situated in Yichang city of central China were studied in this paper. According to biostratigraphic and carbonate carbon isotope stratigraphic correlation, the Yanjiahe Formation and Shuijingtuo Formation were assigned to the Cambrian Terreneuvian Stage 1-2 and Series 2 Stage 3, respectively. In view of this time-frame, the response of inorganic and organic carbon isotopes decoupling to water redox environment transition was systematically studied. The rise of anoxic bottom water during transgression provided sufficient nutrients and essential trace elements for primary productivity in the surface ocean. The increase of net productivity led to oxygen consumption increase and the changes of redox environment in the water column. Photosynthetic oxygen temporarily oxygenated the surface seawater, and dissolved oxygen was gradually consumed by the oxidation of DOC reservoirs, resulting in gradual expansion of oceanic anoxia, which was conducive for the preservation of organic matters. The oxidation of DOC in the surface water accounted for the formation of authigenic carbonates that were 13C-depleted, while the large oceanic DOC reservoir in the Early Cambrian buffered the change in δ13Corg, which eventually manifested as decoupled carbon isotope during the transition time of Cambrian Stage 2-3.
Microscopic characteristics of sulfur depositions in high-sulfur-content reservoirs
LI Nong, ZHAO Liqiang, HUI Dong, LI Tao, PU Zhijin, DENG Xiaohang, ZHOU Keming
2023, 45(1): 168-174. doi: 10.11781/sysydz202301168
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A large number of high-sulfur-content carbonate gas reservoirs have been discovered in the northeastern part of the Sichuan Basin, and their reserve accounts for more than half of the total natural gas reserves in the basin. However, sulfur deposition has been a major problem for a long period which is restricting for the efficient development of high-sulfur-content gas reservoirs. In this paper, sulfur deposition experiments in carbonate rock samples were carried out with high-sulfur-content natural gas by the means of micro-CT scanning, scanning electron microscopy, gas chromatograph and energy spectrum analysis. The changes of the microscopic pore structure of the samples and gas composition before and after the experiments were quantitatively obtained, and the 3D visualization of sulfur deposition in reservoir rocks was achieved for the first time. Results show that after sulfur deposition experiment, the total sulfur content in high-sulfur-content gas decreased by 12.58%, and the organic sulfur component decreased by 9.07%. Sulfur particles were mainly deposited in rock pores with a radius of less than 500 μm, resulting in decreases in rock permeability and porosity by 34.40%-67.80% and 3.77%-7.69%, respectively. This study further improved the understanding of the microscopic characteristics of sulfur deposition and provided support for the formulation of production enhancement measures for high-sulfur-content gas reservoirs.
Experimental dissolution and karst strata association of Middle Permian Maokou carbonate rocks in the northern part of Sichuan Basin
ZHANG Qingyu, JI Shaocong, ZENG Tao, XIA Wenqian, BA Junjie, DONG Hongqi, LIANG Bin
2023, 45(1): 175-184. doi: 10.11781/sysydz202301175
Abstract(613) HTML (251) PDF-CN(43)
Abstract:
Kart reservoir is important for the Middle Permian Maokou Formation in the northern part of Sichuan Basin. Based on field outcrops and drilling core data, combining both macro and micro methods, the characteristics of karst strata association of the Maokou Formation in this area was studied by the means of thin section observation, scanning electron microscope, major and trace elements and experimental dissolution. The first member of the Maokou Formation is dominated by marl and bioclastic micritic limestone, and the content of acid insoluble matter is high. The second and third members of the Maokou Formation are dominated by micritic bioclastic limestone and bright crystal bioclastic limestone, and the content of acid insoluble matter is low. The results of dissolution experiments show that the mineral type and chemical composition of carbonate rock are the main factors affecting the solubility of rock. The change characteristics of the dissolution rate of samples, solution ion concentration and micro dissolution are as follows: bioclastic limestone>micrite limestone>dolomite>argillaceous limestone. The karst strata association types of the Maokou Formation can be divided into 2 groups, 3 types and 9 subtypes. The karstification conditions and rock dissolution degree of different karst formation types are different, and the development scale of fracture cave karst reservoir is also different.
Pore and throat characterization of tight reservoirs based on the methods of alloy injection and large-field image stitching
TANG Xianglu, XIAO Hanmin, JIANG Zhenxue, LIU Xuewei, YANG Zaiquan, LIU Ge, ZHANG Fan
2023, 45(1): 185-192. doi: 10.11781/sysydz202301185
Abstract(464) HTML (186) PDF-CN(48)
Abstract:
Tight oil and gas is an important part of China's oil and gas resources, and the study of related pore-throat structure is conducive to the enrichment and improvement of tight oil and gas geological theory. Taking the tight reservoir of the Triassic Yanchang Formation in the Ordos Basin as an example, combined with large-field image stitching method and image pore extraction method, this paper established a new method for pore-throat structure characterization of tight reservoir with high-temperature and high-pressure alloy injection. Results show that the alloy injected into the pores of tight reservoirs can reflect the characteristics of pore connectivity better, and the pore morphology and structure are easier to be identified. The large-field image stitching method solves the problem of strong heterogeneity of pore distribution and poor representativeness of small-field observation. The tight reservoirs of the Yanchang Formation in the Ordos Basin are mainly composed of intergranular pores, intragranular pores, and intergranular-intragranular mixed pores, among which the intergranular pores are mostly developed. There are five types of throats, including pore shrinkage throat, neck constricted throat, bundle throat, sheet throat and curved sheet throat, among which the curved sheet throat is mostly developed. The porosity of quartz and feldspar is significantly larger than that of other minerals, contributing 74% to the surface porosity of tight reservoir, and is an important occurring space for tight oil and gas.
Characterization of oil-bearing properties in sub-micron shale pores by laser scanning confocal microscopy technology: a case study of shale in Lucaogou Formation, Junggar Basin
LU Feng, LI Zhaoyang, YANG Zhao, ZHANG Liuping, LIU Jin, LI Lulu, LIU Xiangjun
2023, 45(1): 193-202. doi: 10.11781/sysydz202301193
Abstract(999) HTML (386) PDF-CN(134)
Abstract:
Sub-micron pores and their oil-bearing properties are important information for the exploration and development of shale oil. For effective observation on oil-bearing properties of sub-micron pores by laser scanning confocal microscopy, prefilter allocation of main light splitter and detector in laser scanning confocal microscopy was modified in order to conduct combined scanning of reflected light and fluorescent light well, eliminate fluorescence interference from the minerals, and then accomplish accurate observation on sub-micron pores and their oil-bearing properties. This method was applied in the study on Lucaogou Formation shale oil in the Jimsar Sag, Junggar Basin. Most of shale oil occurred in irregular, patchy and star forms. The organic matter maturity of this shale (Ro) was in the range of 0.6%-1.1% and roughly 0.9% in average. However, the distribution of light and heavy components was significantly heterogeneous and controlled by the maturity incompletely. In some positions, both light component content and its ratio with heavy component content were high, while there were mainly heavy components in the places with approximate maturity or in adjacent high places. Hydrocarbon migration in shale sequences is also a crucial controlling factor. The observation on light and heavy components in the shale and the study on their distribution regularities are critical for the exploration and development of shale oil. New technology of laser scanning confocal microscopy could act as an important observation measure.
The publication of "Theory and Simulation Experimental Technology of Oil Generation from Chinese Continental Oil Source Rocks" and the journal "Petroleum Geology & Experimental " continue to achieve excellent results in 2022
2023, 45(1): 203-203.
Abstract(144) PDF-CN(24)
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