石油实验地质

2023, 45(5): .

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Study on the micro mechanism of shale self-sealing and shale gas preservation

GUO Xusheng, HU Dongfeng, YU Lingjie, LU Longfei, HE Chencheng, LIU Weixin, LU Xiancai

2023, 45(5): 821-831. doi: 10.11781/sysydz202305821

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In order to accelerate the exploration and development of shale gas, this paper elaborates on the micro mechanism of shale gas preservation based on shale self-sealing. The self-sealing capacity of shale formations is mainly related to the low connectivity of nano-throats, low speed diffusion caused by bound water occurrence, capillary force sealing, and breakthrough pressure along the layer direction under burial conditions. Results of analysis based on shale pore morphology and connectivity show that the organic pores in shale are mainly composed of nano-throats with poor connectivity and significant retention effects. At the same time, the stacking of multiple relatively dense sealing layers within the top, bottom, and shale layers is conducive to self-sealing in longitudinal direction. Based on experimental and molecular dynamics simulation analysis of the influence of bound water on the diffusion and breakthrough pressure of the shale matrix, it is revealed that the bound water occurrence significantly reduces the effective diffusion capacity of shale matrix pores and could cause high capillary forces, leading to effective sealing of the gas stored in organic pores. In the study, the permeability-breakthrough pressure evolution relationship was constructed, revealing that under deep burial conditions, the effective closure of shale bedding fractures along the layer direction can form high breakthrough pressure sealing. During the uplift stage, under relatively weak tectonic compression, the shale could still maintain a high sealing capacity along the layer direction, which is conducive to shale gas preservation. However, under strong tectonic compression, the shale bedding fractures open and communicate with open fault surface, causing the failure of preservation capacity and large-scale loss of shale gas. This study elucidates the microscopic mechanism of shale gas preservation through experiments and molecular dynamics simulations, which could provide useful guidance for marine shale gas exploration in complex structural areas.

Methods and key parameters for oil and gas resource assessment and distribution characteristics of oil and gas resource: a case study of resource assessment of SINOPEC during the 13th Five-Year Plan period

XU Xuhui, ZHOU Zhuoming, SONG Zhenxiang, YANG Guoqiao

2023, 45(5): 832-843. doi: 10.11781/sysydz202305832

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Based on the research and optimization of oil and gas resource assessment methods, a comprehensive oil and gas resource assessment method process and technical system, which integrated assessment of conventional and unconventional resources as well as assessment of geological, economic and ecological aspects, were established for SINOPEC-controlled exploration areas during the 13th Five-Year Plan period. The study focused on the key parameters for resource assessment, including a complete hydrocarbon generation-expulsion-retention evolution model and productivity model applicable to different types of source rocks in different basins/regions in China based on TSM basin simulation technology, and a key parameter acquisition method and assessment indicators for resource assessment based on typical calibrated unit dissection. Through systematic assessment of all oil and gas bearing layers (from Sinian system to Quaternary system) of nearly 50 basins/regions in SINOPEC-controlled exploration areas, the geological resources of seven different types of oil and gas, including conventional oil and gas, tight oil and gas, shale oil and gas, and coalbed methane were assessed, which clarified the economical recoverability and environmental impact of oil and gas resources. By studying the resource distribution characteristics, the future exploration areas and directions were further clarified. It is indicated that the deep-ultra deep layers are the main focus for conventional oil and gas exploration, while unconventional resources, especially shale oil and gas, are important areas for increasing oil and gas reserves and production in the future. The sea exploration area also has broad exploration prospects. At the same time, the results of resource assessment provide important basis for the planning and exploration deployment of oil companies during the 14th Five-Year Plan period.

Structure and evolution of Precambrian basement in southwestern Tarim Basin

CAO Zicheng, JIANG Huashan, ZHANG Juncheng, GENG Feng, SHA Xuguang, HAO Jianlong, LI Tong, GUO Xiaowen, TAO Ze

2023, 45(5): 844-856. doi: 10.11781/sysydz202305844

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The intensifying exploration for deep-buried oil and gas reserves in the southwestern Tarim Basin accelerates the necessity to understand how the structure and evolution of the Precambrian basement affect the development and distribution of Precambrian strata. However, the lack of systematic and comprehensive understanding of the structural characteristics of the Precambrian basement in the southwestern Tarim Basin restricts further advancements in the exploration of ultra-deep oil and gas reserves in this area. Based on the previous research results, this study analyzed the lithology, age, structure, and evolution of the Precambrian basement in the southwestern Tarim Basin by utilizing integrated research methods of geology and geophysics and in combination with the U-Pb dating results of the Precambrian basement. The results reveal that the structure of the Precambrian basement in the Bachu uplift region is characterized by Paleozoic strata overlying directly on the Paleo-proterozoic metamorphic rocks/granites. In the Maigaiti slope and southwestern depression, it exhibits a typical three-tier structure: the Paleoproterozoic metamorphic rocks/granites at the bottom, overlain by sedimentary build-ups from the rifting to depression evolution of the Nanhua-Sinian system. Evolution of the Precambrian basement in the southwestern Tarim Basin was primarily transpired in five phases: continental rifting in the Columbia supercontinent (> 1.1 Ga), assembly of the southwestern Tarim block and the Australian plate (about 1.0 Ga), assemblage between the northern and southern Tarim blocks (900-800 Ma), Nanhua rifting period (760-640 Ma), and Sinian depression period (< 635 Ma). The uplift and downwarping structures during the Nanhua rifting period dictated the development of the Sinian-Lower Cambrian sedimentary strata. In-depth research into the structure and evolution of the Precambrian basement in the southwestern Tarim Basin, as presented in this study, provides important theoretical basis for deep oil and gas exploration in the region, and helps to promote further development of ultra-deep oil and gas exploration.

Fine-grained sedimentary characteristics and influencing factors of the Lower Cambrian Qiongzhusi Formation in Sichuan Basin and on its periphery

XIONG Liang, DENG Hucheng, WU Dong, WEI Limin, WANG Tong, ZHOU Hua, CAO Kaixuan, XIE Xinhui, MA Ruolong, ZHONG Yutao

2023, 45(5): 857-871. doi: 10.11781/sysydz202305857

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The Lower Cambrian Qiongzhusi Formation in Sichuan Basin and on its periphery is rich in shale gas resources, and it has become one of the important shale gas development intervals in Sichuan Basin. Controlled by complex sedimentary setting, the silty shale reservoir in Qiongzhusi Formation has various lithofacies and strong heterogeneity longitudinally, which restricts the exploration and development of shale gas. Therefore, it is urgent to study the sedimentary characteristics, sedimentary facies and sedimentary models. Based on the previous understanding of tectonic setting and sequence stratigraphy, and through detailed analysis of rock data, well logging data, geochemistry data and paleontology data, this paper clarifies the Early Cambrian sedimentary background of combined ocean-paleoland influence, moderate water depth and widespread reduction of Sichuan Basin and its periphery. The paper also believes that the Qiongzhusi Formation is mainly deposited in shore facies and continental shelf facies. The continental shelf facies are dominated, including shallow water continental shelf subfacies and deep water continental shelf subfacies. Based on the analysis of the combined action of wave and tide, seawater evaporation, suspension, rising ocean current, sedimentary paleogeomorphology and other fine-grained sedimentary influencing factors, and combined with the spatial distribution characteristics of sediments, the fine-grained sedimentary model of the Lower Cambrian Qiongzhusi Formation in Sichuan Basin and on its periphery is established, i.e., in the early stage of sedimentation, the effect of differential sedimentation is significant, the sequence thickness varies greatly, and the sedimentary characteristics and influencing factors differ greatly in different regions. In the middle stage of sedimentation, the effect of differential sedimentation is diminished and the difference of sequence thickness decreases continuously. In the late stage of sedimentation, with steady sedimentation, it enters the "open basin" stage, and the sequence thickness tends to be consistent.

Resource potential in Junggar Basin and SINOPEC's integrated exploration strategy for conventional and unconventional petroleum

ZHAO Yongqiang, SONG Zhenxiang, WANG Bin, QIU Qi, SUN Zhongliang, WU Xiaoqi

2023, 45(5): 872-881. doi: 10.11781/sysydz202305872

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The Junggar Basin is rich in oil and gas resources, with huge exploration potential, which is one of the main arenas of increasing oil and gas reserve and producing in China. In recent years, CNPC has made major breakthroughs in exploration in the Junggar Basin, revealing the orderly symbionic distribution pattern of conventional and conventional oil and gas. In order to further learn from the oil and gas achievements and exploration and development experience in neighboring areas and the new geological theories, for formulating SINOPEC's oil and gas exploration strategy in the Junggar Basin, the potential and distribution characteristics of the oil and gas resources in the Junggar Basin are systematically analyzed in this paper and it is pointed out that no enough attention is paid to natural gas and unconventional petroleum. The key breakthrough direction and exploration strategy of SINOPEC in the Junggar Basin are put forward in the paper after reviewing the major exploration breakthroughs in multiple fields, multiple strata, and multiple types in recent years and combining with the oil and gas geolo-gical conditions and exploration practice of SINOPEC's mining rights area. The research reveals that the Junggar Basin has entered a new period of exploration dominated by the middle and lower assemblage, and the exploration ideas must be adjusted accordingly. We must insist on the transformation from early outside-source exploration to intra-source and near-source exploration, and from early conventional petroleum exploration to both conventional and unconventional petroleum exploration. In order to make big exploration breakthrough of SINOPEC and realize the target of increasing reserves and production in the near and medium term in the Junggar Basin as soon as possible, four exploration strategies are proposed in the paper, including further figuring out the resources, establishing an integrated research team of conventional and unconventional petroleum, strengthening the joint research of geological and engineering integration, and increasing the support of risk exploration.

Effect of low-temperature phase change on the re-equilibration of brine inclusions hosted in quartz minerals

LIU Keyu, ZHANG Shulin, YANG Peng

2023, 45(5): 882-890. doi: 10.11781/sysydz202305882

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Microthermometric measurement of fluid inclusions should follow the sequence of measuring homogenization temperature (T_h) first, followed by the measurement of final freezing point temperature (T_m). However, in practice, this measurement sequence has not always been strictly followed. Many researchers may repeatedly measure the homogenization temperature of fluid inclusions even after the measurement of the final freezing point temperature. This may result in erroneous microthermometric data being obtained. In order to highlight this issue and to emphasize the importance of following the correct microthermometric measurement sequence, this study carried out repeated microthermometric measurements of brine inclusions and quantitatively compared the differences in the microthermomitric data obtained from different microthermomitric measurement sequences. The measurement exercise was aided by petrographic observation of brine inclusions and microscopic laser Raman analysis and fluid inclusion PVTx modelling. The results indicate that prior to the measurement of the final freezing point temperature, the homogenization temperature of brine inclusions can be repeatedly measured with the T_h data showing no obvious deviation. During the measurement of the final freezing point temperature, the brine inclusion may undergo stretching or even leakage, and any subsequent T_h measurements may yield elevated values, but T_m value remains largely unchanged. Therefore, low-temperature (freezing) phase-change measurement may cause resetting of the original PVTx information recorded. Compared to calcite minerals, brine inclusions hosted in quartz have stronger resistance to the influence of re-equilibration, and higher vapor filling degree and high salinity can offset the influence of brine inclusion re-equilibration to a certain extent. In the process of low-temperature phase change temperature measurement, due to the inflation caused by water freezing, it is common for brine inclusions to undergo stretching or even leakage, leading to a general increase in the homogenization temperature of brine inclusions.

Progress in the study of carbonate clumped isotope in the thermal history of marine basins

QIU Nansheng, LIU Xin, XIONG Yujie, LIU Yuchen, XU Qiuchen, CHANG Qing

2023, 45(5): 891-903. doi: 10.11781/sysydz202305891

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The study of thermal history of sedimentary basins is very important for oil-gas accumulation analysis and has decisive influence on oil and gas exploration and oil-gas resource evaluation. However, it is difficult to reconstruct the thermal history of carbonate strata in marine basins due to the lack of effective palaeo-thermometer. The carbonate cluster isotope is an index that can indicate the formation temperature of carbonate rock. It records temperature information in chemical bond through conjugated ¹³C-¹⁸O, and temperature information is revealed through changes in the abundance of conjugated ¹³C-¹⁸O, namely, the abundance of carbonate clumped isotope (Δ₄₇) is closely related to the evolution of strata temperature. This paper reviews the application of carbonate clumped isotope palaeo-thermometer and discusses the existing problems and future development direction based on the basic principle and methods of carbonate clumped isotope thermometry and thermal history reconstruction, and combined with the case studies of thermal history reconstruction in marine basins in recent years. As a new palaeo-thermometer, carbonate clumped isotope shows great potential in the thermal history reconstruction of carbonate strata. Previous studies show that the maximum paleotemperature of the Bird Spring Formation in Arrow Canyon, Nevada, USA is about 175 ℃, and the maximum burial temperature of Permian Maokou Formation in the Sichuan Basin is 213-225 ℃. There is an abnormal high temperature of above 160 ℃ in the Ordovician strata of the Tarim Basin due to the Permian magmatic activity. We believe that the carbonate clumped isotope is still insufficient in the reconstruction of thermal history due to the immaturity of thermal evolution models, the recrystallization, and the change of burial temperature. Thus, we suggest that combining the carbonate clumped isotope, U-Pb dating, and other palaeo-thermometers to accurately reconstruct the thermal history of carbonate strata will be the future development direction.

Molecular carbon isotopic geochemistry records of thermal cracking in the palaeo-reservoir of well Shunnan 1 in Guchengxu Uplift, Tarim Basin

SUN Yongge, KONG Lishu, LU Qinghua, GU Yi

2023, 45(5): 904-911. doi: 10.11781/sysydz202305904

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The highly mature light oil from well Shunnan 1 in the Guchengxu Uplift, Tarim Basin is considered to be a typical residual oil by thermal cracking. The study reveals that the abundant monoaromatic hydrocarbons and polynuclear aromatics in the crude oil of well Shunnan 1 are the by-products of severe in-reservoir oil-cracking during the geological history. The monoaromatic hydrocarbons are the intermediate products of thermal cracking, cyclization and aromatization, while the polynuclear aromatics are indicative of polycondensation reaction. The strong carbon isotopic fractionation induced by hydrocarbon molecule cracking and polycondensation makes the molecule carbon isotope of n-alkanes and the molecule carbon isotope of alkyl naphthalenes in the crude oil of well Shunnan 1 respectively 8‰-10‰ and 6‰-8‰ heavier compared to those from unaltered crude oils in Lower Paleozoic oil-bearing system. The results further proved that the aromatic molecules related to oil cracking and their stable carbon isotopic compositions could be a potentially powerful tool to identify whether thermal alteration occurred in deep and ultra-deep oil reservoirs, and further helps to effectively evaluate the oil and gas phase and resource pattern in the deep and ultra-deep reservoirs.

Nitrogen isotope compositions and organic matter accumulation in terrestrial hydrocarbon source rocks in China

CAO Jian, XIA Liuwen, HU Wenxuan, STÜEKEN Eva E, ZHI Dongming, TANG Yong, XIANG Baoli, HE Wenjun

2023, 45(5): 912-925. doi: 10.11781/sysydz202305912

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As an important biological element, nitrogen is causally linked with organic matter accumulation, but a systematic and regular understanding has not been developed. This paper discusses this issue by taking the widely developed Permian-Paleogene terrestrial (lacustrine) hydrocarbon source rocks in China as an example. The results show that the terrestrial source rocks in China can be classified into three groups according to the nitrogen isotope (δ¹⁵N) compositions and combined with the salinity and evaporative alkali mineral characteristics: the circum-neutral group 1 (average δ¹⁵N=4.0‰±1.5‰), the circum-neutral group 2 (average δ¹⁵N=7.1‰±1.6‰), and the alkaline group (average δ¹⁵N=18.4‰±3.3‰). In the circum-neutral group with δ¹⁵N < 10‰, the δ¹⁵N of the source rocks is positively correlated with organic matter abundance, type, hydrocarbon generating capacity and shale oil potential due to the fact that higher δ¹⁵N characterizes changes in the composition of the hydrocarbon generating bio-precursors. In the alkaline group with δ¹⁵N>10‰, the organic matter types are good and shale oil potentials are higher, but the response relationship between δ¹⁵N of source rocks and organic matter accumulation is not as good as that of the circum-neutral group, reflecting that the organic matter accumulation in alkaline group is influenced by other comprehensive factors other than δ¹⁵N. Accordingly, the organic matter accumulation models of three types of lacustrine source rocks (< 5‰, 5‰-10‰, and >10‰) classified on the basis of δ¹⁵N were established. The δ¹⁵N has the potential to trace organic matter accumulation and quality of source rocks. For example, low δ¹⁵N type (δ¹⁵N < 5‰) is of poor quality and medium-high δ¹⁵N type (δ¹⁵N>5‰) is of good qualityin lacustrine source rocks. This paper enriches the biogeochemical and hydrocarbon source rock geochemical studies of nitrogen by exploring organic matter accumulation in source rocks from the new perspective of nitrogen isotope composition and nitrogen cycling.

Decoding sedimentary environment and synsedimentary tectonic activity information in nodules: a case study of the black rock series in the border area of Guizhou and Hunan

WANG Longzhang, LIU Ling, XIE Xingyou, SHI Rui, FENG Kaiyou

2023, 45(5): 926-935. doi: 10.11781/sysydz202305926

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The black rock series contains energy resources such as oil and gas, coal, shale gas, and various solid resources such as iron, manganese, phosphorus, and barium. The restoration of sedimentary reservoir formation and sedimentary mineralization conditions is the cornerstone of resource prediction, and the restoration of ancient sedimentary environments and synsedimentary tectonics has therefore received additional attention. There are a large number of nodules in the black rock series in the border area of Guizhou and Hunan, and the nodules have sealing properties on the original records. Therefore, based on the observation of exposed profiles in the field and laboratory microscopic analysis, and through the integrated analysis of sedimentation and tectonic processes, it is found that there are significant differences in mineral composition and textural types among nodules in different layers: (1) The nodules of Upper Sinian Series-Cambrian Terreneuvian Series Liuchapo Formation (Z€l) are an assemblage of multi-components such as barite, calcium, phosphorus, and silicon, with texture of gravel or sand fragments. The cementation temperature of the two nodule layers is different, and the one directly covering the barite ore is relatively higher; (2) The nodules of Cambrian Series 2 Palang Formation(€₂p) are mainly composed of pyrite and three stages of growth structure are developed: the core layer shows a framboidal texture, and the outerside two layers show a nephritic shape; (3) The nodule components of Cambrian Series 2 Wuxun Formation (€₂w) are calcium and pyrite, of which calcium is dominated, with scattered pyrite. There are four stages of growth structures, with relatively higher content of pyrite in the first and third stages. Different paleose-dimentary environmental conditions and synsedimentary tectonic activity information can be decoded from nodules at different layers: (1) The barite bearing nodules in the Liuchapo Formation belong to low-temperature hydrothermal origin, and the reduction environment is limited near the fault zone; (2) The pyrite nodules in the Palang Formation are formed near the redox interface, and the environment has medium conditions of high concentration of active iron; (3) The calcareous and pyrite nodules of the Wuxun Formation were formed in a periodic reducing environment of low concentration iron, which is speculated to be caused by frequent relative sea level fluctuation caused by episodic subsidence of the basement, leading to frequent changes in the redox properties of the water medium. Therefore, the study of nodules helps to reconstruct the medium conditions of black rock series and is of great significance for resource prediction.

Characteristics of pressure relief induced by shale brittle fracture in tectonic uplift area and its influence on shale oil enrichment: a case study of Chang 7₃ sub-member of Yanchang Formation in Yan'an area

ZHANG Fengqi, SUN Yue, LIU Siyao, LI Yanxia, SUN Jianbo, WANG Fengqin, LIU Gang, CHEN Hongguo

2023, 45(5): 936-951. doi: 10.11781/sysydz202305936

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In order to gain a deeper understanding of the pressure relief characteristics of shale brittle fracture caused by large-scale tectonic uplift and its impact on shale oil enrichment, this paper, taking the Chang 7₃ sub-member of Yanchang Formation in Yan'an area of Ordos Basin as the research object, quantitatively analyzes the evolution of different lithological properties from overpressure to underpressure or near normal pressure, and further summarizes its influence on the enrichment of shale oil by utilizing the modified overpressure identification plate and numerical simulation techniques and taking into account the effects of temperature reduction, elastic rebound, and brittle rupture of overpressured shale in the process of tectonic uplift. The results show that the origins of overpressure at the end of the Early Cretaceous in the Chang 7₃ sub-member in the study area are mainly hydrocarbon generation and disequilibrium compaction, and the overpressure of the sandstone layer mainly originates from the overpressure transference from the neighboring source rocks; elastic rebound and temperature reduction due to tectonic uplift since the Late Cretaceous dominates the formation of underpressure in the Chang 7₃ sub-member sandstones in the study area, and brittle fracture pressure relief and temperature reduction lead to the formation of weak overpressure or near normal pressure in the Chang 7₃ sub-member shale. The timing of brittle fracture pressure relief of the top and bottom sets of organic-rich shale in the Chang 7₃ sub-member of the study area is not synchronized, resulting in large transport dynamics (source-reservoir excess pressure difference) between the top and bottom sets of organic-rich shale and their intercalated sandstones with magnitudes ranging from 16 to 22 MPa. Meanwhile, fractures in the top shale have closed during oil and gas preservation, and there is an excess pressure differential of about 1.42-6.80 MPa between it and the underlying sandstone, which strengthens the hydrocarbon containment capacity of the shale. Differential evolution of formation pressure under different source-reservoir configurations plays an important role in controlling shale oil enrichment.

Discussion on shale oil enrichment pattern in the Ⅲ submember of the third member of Oligocene Hetaoyuan Formation, Biyang Sag, Nanxiang Basin: a case study of cored interval of well YYY1 in the central deep sag zone

LI Zhiming, JIN Yunyun, LI Chuxiong, HUANG Shuaibo, ZHOU Yuanyuan, JIA Mengyao, LENG Junying, YU Mengli, XU Ershe, LIU Yahui, LIU Peng, HE Jinyi

2023, 45(5): 952-962. doi: 10.11781/sysydz202305952

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The third member of Oligocene Hetaoyuan Formation (Eh₃) in Biyang Sag of Nanxiang Basin is the first strata in which a breakthrough for continental shale oil exploration is made, but scale development has not been realized due to a variety of factors. In order to reveal the target zone of shale oil exploration and development of the third member of Hetaoyuan Formation in the central deep sag zone of Biyang Sag, a case study on the cored interval of the Ⅲ submember of the third member of Hetaoyuan Formation (Eh₃³) from shale oil risk exploration well YYY1 is carried out. Systematic studies on the lithofacies types and the characteristics of shale oil enrichment intervals of the cored interval of Eh₃³ are carried out and the shale oil enrichment pattern and preferred target layers of shale oil exploration and development are defined by taking full advantage of the results of field analysis and combining with previous research results. The findings have shown that seven lithofacies types, i.e. organic laminar mixed shale, organic-rich laminar mixed shale, organic laminar felsic shale, organic-rich laminar felsic shale, organic layered felsic shale and organic massive argillaceous siltstone/siltstone, organic massive fine sandstone, are developed in the middle-lower interval of Eh₃³ of well YYY1. Eight shale oil enrichment intervals are identified according to two key parameters of free hydrocarbon (S₁) and oil saturation index (OSI) and combining with the total porosity of nuclear magnetic logging. The thickness of a single shale oil enrichment interval is 5-20 m and the total thickness is 81 m. The shale oil enrichment intervals developed in the middle interval of Eh₃³ are all controlled by both in-situ retention hydrocarbon and migration hydrocarbon from the adjacent high TOC layer, with two enrichment patterns: source-reservoir integration and source-reservoir coexistence. The shale oil enrichment intervals developed in the lower interval of Eh₃³ are all controlled by in-situ retention hydrocarbon, with source-reservoir integration enrichment pattern. The shale oil enrichment interval in the middle interval is defined the preferred target interval for shale oil exploration and development in Eh₃³ in the central deep sag zone of Biyang Sag. This provides an important basis for shale oil exploration and development in Biyang Sag.

Review on molecular structures of asphaltene macromolecules and instrumental analytical approaches

ZHANG Zhirong, TAO Guoliang, SNOWDON Lloyd R, VOLKMAN John K, CHEN Yan'e

2023, 45(5): 963-972. doi: 10.11781/sysydz202305963

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As one of the major fractions of petroleum and/or source rock extracts, asphaltene is commonly characte-rized by complicate molecular structures as well as relatively higher molecular weight (MW). The asphaltene occluded biomarkers have been well introduced to be valuable for geochemical research whilst the actual molecular weight and molecular structures of asphaltene macromolecules studies show value in petroleum exploration and production, laboratory treatments as well as petroleum chemical storage and transportation etc. The structure, molecular weight and related experimental measurement methods of asphaltenes reported in this paper are reviewed, in an attempt to provide a more comprehensive understanding on the molecular structure, molecular weight and physicochemical properties of asphaltenes, so as to provide basic data for applying asphaltene geochemical research in the exploration and development study of conventional and unconventional reservoirs. In view of the different views on the structure and molecular weight of asphaltene molecules, the results are reviewed, the main reasons for the controversy are discussed based on the current status of experimental measurement technology, and the future development trend of related researches is pointed out. The current understanding of the structure of asphaltene macromolecules is mainly based on two completely different models, i.e., the island model and archipelago model. The former suggests that asphaltenes have a single aromatic nucleus and the molecular weight characterized is relatively lower. The latter believes that asphaltenes have multiple aromatic nuclei and the molecular weight characterized is relatively higher. The above two models of asphaltene molecular structure are supported by their own experimental measurement results, among which the evidences of the "island" model mainly come from fluorescence spectroscopy, laser vaporization/laser ionization mass spectrometry (L2MS) analysis and stem from various ionization high-resolution (Fourier transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) analysis technologies. The evidences for the archipelago model mainly come from gas phase permeation measurement, size exclusion chromatography and chemical degradation.

Basis of petroleum geological theory and exploration direction for ultra-deep exploration of 10 000-meter depth

JIN Xiaohui, MENG Qingqiang, SUN Dongsheng, TIAN Jinqiang, LIANG Shiyou, LI Qiang

2023, 45(5): 973-981. doi: 10.11781/sysydz202305973

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Oil and gas resources are of important strategic importance for China's economic development. After more than 50 years of mining in main oil fields, the water cut of oil fields in China has exceeded 90%. China's external dependence on oil and natural gas has increased from 58.9% and 29.2% in 2013 to 71.2% and 40.2% in 2022 respectively, reaching the oil security warning line and impeding national security. In order to find more replacement reserves, oil and gas exploration has been promoted to the "deeper and more complicated" ultra-deep exploration area of 10 000-meter depth. Recently, domestic oil companies have successively implemented the "Deep Sea No. 1" and "Deep Earth No. 1" oil and gas exploration plans, which have explored and practiced new technologies of intelligent drilling and promoted the implementation of the 10 000-meter exploration plans, and reservoirs have been encountered by wells over 9 000 m deep. Based on recent exploration practice, this paper demonstrates the basis of petroleum geological theory for exploration of 10 000-meter depth from the aspects of ultra-deep hydrocarbon generation, storage and accumulation mechanisms, and it suggests that multi-source hydrocarbon generation can be identified in ultra-deep strata and the hydrocarbon generation threshold has exceeded 120-160 ℃. The "three-element controlling reservoir" theory points out that deep and ultra-deep reservoirs can be developed at depths greater than 4 500 meters and multi-phase hydrocarbon accumulation and preservation constitute a favorable condition for the development of ultra-deep large-scale oil and gas reservoirs. The Sichuan Basin has the total resources of about 15.22×10¹² m³ in deep marine strata, wherein the Sinian system on the margin of Mianyang-Changning taphrogenic trough, the Longwangmiao Formation on the margin of central Sichuan palaeo-uplift, the Longwangmiao Formation in Micang Mountain-Daba Mountain areas to the east of Chengkou oceanic trough, and the Upper Paleozoic in western Sichuan have a resource potential of more than hundreds of billions of cubic meters. The Tarim Basin has about 15.262×10⁹ tons of equivalent oil in deep marine strata, wherein the Middle-Lower Ordovician karst area in the Tabei uplift, Tazhong uplift and Maigaiti slope, the Middle-Lower Cambrian platform marginal zone on the western margin of Manjiaer Sag, and the Middle and Lower Cambrian intraplatform shoal of Awat-Shuntuoguole are all oil potential areas of hundreds of millions of tons. There are important oil and gas discovery in the Middle Neoproterozoic in the southwest edge of Ordos Basin. Additionally, the field of clastic rocks such as the deep strata in the central deep depression zone in Junggar Basin and the Kuqa Depression in Tarim Basin are also important areas of exploration of 10 000-meter depth.

Characteristics, controlling factors and exploration prospects of microbial dolomite reservoirs in the second member of Dengying Formation, Penglai-Zhongjiang area of central Sichuan Basin

WEN Long, ZHANG Jianyong, PAN Liyin, MA Hualing, LI Zeqi, LI Wenzheng, XIE Wuren, Wang Zecheng, FU Xiaodong, WANG Yongxiao, LI Rong

2023, 45(5): 982-993. doi: 10.11781/sysydz202305982

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Abstract:
The formation and evolution of dolomite reservoirs in the Sinian Dengying Formation in the Sichuan Basin have attracted much attention recently. Due to their economic significance for hosting natural gas resources, determining the key factors that govern the formation and evolution of these dolomite reservoirs are important for hydrocarbon exploration and development. Based on the drilling and seismic data in the Penglai-Zhongjiang area of the Sichuan Basin, the main controlling factors for the development of high-quality reservoirs in the second member of Dengying Formation are investigated by integrating core and thin section observation and geochemical analysis. Lithologies of the reservoirs in the second member of Dengying Formation are dominantly microbial dolomites, dolo-grainstones, and brecciaed dolomites. Moreover, reservoir spaces in these dolomites are mostly dissolution pores, residual framework pores, and breccia pores. The porosity ranges from 2.0% to 8.0% with an average of 4.39%, and the average permeability is 0.53×10^-3 μm², and the reservoir thickness is 170-320 m. Synsedimentary faulting had caused the differentiation of depositional paleo-geomorphology and led to the formation of submarine high barriers, which provided favorable conditions for the development of microbial mound (reef) and shoal complexes. Furthermore, fault activities could have broken the weakly consolidated carbonate sediments into breccias, thus resulting in the brecciaed dolomite reservoir. Penecontemporaneous dissolution is the key to the development of high-quality reservoirs in the second member of the Dengying Formation, which is consistent with their occurrence in the upper part of the shallowing-upward cycles. Differentiation in microfacies leads to distinct diagenetic pathways and porosity evolution of these microbial-dominant carbonate sediments. Overall, dolomite cementation during shallow burial had controlled thedegree of pore preservation. It is predicted that three fault-controlled, platform-margin mound regions in the study area, i.e., Penglai-Zhongjiang, Yanting-Mianyang, and Cangxi-Guangyuan, with areas of 1 600 km², 1 870 km² and 2 280 km², respectively, had been developed in the second member of Dengying Formation in the north slope of the central Sichuan paleo-uplift, showing multi-stage and multi-zone characteristics. Microbial mounds in the platform margin of the second member of Dengying Formation in the Yanting-Mianyang and Cangxi-Guangyuan areas, with three sets of high-quality source rocks, may form a variety of favorable source-reservoir combinations. Additionally, the accumulation conditions may have been superior. The estimated natural gas resources in the study area, over one trillion square meters, makes it a favorable area for ultra-deep carbonate exploration and is expected to be a breakthrough site for the next one trillion square gas field in the Sichuan Basin.

Accumulation conditions and rolling exploration results in the periphery of "Deep Sea No. 1" giant gas field in central canyon of Qiongdongnan Basin

CHEN Kui, WANG Wenjuan, XU Wanxing, JIANG Zhandong, SHU Kedong, HUANG Anmin

2023, 45(5): 994-1006. doi: 10.11781/sysydz202305994

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Abstract:
In order to explore high-quality proven geological reserves for the "Deep Sea No.1" giant gas field in the central canyon of the Qiongdongnan Basin and promote the sustainable development of the giant gas field, the rolling exploration technology is innovatively introduced into the deep-water natural gas exploration in the central canyon. The main gas-bearing strata such as the Upper Miocene Huangliu Formation and the Pliocene Yinggehai Formation on the periphery of mature exploration area of the gas field are studied. Five potential blocks are searched for rolling exploration, and Block 4 with superior reservoir forming conditions and the ability to control geological reserves is selected. The reservoir forming conditions of T₂₉B and T₂₉E sand bodies in the Yinggehai Formation, upper HL-Ⅰ, HL-Ⅲ and HL-Ⅳ gas groups in the Huangliu Formation are studied. By analogy with drilling data and accumulation rules of the surrounding wells, the accumulation conditions such as trap, reservoir deposition and oil and gas migration and accumulation of Block 4 are studied. The T₂₉E sand bodies of the Huangliu Formation and the Yinggehai Formation in Block 4 are all lithologic traps formed by the joint closure of the central canyon channel bank and the weak-amplitude muddy channel in the later stage of the canyon. The T₂₉B sand body is an anticline trap formed by the local high point of the submarine fan under the canyon background. Through the amplitude attribute sand body characterization and forward modeling, the reservoir-seal combination of the target layer of the Huangliu and Yinggehai formations in Block 4 is well implemented. The vertical migration modes such as diapirs, high-angle fractures and micro-fractures are migrated to the target layer traps, and the effectiveness of the top and bottom plates of the traps is good, thus playing a good role in preserving oil and gas. According to the accumulation factors, the resource parameters and potential resources are implemented, and the deployment of deep-water rolling exploration wells is carried out. The deep-water directional drilling technology is innovatively introduced, and the well L17-10d is deployed to implement the gas-bearing strata and reserve scale as much as possible, and good drilling results are achieved. The well L17-10d encountered a pure gas layer in the T₂₉B, T₂₉E sand bodies and the HL-Ⅲ gas group. The vertical thickness of the gas layer was nearly 45 m, and the proven geological reserves were nearly 7 billion cubic meters. It further confirmed the accumulation law of the deep water area of the central canyon, pointed out the direction for the rolling exploration of the surrounding area, promoted the production of proven geological reserves and controlled geological reserves of some difficult-to-produce natural gas, and provided a strong guarantee for the subsequent sustainable development of the "Deep Sea No.1" giant gas field.

Characterization parameters of the evolution degree of strike-slip faults

DENG Mingzhe, CAI Pengrui, LU Jianlin, DENG Peng

2023, 45(5): 1007-1015. doi: 10.11781/sysydz2023051007

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Abstract:
Strike-slip faults and their related structures have great value for oil and gas exploration. Accurate understanding of the evolution degree of strike-slip faults is crucial to oil and gas exploration in strike-slip structural belts. Previous studies have had a qualitative understanding of the growth and evolution process of strike-slip faults, but there is still a lack of objective and quantitative basis to determine the evolution degree of strike-slip fault zone. The key is the lack of parameters that can characterize the evolution degree of strike-slip faults. In order to solve this problem, this paper, based on the independent physical simulation experiment of Riedel shear structure, collected plane images and carried out a statistical analysis of fault geometry parameters in strike-slip fault zone. By analyzing the corresponding relationship between the periodic change of plane structure and the change of geometric statistical parameters during the evolution of fault zone, the change law of geometric parameters during the evolution of fault zone is revealed, and a new understanding is proposed that the standard deviation of the angle data between adjacent faults in the same fault zone is taken as the characterization parameter of the evolution degree of strike-slip faults. The main conclusions are as follows: (1) The evolution degree of strike-slip fault zone is significantly related with the standard deviation of the angle data between adjacent faults in fault zone, and the plane structural style in different evolution degrees corresponds to the distribution interval of the standard deviation of the angle data between adjacent faults in fault zone; (2) According to the combination pattern of fault planes, the evolution stage of strike slip fault can be divided into shear unidirectional fracture stage, shear en echelon fracture stage, and shear connected fracture stage, and the corresponding standard deviation (S) of the angle data between adjacent faults in fault zone is 1 < S < 4, 4≤S < 8, and S≥8, respectively; (3) For the actual strike-slip fault zone, the evolution degree of strike-slip faults can be revealed by statistical analysis of the standard deviation of the angle data between adjacent faults in fault zone.

In-situ thermal simulation experiment of single organic matter pore evolution in shale

XI Binbin, PAN Anyang, BAO Fang, LU Longfei, CAO Taotao, WANG Ye, MA Zhongliang, LIU Xian

2023, 45(5): 1016-1025. doi: 10.11781/sysydz2023051016

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Abstract:
Organic matter pores is the most important shale reservoir space in South China, and the types of organic matter influence the formation and evolution process of organic matter pores. In order to achieve in-situ observation of the pore evolution process of single organic matter and reveal the pore evolution process during the pyrolysis of the macerals of the Lower Paleozoic shale, this paper focuses on the low-maturity Upper Ordovician shale from Ohio, USA and the Lower Silurian shale in the eastern Baltic Sea, Europe. By using femtosecond laser, cooling-heating stage, microscope and SEM techniques, the in-situ observation of pore evolution of the single organic matters in shale can be realized, and the pore development process and evolution law of different types of organic matters can be distinguished. The in-situ observation results of pore thermal evolution process of low-maturity "gas-prone" graptolite and "oil-prone" lamalginite show that: (1) The initial temperature of obvious volume change of graptolite is higher than that of lamalginite, which infers that the hydrocarbon generation of graptolite is later than that of lamalginite. (2) Both graptolite and lamalginite generated obvious shrinkage cracks during thermal evolution process, but the scale of shrinkage cracks of graptolite is smaller than that of lamalginite, indicating that the hydrocarbon-generating transformation ratio of graptolite is lower than that of lamalginite. (3) There is an obvious difference between graptolite and lamalginite in the evolution of internal pores: the intrinsical biopores of graptolite were enlarged and some new internal pores were formed during the high temperature stage, but no new internal pores of lamalginite were observed during the whole process of thermal evolution, which infers that "gas-generating window" is the main stage of organic matter pore development. Moreover, the difference in the composition and structure of organic matters may be the main reason for the difference in pore evolution of graptolite and lamalginite.

Application of sparry grain limestone petrographic analysis combining image processing and deep learning

YU Xiaolu, LI Longlong, JIANG Hong, LU Longfei, DU Chongjiao

2023, 45(5): 1026-1038. doi: 10.11781/sysydz2023051026

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Traditional carbonate rock slice identification is based on manual observation and description, which is highly subjective, mainly qualitative and difficult to quantify. In this paper, an intelligent rock thin-section image information mining model covering the process and technology is designed. The mapping relationship between lithofacies characteristics and thin slice images is constructed through a petrographic analysis framework. A full-flow feature extraction algorithm is also designed by combing image processing and deep learning. Qualitative recognition of grain type in structural component features is obtained by convolutional neural network, that is, the classification of grains (intraclasts, bioclasts, envelopes, spherulites and agglomerates) based on the improved ResNet50 model. Quantitative recognition of grain content, size, shape and contact mode in structural component features are obtained by digital image processing, that is, grain content is calculated based on threshold segmentation, grain morphology parameters are calculated based on minimum peripheral circle/minimum peripheral rectangle and by combining with area ratio/aspect ratio, intersection ratio (IoU) and other algorithms. And the qualitative and quantitative identification of calcite and other minerals in mineral component features are obtained by HSV colour space processing of the stained images. A thin section example from Shun X well is given as an example, and the validity of each feature extraction algorithm is verified through the complete image recognition process and comparison with the manual identification report. The results show that the petrographic analysis framework is effective in representing meaningful information in sparry grain limestone. Through the model of combing petrographic analysis framework with image analysis algorithm, a standard and intelligent identification of this type of carbonate rocks has been achieved, which can provide effective method support for the study of intelligent identification of rock slice images.

Great achievements of the journal in the past 60 years

2023, 45(5): 1039-1039.

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2023 Vol. 45, No. 5

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