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2024, Volume 46,  Issue 2

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2024, 46(2): .
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Development model and exploration practice of fault-lithologic composite traps in Oligocene Weizhou Formation, Weixinan Sag, Beibuwan Basin
HU Desheng, SUN Wenzhao, MAN Xiao, CHEN Kui, CHEN Lijun, GONG Liyuan, LI Shanshan
2024, 46(2): 215-227. doi: 10.11781/sysydz202402215
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In order to address the challenges related to exploration target search and sustainable development of oilfields in the Weixinan Sag of Beibuwan Basin, this study focused on fault-lithologic composite traps within the structural trap development section of the third member of Oligocene Weizhou Formation in the No.2 fracture zone of Weixinan Sag. By examining the relationship between the evolution of depression-controlling faults and the sedimentary characteristics of the third member of Weizhou Formation, it was observed that the No.2 fault zone exhibited weak activity during the sedimentary period of the third member, facilitating the continuous transportation of underwater distributary channels of the braided river delta along the long axis direction of the sag towards the depression. Conversely, the regional tectonic activities were intense during the sedimentary periods of the second and first members of Weizhou Formation, leading to the formation of a series of en-echelon faults. The interaction between relatively isolated underwater distributary channels of the braided river delta front and en-echelon faults resulted in angled staggered formations and intersecting channels, ultimately creating a distinctive channel-type structural-lithologic trap group characterized by NE-SW trending fault sealing and NW-SE trending sand body pinching. The research findings indicate the development of a three-factor coupling fault-lithologic trap model in the third member of the Weizhou Formation. This model includes the depression faulted uplift background of the No.2 fracture zone, low-frequency oscillating channel, and en-echelon tension-torsion fault. The intersection area of the main stream line of underwater distributary channel in a braided river delta and the No.2 fracture zone is identified as a favorable zone for structural-lithologic traps. Additionally, composite structural-lithologic traps are predominantly found in the near-source intersection area, while isolated structural-lithologic traps are more common in the far-source intersection area, leading to the formation of an accumulation model involving axial channel sand control, fault, and sand body composite control. The fault-lithologic trap model and accumulation model have demonstrated effective application in practice. Several favorable fault-lithologic composite trap groups with distinct fault sealing and sand body pinching orientations have been identified, particularly in the Wei B-E structure. Notably, well WB-E 1 has successfully drilled nearly 100 meters of oil layer, with subsequent wells yielding positive results. The exploration efforts in the third member of the Weizhou Formation within the No.2 fault zone of the Weixinan Sag have led to the discovery of geological reserves exceeding 10 million cubic meters, underscoring the significance of fault-lithologic composite traps in this region.
Fluid evolution of fracture veins of Paleogene Funing Formation in well HY1 in Subei Basin and implications for shale oil filling
LI Chao, LUO Tao, HUANG Yahao, LIU Yicheng, CHEN Junlin, WANG Chuan
2024, 46(2): 228-237. doi: 10.11781/sysydz202402228
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A significant breakthrough has been made in the exploration and development of shale oil in the Subei Basin. Cores from well HY1 in the Gaoyou Sag of Subei Basin indicate the presence of well-developed natural fractures within the organic-rich shale of the second member of Paleogene Funing Formation. These fractures contain compelling evidence of shale oil, gas, and fluid activities and migration. This study specifically examines the calcite veins filling the shale fractures within the second member of Funing Formation. Through the integration of methodologies from mineralogy, elemental geochemistry, geochronology, and geological fluid analysis, the originof paleo fluids, the timing of vein formation, and fluid inclusions within the shale fracture veins are investigated. This study also sheds light on paleotemperature conditions and corresponding geological timeframes during the burial, uplift, and denudation of the shale oil-bearing layers. The primary fracture types observed in the second member of Funing Formation include bedding fractures, detachment fractures, shear fractures, shrinkage fractures, and tensile fractures, predominantly filled with fibrous calcite veins indicating a source linked to hydrothermal fluids in a reducing environment. Notably, three distinct stages of shale oil injection are evident within the organic-rich shale fracture veins of the second member of Funing Formation, marked by primary green fluorescent oil inclusions from the Middle Eocene, secondary blue fluorescent oil inclusions from the Late Eocene, and secondary green fluorescent oil inclusions from the Late Quaternary period. The primary accumulation of shale oil occurred during the period of maximum burial depth, emphasizing the significant role of tectonic movements, especially the Sanduo tectonic event, in the redistribution and migration of shale oil in the region.
Reservoir characteristics and hydrocarbon accumulation significance of metamorphic zones of intrusive rocks and surrounding rocks in second member of Paleocene Funing Formation in Qintong Sag, Subei Basin
ZANG Suhua
2024, 46(2): 238-246. doi: 10.11781/sysydz202402238
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Volcanic reservoirs play a crucial role in oil and gas exploration. The intrusive rocks in the Qintong Sag of the Subei Basin are widely distributed within active oil and gas deposits, making them a significant area for further exploration and reserve expansion in East China's continental basins. Despite the promising exploration prospects, there is a research gap regarding the impact of volcanics on shale surrounding rock reservoirs and their accumulation potential. It is essential to evaluate whether the shale metamorphic zones could serve as a key exploration target. This study focuses on the metamorphic zones of intrusive rocks and surrounding rocks in the Qintong Sag's second member of the Funing Formation. By integrating core, thin section, whole rock, and well logging data, this work systematically reveals the reservoir characteristics of the mud shale metamorphic zones and defines the contribution of magma intrusion to hydrocarbon accumulation in the metamorphic zones. The results indicate that magma intrusion alters the mud shale of the Funing Formation, resulting in a circular metamorphic zone reservoir with enhanced reservoir properties. This reservoir exhibits two types of spaces, pores and fractures, with development dependent on the thickness and intensity of intrusive rocks. Magma intrusion also leads to a "ripening effect", increasing the Ro of mud shale by 0.4%-0.5% at the same depth. The intrusive rocks of the Funing Formation were formed during the Wubao to Sanduo periods. The intrusion of igneous rocks created a circular metamorphic zone prior to the hydrocarbon charging stage, indicating a favorable accumulation match. Influenced by magma intrusion, spatial distribution of intrusive rocks, and the source-reservoir relationship, the metamorphic zones of intrusive rocks and surrounding rocks in the second member of Funing Formation in the Qintong Sag exhibit characteristics of integrated source-reservoir and near-source enrichment. This presents a new opportunity for significant reservoir enhancement on a large scale.
Lithofacies characteristics and reservoir capacity of fine-grained sedimentary rocks of second member of Qianfoya Formation in Yuanba area, Sichuan Basin
PENG Jun, ZHANG Xinyi, XU Tianyu, CHENG Lixue, ZHANG Kun, LI Bin
2024, 46(2): 247-262. doi: 10.11781/sysydz202402247
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The shale oil and gas exploration potential of the Jurassic Qianfoya Formation in the Sichuan Basin is promising. However, there is still a lack of understanding regarding the basic sedimentary characteristics and development patterns of fine-grained sedimentary rocks. Therefore, utilizing the principles of fine-grained sedimentology, this study thoroughly examines the geological features of the fine-grained sedimentary rock reservoir in the second member of the Qianfoya Formation in the Yuanba area. This investigation includes the analysis of material composition, sedimentary structures, reservoir space types, and classification of lithofacies types. Subsequently, the physical characteristics and reservoir space types of the main lithofacies reservoirs are compared and analyzed. Finally, the study predicts the favorable intervals for development. The results reveal that the fine-grained sedimentary rocks in the second member of the Qianfoya Formation in the Yuanba area consist of two rock types: clay rock and siltstone, with a total of 15 rock facies types. Organic-rich massive clay rock, organic-rich massive silty clay rock, organic-rich laminated silty clay rock, organic-rich banded silty clay rock, organic-rich massive silty clay rock, and organic-rich laminated silty clay rock are the six major lithofacies types of fine-grained sedimentary rock. Additionally, various types of reservoir spaces exist, such as interlayer fractures among clay minerals, shale bedding fractures, intercrystalline pores among clay minerals, intercrys-talline pores among iron pyrite, dissolved pores, and organic matter pores. Among these, intercrystalline pores of clay minerals are the most widely distributed. The average porosity and permeability of the six major lithofacies types were found to be 2% and 0.023×10-3 μm2, respectively. The lithofacies of organic-rich laminated silty clay rock, organic-rich laminated silty clay rock, and organic-rich massive silty clay rock exhibit rich reservoir space types, wide distribution, large quantity, higher porosity and permeability compared to other lithofacies, as well as higher TOC values, making them the most favorable reservoir lithofacies types. The dominant stratum of the second member of the Qianfoya Formation in the study area is situated in substratum 1, the bottom of substratum 2, and the middle and lower parts of substratum 3.
Pore structure and fractal characteristics of deep shale gas reservoirs in the Permian Dalong Formation, northeastern Sichuan Basin
HE Chencheng, ZHAO Yongqiang, YU Lingjie, LU Longfei, LIU Weixin, PAN Anyang, LI Chuxiong
2024, 46(2): 263-277. doi: 10.11781/sysydz202402263
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The characteristics of shale reservoirs play a crucial role in the enrichment and exploitation of shale gas. The Upper Permian Dalong Formation in the northern Sichuan Basin is a significant marine-derived high-quality hydrocarbon source rock that requires further detailed investigation. This study conducted qualitative and quantitative analyses on the pore structure of deep shale reservoirs within the Permian Dalong Formation in northeastern Sichuan. Various methods such as high-resolution field emission scanning electron microscopy, CO2 adsorption, N2 adsorption, and high-pressure Hg injection measurements were employed. Fractal fitting using the V-S model based on CO2 adsorption, the FHH model based on N2 adsorption, and the fractal geometry model based on high-pressure Hg injection was utilized to characterize the pore structure of deep shale. The findings revealed that deep shale reservoirs in the Dalong Formation in northeastern Sichuan Basin contain abundant nano-scale organic pores and limited inorganic pores. The development of organic pores exhibits strong heterogeneity influenced by maceral differences and organic matter distribution. Similar to Longmaxi Formation shale, the pores in the Dalong Formation are predominantly micropores and mesopores, comprising over 90% of the total pore volume and primarily influenced by organic matter abundance. The research on fractal dimension characteristics suggests that macropores in deep shale exhibit stronger heterogeneity compared to mesopores and micropores. Due to their smaller size and minimal diagenetic impact, micropores and mesopores have simpler structures with lower fractal dimensions. On the other hand, macropores display significant heterogeneity due to their larger size, wide distribution range, diverse genesis, and susceptibility to diagenesis. Due to their abundant reservoir space and strong self-similarity, micropores and mesopores in deep shale are conducive to the occurrence, seepage and exploitation of shale gas.
Stratigraphic and sedimentary characteristics of Middle Permian Qixia Formation in Beichuan Tongkou section, middle Longmen Mountain
WANG Dong, WANG Haijun, YANG Cheng, ZHANG Junhui, ZHAO Shanshan, HU Yongliang
2024, 46(2): 278-287. doi: 10.11781/sysydz202402278
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The Qixia Formation in the foothills of Longmen Mountain in the western Sichuan Basin is gaining attention for oil and gas exploration. However, exploration of the Qixia Formation in the middle Longmen Mountain is insufficient. Consequently, it is essential to understand its stratigraphic sedimentary characteristics for guiding natural gas exploration. This study focuses on the Beichuan Tongkou section, combines field measurements, thin-section observation and trace element analyses to investigate the sequence stratigraphy and sedimentary features of the Qixia Formation, thereby defining its sedimentary environment. Within the Beichuan Tongkou section, which spans from the Liangshan Formation to the Qixia Formation, researchers have identified two third-order and one fourth-order sequence boundaries, delineating one third-order sequence and two fourth-order sequences. The sedimentary facies found in this interval include shore-swamp, open platform, and platform edge environments, which are further classified into four subfacies including open sea, intertidal sea, intra-platform beach, and platform edge beach. Additionally, three microfacies are identified including intra-platform bioclastic beach, platform edge bioclastic beach, and platform edge sand debris and bioclastic beach. The Qi1 member and lower part of the Qi2 member is located on an open platform, while the upper-middle part of the Qi2 member is found at the platform edge. This setup defines a sedimentary model for a carbonate-rimmed platform in the Qixia Formation of the middle Longmen Mountain. During the sedimentary period of the Qixia Formation, a warm, humid, and oxygen-rich marine environment supported abundant ancient organisms. In the highstand system tract of the upper-middle part of the Qi2 member, the Sr/Ba ratio is lowest, while the MgO/CaO ratio is highest, indicating elevated ancient water temperatures and specific chemical compositions. The m value, reflective of paleosalinity, is notably high, suggesting a shallow, high-energy platform edge environment away from the mainland. Thick, high-energy platform edge sand debris and bioclastic beaches in this area promote the development of high-quality porous dolomite reservoirs of beach facies. The sedimentary facies and reservoir conditions of the Qi2 member in the middle Longmen Mountain are comparable to those in other sections, making it a promising target for oil and gas exploration within the Qixia Formation and a potential strategic replacement area for reservoir expansion.
Origin of dolomite in Middle Permian Qixia Formation in northwestern Sichuan Basin: a case study of Chejiaba section
ZENG Jie, DONG Shaofeng
2024, 46(2): 288-298. doi: 10.11781/sysydz202402288
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This study focuses on the origin of dolomite within the Middle Permian Qixia Formation in the northwestern Sichuan Basin by the approaches of petrographic examination and stable isotopic as well as clumped isotopic analysis. Two types of dolomite (matrix dolomite and dolomite cement) in the upper part of the Qixia Formation at the Chejiaba (CJB) section were classified. The gray matrix dolomite shows a clear inter-finger contact with limestone, while the milky white dolomite cement lines the edges of dissolved pores. Microscopic examination revealed three types of matrix dolomite including floating fine crystalline, planar-e(s) matrix dolomite (Md1); fine to medium crystalline, planar-e(s) matrix dolomite (Md2); and medium to coarse crystalline, planar-a matrix dolomite (Md3), along with saddle dolomite cement (Sd). The presence of Md1 dolomites along stylolites suggests they were formed due to pressure dissolution during shallow burial. The similar isotopic signatures of Md2 and Md3 with the host limestone indicate that the dolomitizing fluids likely originated from Permian seawater. However, elevated diagenetic temperatures suggest an influence from the Late Permian volcanic activity. The Sd dolomite and subsequent calcite cementation with negative δ18O values and radioactive 87Sr/86Sr ratios point to formation in high-temperature, externally-derived, radio-genic strontium-rich dolomitizing fluids, possibly from deep-seated thermal fluids within siliciclastic strata. The formation of dolomite in the Qixia Formation of the northwestern Sichuan Basin was influenced by both original sedimentary facies and faulting activities. The influx of hydrothermal fluids triggered widespread dolomitization, resulting in the creation of numerous dissolved and inter-crystalline pores, which played a beneficial role in reservoir development.
Characteristics and sedimentary geological significance of tempestites in first member of Lower Cambrian Canglangpu Formation in north central Sichuan Basin
TIAN Lizhou, SONG Jinmin, YE Yuehao, LIU Shugen, LI Zhiwu, JIN Xin, YANG Di, ZHAO Lingli, DING Yi, REN Jiaxin, WANG Han, LI Keran, DENG Haoshuang
2024, 46(2): 299-310. doi: 10.11781/sysydz202402299
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Tempestite has been discovered in the Lower Cambrian Canglangpu Formation in the north central Sichuan Basin. However, there has been limited research on storm sedimentation and the Lower Cambrian sedimentary environments, ancient geography, and ancient sedimentary geomorphology in this region. By conducting detailed core observations and thin section identifications, the storm sedimentation sequence and sedimentation model of the Canglangpu Formation have been investigated, revealing its geological significance. Various storm sedimentation structures, such as gravel sections, graded sections, parallel laminated sections, and hummocky cross-stratifications (HCS), are present in the Canglangpu Formation. The tempestites within this formation can be categorized into five types of storm sedimentation sequences: Sequence Ⅰ includes gravel sections (A), parallel laminatedsections (C), and HCS (D) above the fair-weather wave base (FWB). Sequence Ⅱ comprises parallel laminated section (C), HCS (D), and horizontally laminated mudstone section (E) deposited near the storm wave base (SWB). Sequence Ⅲ includes scouring-fill structures and gravel section (A), as well as a graded section (B) deposited between the fair-weather wave base (FWB) and SWB. Sequence Ⅳ is characterized by a graded section (B) and parallel laminated section (C) located far from the storm center, above and near SWB. Sequence Ⅴ consists of scouring-fill structures and gravel section (A) located between the mean sea level (MSL) and FWB, close to FWB. The analysis of tempestites in north central Sichuan Basin suggests that the initial member of the Canglangpu Formation was deposited on the shelf-ramp-platform margin and shelf, indicating that the Upper Yangtze Plate was situated in a low latitude region during the sedimentation period of the Canglangpu Formation. This information provides valuable insights into lithofacies paleogeography reconstruction. Storm events have resulted in the formation of storm mudflats in the study area, which are favorable for reservoir development. The investigation of storms in the Canglangpu Formation aids in predicting the distribution of shoal facies in the study area.
Genetic mechanism of dolomite in third member of Maokou Formation in Middle Permian, southeastern Sichuan Basin
YANG Hao, PAN Lei, QUAN Li, LI Rangbin, WANG Guangwei, GAO Junjie, ZHOU Ling, XU Rui
2024, 46(2): 311-318. doi: 10.11781/sysydz202402311
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Dolomite formation is a crucial aspect of natural gas reservoirs in the marine strata of the Sichuan Basin, particularly in the exploration of deep carbonate reserved oil and gas resources. Understanding the genetic mechanism of dolomite is essential for predicting the distribution of favorable dolomite reservoirs in deep formations. Taking the third member of the Maokou Formation in the southeastern Sichuan Basin as an example, present work systematically investigates the genetic mechanism of dolomite by analyzing geochemical characteristics such as carbon and oxygen isotopes, strontium isotopes and rare earth elements. The analysis is based on data from outcrops, core samples and thin sections. The results reveal that the dolomites of the third member of the Maokou Formation in the southeastern Sichuan Basin can be categorized into laminated fine-crystalline dolomite, laminated fine to medium-crystalline dolomite, and saddle dolomite cement. Among these, the laminated fine to medium-crystalline dolomite is the most common type. The δ18O values of this dolomite type (-7.04‰ to -6.08‰) appeared unaffected by hydrothermal fluids, and are similar to those of mud-crystal limestone. Additionally, the 87Sr/86Sr ratio (0.707 550 to 0.707 740) locate within the range of contemporaneous seawater, indicating a diagenesis process of quasi-syngenetic to burial-induced restricted marine water infiltration dolomitization. The δ18O values of hydrothermally influenced dolomite range from -7.33‰ to -7.11‰, showing slightly negative compared to those of mud-crystal limestone. These dolomites also showed relatively higher 87Sr/86Sr ratios (0.707 950 to 0.708 171) than those of contemporaneous seawater. The laminated fine-crystalline dolomite, which was not affected by hydrothermal fluids, shares a common genetic mechanism with the previously mentioned laminated fine to medium-crystalline dolomite. Saddle dolomite cement found in caves and fractures is either directly precipitated or recrystallized from hydrothermal fluids. This type of dolomite is characterized by higher 87Sr/86Sr ratios (with a mean value of 0.707 970), significantly negative δ18O values (with a mean value of -7.32‰), and positive Eu anomalies. A gradient change from laminated fine-crystalline dolomite to laminated fine to medium-crystalline dolomite and finally to saddle dolomite cement is observed, controlled by a transition from marine water to hydrothermal fluids. The dolomite in the study area is proposed to be the product of multi-stage fluid-induced diagenesis.
Main controlling factors for oil and gas enrichment in Jurassic laminated shale in Fuxing area of Sichuan Basin
WANG Daojun, CHEN Chao, LIU Zhujiang, YANG Shufan, LIU Miaomiao, XIE Jiatong
2024, 46(2): 319-332. doi: 10.11781/sysydz202402319
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The Fuxing area of the Sichuan Basin is known for its multiple sets of organic-rich shale in the Jurassic period, with several wells yielding significant amounts of industrial oil and gas. Understanding the characteristics and factors influencing the enrichment of different shale facies sedimentary reservoirs is crucial for effective exploration. Through detailed rock core descriptions, experimental analysis, and drilling exploration, the author has classified and compared fine-grained sedimentary rock facies in terrestrial strata, characterized reservoirs, identified types of oil and gas reservoirs, and analyzed the main factors influencing enrichment and productivity. The study reveals that the Jurassic strata in the Fuxing area contain two sets of high-quality shale with semi-deep lacustrine facies: the first sub-section of the Dongyuemiao section of the Ziliujing Formation and the lower sub-section of the second section of the Lianggaoshan Formation. These shale layers exhibit multi-layer stacking, moderate TOC content, well-developed bedding, integrated source storage, and high gas/oil ratio characteristics. The lithology of these target shale layers is complex, leading to the establishment of a four-element lithofacies classification scheme based on "color-TOC-sedimentary structure-mineral composition". This scheme identifies the gray-black high carbon sand-laminated clay shale as the favorable lithofacies for the lower sub-section of the second section of the Lianggaoshan Formation, and the gray-black high carbon medium debris-laminated and fine sand medium debris-laminated clay shale as the favorable lithofacies for the eastern sub-section. The three lithofacies in the study area are horizontally distributed in a stable manner. The gray-black high carbon fine sand layer, medium debris layer, and fine sand-medium debris layer type clay shale exhibit reservoir characteristics of "porous coexistence, large pores and throats, and integrated pores and fractures". These reservoir properties, including good permeability, facilitate the efficient storage and flow of shale oil and gas in the pore-fracture storage system. The first sub-section of the Dongyuemiao section is a condensate gas reservoir, while the lower sub-section of the Lianggaoshan Formation's second section is a volatile oil reservoir. Key factors contributing to the enrichment of continental shale oil and gas have been identified, including medium to high hydrocarbon generation potential and strong retention rates supporting the accumulation of laminated shale oil and gas. A favorable hydrocarbon-diagenesis-reservoir formation configuration promotes oil and gas accumulation. Additionally, the development of micro cracks and suitable thermal evolution aids in hydrocarbon enrichment and flow, while large, wide, gentle synclines and high-pressure overpressure conditions support the sustained preservation of hydrocarbons.
Development characteristics of Ordovician ancient subterranean river system in thrust anticline area of Tahe Oilfield, Tarim Basin
ZHANG Changjian, JIANG Lin, WEN Huan, LÜ Jing, CHANG Qi
2024, 46(2): 333-341. doi: 10.11781/sysydz202402333
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The study of the ancient subterranean river system in the Tahe Oilfield in the Tarim Basin is still in its initial stage. Subterranean river caves are mainly classified based on the characteristics of the depth distribution and structural patterns of the subterranean rivers. However, there is a lack of comprehensive analysis regarding the spatial development patterns of complex subterranean river systems from geological perspectives such as structure, fault, paleogeomorphology, and water table. This hinders the understanding of primary and secondary relationships of subterranean rivers, spatial superposition patterns, and original connectivity relationships, thereby impeding comprehensive management research in the later stages of Tahe Oilfield development. To address this gap, an investigation was carried out in the S67 well block to clarify the developmental characteristics of the Ordovician ancient subterranean river system in the thrust anticline area of the main Tahe Oilfield. The study utilized methods like structural fault analysis, paleogeomorphology restoration, seismic attribute characterization, and vertical section interpretation to identify river types, classify systems, and examine geological origins. Noteworthy findings include the identification and analysis of a phreatic loop river for the first time. Results show that the S67 well block is located in a low-lying region at the southern edge of the karst platform within the main Tahe Oilfield area. This region features peak cluster depressions and karst hill depressions with minimal amplitude differences and shallow incision depths of surface water systems. The low-angle thrust structural style of the thrust anticline, combined with a network of faults, provides favorable dissolution channels for the multi-layered subterranean river system. In the study area, the Ordovician period is characterized by the emergence of distinct water-table subterranean river systems and phreatic loop river systems, all of which display a dendritic structure. The water-table subterranean rivers are divided into main, branch, and abandoned types, while the phreatic loop rivers are classified as ascending or symmetrical. The formation of ancient subterranean rivers in the thrust anticline area is primarily influenced by factors such as paleogeomorphology, water table levels, thrust anticline structure, and secondary fault networks.
Characteristics of Lower Paleozoic strike-slip faults and their significance for oil and gas exploration in Xunyi-Yijun area, Ordos Basin
WANG Qichao, LIU Guangxiang, WU Jiang, YUE Xinxin, SUN Ziming, ZHANG Juntao, GAO Xiaopeng, LIU Ling, LU Kai, ZHANG Zhongpei
2024, 46(2): 342-353. doi: 10.11781/sysydz202402342
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In recent years, the Ordos Basin has undergone a reevaluation, transitioning from a perception of a single block to a focus on understanding the influence of multiple stages and fault systems on hydrocarbon accumulation. This study utilizes the latest 3D seismic and drilling data alongside a theoretical model of strike-slip fault to analyze fault geometry and kinematics in the Xunyi-Yijun area of the basin's southern region. The mechanisms of fault formation are predicted and the related impacts on the formation and accumulation of deep carbonate rocks are also discussed. The results indicate the presence of three fault systems in the Xunyi-Yijun area, with decoupling between deep and shallow layers. The Lower Paleozoic strike-slip faults exhibit distinct characteristics, including high vertical structure, dip swing, and a "flower-shaped" structure in the section. The faults display "compression shear in the northwest, tension shear in the center, and segmental deformation on trunk fractures" on their planes.Moreover, the Lower Paleozoic strike-slip faults experienced two stages of tectonic activities with greater intensity observed from the Late Ordovician to the Silurian, marking the primary period of fault development. The activity intensity during the Middle to Late Permian was relatively weaker, with faults exhibiting inherited strike-slip characteristics. These faults formed two groups, one with a NE strike and the other with a NW strike. Additionally, during the second stage of the Caledonian period, ocean subduction and closure lead to stress being transferred to the basin interior. It is believed that the NE and NW strike basement weak zones existed in the southern Ordos Basin. As a result, the Lower Paleozoic strike-slip fault system likely formed due to the activation of oblique compression. This fault system has the potential to enhance the physical properties of deep carbonate reservoirs and create high-quality karst or tectonic fracture reservoirs in the Xunyi-Yijun area. Moreover, The formation of the "upper source and lower reservoir-lateral source and lateral reservoir" combination along the strike-slip fault belt is an area of interest for deep oil and gas exploration in the Xunyi-Yijun region.
Oil-bearing potential and hydrocarbon occurrence characteristics of shale in Paleogene Hetaoyuan Formation in Biyang Sag, Nanxiang Basin
JIN Yunyun, LI Chuxiong, WANG Yong, YAN Yongxin, LUO Xi, HUANG Shuaibo, LI Zhiming, ZHOU Yuanyuan, SUN Zhongliang, LIU Yahui, JIA Mengyao, LENG Junying
2024, 46(2): 354-365. doi: 10.11781/sysydz202402354
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High-quality lacustrine shale beds within the Paleogene Hetaoyuan Formation in the Biyang Sag, Nanxiang Basin, exhibit significant potential for shale oil resources. Previous studies on the oil-bearing potential and hydrocarbon occurrence of shales in the Biyang Sag were relatively limited, potentially hindering advancements for shale oil exploration. This study focuses on the shales found in the third sub-member of the third member within the Hetaoyuan Formation in well Y1, which locates in the southern area of the Biyang Sag. Through various geochemical analyses such as Rock-Eval pyrolysis, multi-temperature pyrolysis and X-ray diffraction, this research systematically investigates the shale oil contents, hydrocarbon occurrence characteristics, and influencing factors. These results indicate that the lithofacies assemblages of the shale primarily consist of felsic shale facies, dolomitic shale facies and mixed shale facies, with a well-developed laminar structure. The source rock quality is classified as good to excellent, and the thermal maturity stage of oil window. The predominant organic maceral is sapropelic, with organic matter types ranging from Ⅰ to Ⅱ1. Oil content increases with burial depth, ranged from adsorbed hydrocarbons in the upper section to free hydrocarbons in the lower section. Detrital mineral content and total organic carbon (TOC) content are identified as the main factors influencing free and adsorbed hydrocarbon content, respectively. Overall, the oil saturation index (OSI) of the shale in the lower section of the Hetaoyuan Formation exceeds 100 mg/g, with an average free hydrocarbon content over 3 mg/g, indicating promising prospects for shale oil exploration and development.
Geochemical characteristics of light hydrocarbons in Upper Paleozoic tight gas from Daniudi Gas Field, Ordos Basin
NI Chunhua, WU Xiaoqi, WANG Ping, WANG Fubin, JIA Huichong, ZHU Jianhui, ZHANG Yi, JIANG Haijian
2024, 46(2): 366-379. doi: 10.11781/sysydz202402366
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The Daniudi Gas Field is one of the typical tight sandstone gas fields in the Ordos Basin. In order to better understand the genetic types and source of natural gas and reveal the migration phase of natural gas, the geochemical analysis of light hydrocarbons in the Upper Paleozoic tight gas from the Daniudi Gas Field has been conducted in this study. The results indicate that, the C5-7 light hydrocarbons in the Upper Paleozoic tight gas are dominated by iso-alkanes, and the C6-7 light hydrocarbons display low contents of aromatics (< 10%), which are even undetectable. The C7 light hydrocarbons are dominated by methylcyclohexane (MCH) with the relative MCH contents exceeding 50%. The K1 and K2 values of natural gas from the Lower Shihezi Formation (P1x) are consistent with those from the Shanxi (P1s) and Taiyuan (C3t) formations, respectively. The δ13C1 values of the P1x gas are consistent with those of the P1s gas and significantly different from those of the C3t gas. Compared with the P1s gas, the P1x gas mainly displays lower benzene/n-hexane, benzene/cyclohexane and toluene/n-heptane ratios, as well as significantly higher n-heptane/methylcyclohexane ratios. Geochemical characteristics of light hydrocarbons and carbon and hydrogen isotopic compositions of alkanes indicate that, the Upper Paleozoic natural gas from the Daniudi Gas Field is typical coal-derived gas. The P1s and C3t gases were in-situ self-generated and self-accumulated, whereas the P1x gas was mainly derived from the underlying P1s source rocks and accumulated via vertical migration in free phase, with insignificant contributions of the C3t source rocks. Due to the effect of migration and water solution of natural gas, the deviation exists in the identification of thermal maturity for tight gas from the Daniudi Gas Field using the light hydrocarbon indexes such as heptane and isoheptane values and benzene/n-hexane ratios.
Source rock evaluation and prediction of effective hydrocarbon kitchen distribution of Upper Triassic in Junggar Basin
YANG Fan, CAO Zhenglin, LIU Hailei, TAO Shu, LU Shan, GONG Deyu, CHEN Gang, XIAO Meng
2024, 46(2): 380-392. doi: 10.11781/sysydz202402380
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The Junggar Basin is known for its high potential of petroleum resources, showcasing significant oil and gas discoveries across various layers. However, the potential of utilizing the thick mudstone of the Upper Triassic as a viable source rock in the basin remains uncertain. Therefore, an urgent need exists to assess the Upper Triassic source rock across the basin and predict the distribution of effective source rocks. This evaluation is crucial for informing future exploration strategies in this layer. To shed light on the spatiotemporal distribution characteristics and hydrocarbon generation potential of the Upper Triassic source rocks in the Junggar Basin, as well as to determine the hydrocarbon generation intensity within the effective hydrocarbon kitchen's distribution range, a comprehensive evaluation was conducted. This evaluation involved assessing organic matter abundance, type, and maturity of the Upper Triassic source rock across the basin using various geochemical parameters derived from data collection, profile measurement, drill cuttings observation and sampling, and organic geochemistry testing. Key parameters were then selected, and a multi-level fuzzy mathematical evaluation method was employed to map out the development range of the effective hydrocarbon kitchen. The findings indicate that the Upper Triassic source rock exhibits a broad distribution range spanning the entire basin, with considerable thickness (reaching up to 350 m) and significant burial depth in the central region (up to 11 000 m). The total organic carbon (TOC) content of source rocks varies from 0.75% to 8.30%, with an average value of 1.74%. Additionally, the average values of S1+S2 and chloroform asphalt "A" are 2.78 mg/g and 0.354%, respectively. The predominant organic matter types in the area are type Ⅲ and type Ⅱ2, indicating a dominance of terrestrial higher plant inputs. The favorable kerogen types are primarily located from the western sag to the Fukang Sag. The thermal evolution of the organic matter ranges from low-maturity to mature stages, with some areas reaching high to over-maturity levels. As per the evaluation criteria for hydrocarbon generation potential of coal-bearing mudstone in China, the Upper Triassic mudstone generally represents poor to medium-quality source rocks with significant hydrocarbon generation potential, particularly for gas. The key hydrocarbon source kitchens are situated from the western part of the Fukang Sag to the eastern part of the Shawan Sag in the central depression, and in the Huomatu Anticline of the southern margin thrust belt. Within these areas, the Ro value of the source rock exceeds 1.3%, with oil and gas generation intensities reaching 50×104 t/km2 and 3×108 m3/km2, respectively.
Effectiveness of nitrogen compound indices of oil-bearing sandstone extracts
ZHU Nan, WANG Aiguo, GUO Kai, WU Chunyan, LI Chunyu, YANG Zeguang, HE Sensen
2024, 46(2): 393-401. doi: 10.11781/sysydz202402393
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Reports on the utilization of nitrogen compounds to trace oil and gas migration in oil-scarce regions are limited. This scarcity can be attributed to the lack of clarity regarding the effectiveness of nitrogen compound indices in oil-bearing sandstone extracts. To address this knowledge gap, two sets of experiments were designed, encompassing five extraction methods, to evaluate the validity of nitrogen compound indices. The results showed that as the extraction intensity increased, the ratios of 1-/4-methylcarbazole and 1, 8-/2, 7-dimethylcarbazole in the extracts gradually decreased but remained consistently elevated compared to the control oils. The benzocarbazole [a]/[c] ratio remained relatively stable and closely aligned with the control oils. Upon analysis, it was concluded that the observed variations could be attributed to the differing adsorption capacities of carbazole isomers. These differences lead to the fractionation of carbazole isomers between free and adsorbed hydrocarbons under surface conditions, as well as between the extracts and rocks during the extraction process. In comparison to 1-/4-methylcarbazole and 1, 8-/2, 7-dimethylcarbazole, the benzocarbazole [a]/[c] ratio is less susceptible to these two fractionations, with its variations primarily governed by migration fractionation. Additionally, the indicated direction of oil and gas migration aligns with geological realities. Therefore, the benzocarbazole [a]/[c] ratio can be considered an effective nitrogen compound index in oil-bearing sandstone extracts, suitable for oil migration tracing studies in oil-scarce regions.
Displacement simulation experiments of migration effect on carbazole parameters of crude oil
ZHAO Shouyu, XU Yaohui, YAN Gang, HAN Xin, LIU Baolei, ZHONG Ming, LI Shanshan
2024, 46(2): 402-411. doi: 10.11781/sysydz202402402
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The investigation of oil and gas migration plays a vital role in the field of oil and gas exploration and development. Determining the direction and path of crude oil migration is challenging due to factors such as crude oil viscosity, complex chemical composition, and geological influences. Molecular geochemical parameters are a valuable tool for studying petroleum migration, although they can be affected by factors like compound formation, distribution, and maturation. One approach involves simulating these parameters under controlled laboratory conditions to isolate their individual effects. This study establishes a sandy-packed system to simulate oil-driven experiments and uses a C18 solid-phase extraction column to analyze carbazole compound fractions at different migration distances. The findings indicate that certain carbazole parameters, such as 1-/4-MCA, 1, 8-/1, 7-DMCA, 1, 8-/2, 4-DMCA, 1, 8-/2, 5-DMCA, and 1, 8-/2, 7-DMCA, increase significantly with transport distances. Notably, fully shielded and fully exposed isomers like 1, 8-/2, 4-DMCA and 1, 8-/2, 5-DMCA show enrichment levels of up to 63.97% and 35.50%, making them potential indicators of oil migration. However, parameters related to benzo [a]/[c] carbazole show inconsistencies with previous findings, likely due to intrinsic configurations and varying geological conditions that must be taken into account. The experimental results presented in this study offer insights for assessing specific reservoirs, with a recommendation to also consider molecular kinetics when selecting molecular parameters.
Enhancing oil recovery of ultimate water-cut reservoirs with a novel methane-producing bacterial strain
DING Mingshan, LIN Junzhang, FENG Yun, SUN Nan, WANG Guan, BA Yan, WANG Weidong
2024, 46(2): 412-419. doi: 10.11781/sysydz202402412
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Most of the old oil fields in eastern China are now in the ultra-high water-cut development stage characte-rized by a rapid increase in water-cut, low oil recovery rate and inefficient water flooding. The existing technologies for enhancing oil recovery are no longer economically viable for crude oil extraction, necessitating the development of alternative techniques.This study focuses on a post-polymer-flooding oil reservoir in the Shengli Oil Field as an experimental area. The analysis of the reservoir microbial community structure and the study on the activation, reservoir adaptability and oil displacement performance of the novel methane-producing bacterial strain were conducted to explore its application potential in such reservoirs. The results showed a rich population of petroleum hydrocarbon degrading bacteria in the experimental area, which is conducive to the implementation of biogasification technology. Under simulated reservoir conditions, the novel methane-producing bacterial strain had good compatibility with the indigenous microorganism in the reservoir, achieving a gas production rate of 3.12 mmol/g oil after 90 days. This rate was 4.5 times higher than that from activating reservoir microorganisms alone, with methane accounting for 78% of the generated gas. Microbial community structure analysis revealed that the newly discoverd methane-producing bacterial strain comprised 35.9% of the community, playing a vital role in the significant increase in gas production rate. An adaptability study demonstrated that this bacterial strain exhibited exceptional gas production performance at reservoir temperatures below 65 ℃ and crude oil viscosities less than 1 356 mPa·s. Utilizing a laboratory-designed physical model, the impact of the bacterial strain on enhancing oil displacement performance was assessed. These results showed effective mobilization of residual oil at the model's top after bacterial strain injection, leading to a 5.4 percentage point increase in oil displacement efficiency under ultimate water-cut conditions. These findings support the proposal of using biogasification technology to enhance oil recovery in ultimate water-cut reservoirs.
Technique and preliminary application of field analysis of water content for continental shale cores
JIA Mengyao, BAO Yunjie, LI Zhiming, LENG Junying, LIU Wangwei, HE Jinyi, LU Longfei, LI Maowen
2024, 46(2): 420-427. doi: 10.11781/sysydz202402420
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Fluid analysis of continental shale layers plays a crucial role in evaluating shale oil layer selection. Given the current state of technologies of fluid characterization for continental shale layers and well-site requirements, a rapid method for testing water content of rock sample was developed in this study which uses microwave and is ideal for well site. After removing drilling fluid from the core surface, rock samples are promptly collected and crushed into particles using a mill. These granular samples are then placed in the sample cups of a microwave water content tester. Within 2 seconds, mass percentage data for the rock water sample can be obtained, with an average absolute error of ±0.15%. Initial application studies demonstrate promising prospects for moisture detection data in rock samples. Firstly, the water detection results can be used to calculate the pore space occupied by water in the sample's pore system, enabling quick determination of the sample's apparent porosity when combined with well site pyrolysis data. Secondly, the apparent oil saturation and apparent water saturation of core samples can be calculated based on rock sample density and pyrolysis data, facilitating rapid evaluation of rock sample fluids at the well site. This method eliminates the washing and drying processes for rock samples, reduces analysis time, and consequently enhanced the experimental techniques for core porosity and fluid analysis at well sites.
Prediction of petroleum resource abundance based on artificial neural network method: a case study of third member of Paleogene Shahejie Formation in Wenliu area of Dongpu Sag, Bohai Bay Basin
YANG Zijie, CHEN Dongxia, WANG Qiaochu, WANG Fuwei, LI Sha, TIAN Ziye, CHEN Shumin, ZHANG Wanrong, YAO Dongsheng, WANG Yuchao
2024, 46(2): 428-440. doi: 10.11781/sysydz202402428
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The abundance of petroleum resource is influenced by various factors and involves complex parameters and extensive data. Consequently, traditional geostatistical methods often lack precision in quantitative prediction. To address this issue, this study focuses on the third member of Paleogene Shahejie Formation (member Es3) in the Wenliu area of the Dongpu Sag and utilizes a multi-layer perceptron neural network (MLP) for predicting petroleum resource abundance and employed the Boosting ensemble learning algorithm to optimize the prediction model. The MLP and MLP-Boosting algorithm models were test on 66 sample groups, yielding correlation coefficients of 0.789 and 0.989 for the training set, 0.618 and 0.825 for the validation set and 0.689 and 0.845 for the test set. The analysis identified effective thickness, average permeability and effective porosity are the most significant geological factors influencing petroleum resource abundance, with importance coefficients of 33.93%, 20.12% and 19.53%, respectively. Other factors such as trap area, surface crude oil density and sedimentary facies assignment were found to be less influential. Overall, the Boosting ensemble learning algorithm significantly enhanced the prediction accuracy of the multi-layer perceptron model, offering valuable insights for target optimization, exploration planning and petroleum resource evaluation in other blocks in the sag.
2024, 46(2): 441-441.
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