Table of Content

14 January 2023, Volume 44 Issue 1

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Petroleum Geology

Differences and main controlling factors of large-scale gas accumulations in typical giant carbonate gas fields： A case study on Anyue gas field in the Sichuan Basin and Jingbian gas field in the Ordos Basin

Caineng ZOU, Zengye XIE, Jian LI, Lu ZHANG, Chunlong YANG, Huiying CUI, Xiaobo WANG, Zeqing GUO, Songqi PAN

2023, 44(1):  1-15.  doi:10.11743/ogg20230101

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Anyue gas field in the Sichuan Basin and Jingbian gas field in the Ordos Basin are two giant marine carbonate gas fields with the largest overall scale and the largest single-layer scale respectively discovered in China so far. Based on the analysis of the source-location structure of gas reservoirs， the process of gas accumulation and the space-time configuration of key play elements during reservoir generation， we consider that Anyue and Jingbian gas fields are characterized by a variety of source-location structure types and source-trap configuration of high efficiency. However， Anyue gas field is of “in-situ” accumulation of pyrolysis gas from paleo-oil reservoirs in inherited paleo-uplift， while Jingbian gas field is of adjusted accumulation in the structural transformation zone of a slope. There are three key factors controlling the large-scale enrichment of Anyue and Jingbian gas fields. First， the hydrocarbon source rocks underwent abnormally thermal events， with the duration of thermal events with a heat flow value over 70 mW/m² being about 70 Myr. Different types of kerogen got fully cracked to generate methane-dominated gases in large amount. Second， the multi-layered reservoirs of large scale and high quality are well developed due to the modification of favorable lithofacies by karstification. Third， many types of large-scale traps are well developed including structural， structural-lithologic， stratigraphic-lithologic and lithologic traps， being favorable for large-scale and effective accumulation of gas. The areas with ideal configuration relationship between high-energy beach body and lithologically tight zone within paleo-oil reservoirs or adjacent to hydrocarbon source rocks are favorable for discovering large-scale gas accumulation in the near future.

Progress and direction of geological modeling for deep and ultra-deep carbonate reservoirs

Zhiliang HE, Xiangyuan ZHAO, Wenbiao ZHANG, Xinrui LYV, Dongya ZHU, Luanxiao ZHAO, Song HU, Wenbo ZHENG, Yanfeng LIU, Qian DING, Taizhong DUAN, Xiangyang HU, Jianfang SUN, Jianhua GENG

2023, 44(1):  16-33.  doi:10.11743/ogg20230102

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Exploration and development of deep and ultra-deep carbonate reservoirs have been a hot and key research topic in the industry. Accurately depicting the spatial distribution and physical property parameters of the reservoirs has been a major challenge for an efficient oil and gas exploration and development. Based on an analysis of current development of reservoir geological analysis， logging evaluation， seismic prediction， geological modeling and other related technologies， this study is focused on figuring out the development mechanisms and distribution patterns of high-quality deep carbonate reservoirs by overcoming the data issues （scarcity， low quality and inaccuracy） and the high heterogeneity nature of the reservoir. A series of key technologies for characterization and modelling of the deep carbonate reservoirs have been developed， including technologies for construction of multi-scale and multi-attribute deep carbonate reservoir knowledge base； new technologies for geological analysis such as macroscopic to microscopic geological observation， in-situ micro-area qualitative and quantitative analysis for reservoir sedimentation and diagenetic environment， experiment and numerical simulation technologies for mechanism and process of reservoir development； new logging interpretation technologies， such as reservoir type identification and quantitative parameter evaluation based on global logging simulation， and sedimentary microfacies identification based on machine learning； new seismic prediction methods， such as seismic petrophysical modeling， machine learning technologies for rock physics guided reservoir parameter prediction and uncertainty evaluation； new geological modeling technologies such as new algorithm of multipoint geostatistics， geological process simulation， and geological modeling based on artificial intelligence. The technological processes of geological modeling of carbonate reservoirs under the control of karst unconformity， fault and sedimentary facies have been established respectively and applied to oil and gas reservoirs in Tahe， Shunbei and Yuanba blocks in the Tarim Basin and the Sichuan Basin， providing scientific basis for exploration and development deployment. The future research direction of geological modeling for deep and ultra-deep carbonate reservoirs is also predicted： updating geological knowledge base to support geological modeling； expanding the modeling technology based on geological process and improving its application； developing geophysical interpretation and prediction technologies based on artificial intelligence to improve the ability to depict complex reservoirs； developing new modeling methods based on artificial intelligence to continuously improve the accuracy of reservoir characterization and the reliability of models； and establishing rapid updating technology of geological models for deep reservoirs to continuously improve the efficiency and accuracy of model updating.

Differences of natural gas accumulation and play fairways in the marginal zone and interior of Sinian platform in Anyue gas field， Sichuan Basin

Jian LI, Chunlong YANG, Wuren XIE, Yurun RUI, Xiaobo WANG, Lu ZHANG, Zengye XIE, Zeqing GUO

2023, 44(1):  34-45.  doi:10.11743/ogg20230103

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Anyue gas field in the Sichuan Basin is the largest marine carbonate gas field discovered so far in China， with discovered gas-in-place of 1.03×10¹² m³， of which gas in Dengying Formation accounts for 56.3 %. In recent years， the exploration and development of Dengying Formation mainly focus on its 4^th member （Deng 4 Member） in the marginal zone of platform， and the single well productivity of the platform marginal zone is markedly higher than that of the platform interior. In the study， we discuss the differential gas accumulation based on the geochemical characteristics of natural gas and the correlation between gas reservoirs in the platform marginal zone and the interior， combined with the analysis of accumulation conditions. The results show that the hydrogen isotope value in methane of natural gas in the platform margin zone is obviously larger compared with the platform interior， and the salinity of the paleowater medium of the Cambrian source rocks is correspondingly higher than that of the Sinian source rocks， which reflects the contribution increase of the Sinian source rocks from the platform marginal zone to interior. Second， the reservoirs in the two zones are consistent in multiple aspects like the rock type and reservoir space， but the dissolved pores or cavities of reservoirs in the platform marginal zone is better developed than that in the platform interior， which is mainly controlled by the development extent of mound-shoals and the intensity of epigenetic karstification. Third， there are differences in source-reservoir assemblages between the two zones. The platform marginal zone is adjacent to the source rocks in the rift with its reservoir laterally juxtaposed with the source rocks， resulting in various source-reservoir assemblages such as the side source rock-lateral reservoir assemblage as well as the upper source rock-lower reservoir assemblage. However， the platform interior is located far away from the source rocks in the rift， and faults rooted in source rocks are needed to transport the hydrocarbons， resulting in single-type source-reservoir assemblage. Fourth， the difference in the contribution between the Sinian and Cambrian source rocks and the differences in the development position and assemblage type of high-quality reservoirs are the main reasons for the huge differences in productivity between the two zones. Penglai-Jian’ge area in northwestern Sichuan Basin is a favorable play fairway for deep hydrocarbon exploration in the Dengying Formation of platform marginal zone in the future， while the surrounding slope areas of paleo-uplift are favorable for exploration of structural-lithologic gas reservoirs in the Dengying Formation of platform interior.

Experimental study on late gas generation characteristics of the Middle Devonian sapropelic source rocks of in Northwestern Sichuan Basin

Hui TIAN, Zijin WU, Haifeng GAI, Xing WANG

2023, 44(1):  46-54.  doi:10.11743/ogg20230104

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The Paleozoic source rocks in southern China contain mainly Type Ⅰ-Ⅱ kerogen and are currently over-mature with vitrinite reflectance （R_o） greater than 2.0 %. Accurate evaluation of the gas generation potential of Type Ⅰ-Ⅱ kerogen after oil generation and expulsion and geochemical characteristics of over-mature gases are of great significance to natural gas exploration of deep and ultra-deep Paleozoic gas pools in southern China. In view of this， the mature Devonian source rock samples （R_o≈1.1 %） collected from the Guanwushan Formation in Northwestern Sichuan Basin are applied for gold tube pyrolysis experiments， and the gas generation potential of Type Ⅰ-Ⅱ kerogen at the high-to-over mature stage and the geochemical characteristics of over-mature gas generated are investigated in combination with existing documents. The results show that the residual oil content of the studied samples is 140 mg/gTOC and has a gas generation potential of 220 mL/gTOC at present， indicating high gas-generation potential. Furthermore， the amount of gas from kerogen cracking is at least 140 mL/gTOC and that of gas from residual oil cracking is at best 80 mL/gTOC， indicating that kerogen-cracking gas is the main source of late gas generated from the sapropelic source rocks under high oil expulsion efficiency. Meanwhile， the geochemical characteristics of the late gases generated from the sapropelic source rocks are obviously affected by oil expulsion efficiency. For example， the dryness coefficient of the gases is high， and the methane carbon isotope grows heavier rapidly with increasing methane yield. At R_o≈3.50 %， the carbon isotope of methane is close to that of the original kerogen （with a mere carbon isotopic fractionation difference of 0.5 ‰）. The results obtained above will be of scientific basis to the evaluation of natural gas exploration potential and the identification of natural gas sources in the deep and ultra-deep Paleozoic reservoirs in southern China.

Diagenesis and porosity evolution of microbial carbonate rocks undergone a deep burial history： Taking the Leikoupo Formation of Middle Triassic in western Sichuan Basin as an example

Yixiong QIAN, Hengzhi WU, Lingfang ZHOU, Shaofeng DONG, Qiongxian WANG, Xiaobo SONG, Meizhou DENG, Yong LI

2023, 44(1):  55-74.  doi:10.11743/ogg20230105

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The microbial dolomite reservoir in the Middle Triassic Leikoupo Formation， western Sichuan Basin， has been one of the exploration targets with a buried depth excessing 5 000 m and it serves as an excellent object of study for diagenetic fluids and pore evolution that are probably accountable for the formation of deeply-buried microbial carbonate reservoirs. Samples and data from more than 10 wells and several outcrops and seismic sections were studied with multi-technical methods， such as cast thin section observation， cathodoluminescence microscopy （CL）， scanning electron microscopy+energy-dispersive X-ray spectroscopy （SEM-EDS）， fluid inclusion microthermometry， carbon and oxygen isotope analysis via microsampling， carbonate clumped isotopes thermometer （△47）， U-Pb dating of calcite， FIB-SEM and so on. The results show that there are three early diagenetic subsystems： open， closed and semi-open （transition）， which respectively correspond to the intensive meteoric water influx in algal dolostone in intertidal-subtidal zones under humid climate， the weak meteoric water influx in evaporate rock-algal lamellated micritic dolostone in supratidal-intertidal zones under arid climate and the interaction between bacteria & microbial and carbonate mud druing an early supergene-shallow burial period. Large-scale dolomitzation and de-dolomitization during the burial-tectonic epochs are believed to take place repecitvely in the Carnian（226.50 ± 9.68 Ma）and Norian（211.50 ± 1.50 Ma）stages of Late Triassic， with temperatures of 43 ℃ to 54 ℃ and 50 ℃， respectively. Four to five successive dolomitization and calcite cementation stages have been identified， showing that the δ¹⁸O_water （PDB‰） of parent fluids varies between -0.83 ‰ and 9.70 ‰ for dolomite， and between -1.16 ‰ and 12.94 ‰ for calcites， while with the enlargement of cement crystalline， the overall δ¹⁸O_water slowly reduces， indicating an increase in temperature and fluctuationg salinity. Statistics reveal that vugs and solution-enlarged fractures （43.69 %）， framework pores （32.38 %） and micro-poros are the major pore types of microbial carbonate rocks. The micropores are well connected as organic acid-rich pore fluid inhibiting large-scale cementation and result in the partial preservion of pores with a good connectivity. Thereby， the shallow subtidal to supratidal zones with a development of shallow-up Mirokovich sedimentary cycle for microbial carbonate rocks， combined with the open to semi-open early diagenetic system， are the basis of reservoir porosity development and preservation， and the superposition and transformation of burial diagenetic fluids leads to the overall reduction of porosity.

Characteristics and genesis of the Middle Permian Qixia Formation dolostone in western Sichuan Basin

Yueqing HAN, Juntao ZHANG, Zhiliang HE, Zhenkui JIN, Wenbiao HAN, Ping GAO, Yunqing HAO, Wei SUN, Chongyang WU

2023, 44(1):  75-88.  doi:10.11743/ogg20230106

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Recent oil and gas discoveries in the Middle Permian Qixia Formation dolostone in western Sichuan Basin reveal the exploration potential of dolostone reservoirs. However， the mapping of paying dolostone reservoirs has been difficult due to their high heterogeneity and puzzling dolomitization mechanisms. In this study， the lithological and geochemical characteristics of different types of dolostone in the Qixia Formation of western Sichuan Basin are comprehensively analyzed and combined with sedimentary facies and thermal events of the Emeishan Large Igneous Province （ELIP） to summarize the diagenetic stages and genetic mechanisms of the dolostones. The results show that the Qixia Formation in western Sichuan Basin hosts mainly porphyritic or stratified dolostone that are mostly fine crystalline， sometimes medium to coarse crystalline， or micrtic to very fine crystalline， occasionally with some saddle shaped dolomite cement. The micritic to very fine crystalline dolostones， largely developed at the bottom of the Qixia Formation， consist of mainly subhedral-anhedral crystalline often accompanied by framboidal pyrite. Some micritic dolostones consist of sphaerolitic， dumbbell- or cauliflower-shaped crystalline， and have negative δ¹³C（VPDB） values and δ¹⁸O（VPDB） values in the range of that of normal seawater， all indicating possible products of microbial-mediated dolomitization process. Fine crystalline dolostones are developed both in the first and second members of the Qixia Formation， where they consist of euhedral to subhedral crystalline with mist-centered bright-edge structure and show change of color from brown-dark red inside to bright red outside and then dark again under cathode luminescence. Their obvious residual grain structure suggests a grainstone origin and their δ¹⁸O（VPDB）， δ¹³C（VPDB） as well as ⁸⁷Sr/⁸⁶Sr values all indicate seawater as the dolomitization fluid possibly formed by reflux seepage dolomitization with medium salinity during shallow burial period. The medium to coarse crystalline dolostones are generally developed in the second member of the Qixia Formation. They contain mainly anhedral nonplanar crystalline which are dark from inside and bright to the outside under cathodoluminescence. Their δ¹⁸O（VPDB） values are lower than that of the normal sea water， indicating a modification by later fluids. Some medium to coarse crystalline dolostones with ⁸⁷Sr/⁸⁶Sr values higher than that of seawater of the same period consist of porphyritic or zebra-like crystalline and often have saddle dolomites in vugs. These characteristics indicate a direct influence of thermal fluids that originated from or passed through certain clastic rocks. Those with ⁸⁷Sr/⁸⁶Sr values still within the seawater range but with no saddle-shaped dolomites being observed may be the products of recrystallization or thermal adjustment of fine crystalline dolostones. In general， the dolostones of the Qixia Formation in western Sichuan Basin are mostly likely the results of a successive syngenetic microbial-mediated dolomitization， shallow burial reflux seepage dolomitization， and late thermal fluid modification， among which shallow burial reflux seepage dolomitization could be the main genetic mechanism.

Tectonic evolution of Qingyang and Wushenqi paleo-uplifts during the Early Paleozoic and its control on Cambrian lithofacies paleogeography in the Ordos Basin

Chunlin ZHANG, Fengcun XING, Yueqiao ZHANG, Fujie JIANG, Wanglin XU, Amin ZHANG

2023, 44(1):  89-100.  doi:10.11743/ogg20230107

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The tectonic evolution process of Paleozoic paleo-uplifts in the north and south of the Ordos Basin is still not quite clear and this restricts the oil and gas exploration and the study on sedimentary filling evolution process in the basin. Based on the latest drilling， outcrop and seismic data， the study characterizes and defines the Qingyang and Wushenqi paleo-uplifts by means of seismic identification of reflector contact relationship and shore facies. The results show that the two paleo uplifts have experienced six major stages of evolution， namely the gradual submersion in the Cambrian， uplifting and denudation from the Cambrian to the Ordovician， stabilization of the Qingyang paleo-uplift and rapid subsidence of the Wushenqi paleo-uplift after the Yeli-Liangjiashan period， the compression， denudation and finalization of the paleo-uplifts during the Late Caledonian， the extinction of the paleo-uplifts in Carboniferous-Permian， and the tectonic tilting and finalization after the Permian. The two paleo-uplifts were similar during the early stages and differentiated during late stages with the south uplift rising and north uplift subsiding. The two paleo-uplifts together formed an alternative rising-subsiding framework that had obviously controlled the sedimentary reservoirs of the Cambrian system. The paleo uplifts are gentle in the east and steep in the west. The east is more favorable for the large scale development of dolomite reservoirs. During the process of Cambrian lithofacies paleogeography evolution， both paleouplifts controlled the lithofacies paleogeography framework but with the Qingyang paleouplift working more continuously on the uplift-depression pattern and sedimentation. The Huaiyuan tectonic movement in the late Cambrian led to the uplift and denudation of the Ordos Basin， resulting in a widespread development of karstification at the top of the Cambrian.

Unconformity characteristics of Huaiyuan movement in the northeast of Wushenqi paleo-uplift in Ordos Basin and its implications for karst reservoir generation

Juntao ZHANG, Yuyin ZHANG, Ning GU, Xiaohui JIN, Tao ZHANG, Sihong LIU, Huichong JIA, Jiaqi YANG, Ling LIU, Xiaopeng GAO

2023, 44(1):  101-109.  doi:10.11743/ogg20230108

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Huaiyuan movement during the first and second episodes of the Caledonian was a major tectonic activity in the Early Paleozoic Ordos Basin， featuring long-term sedimentary hiatus and forming a more effective karst reservoir near the paleo-uplift， with the impact second only to the Caledonian （the third episode） - Hercynian movement. To the east side of Wushenqi paleo-uplift， the first member of the Ordovician Majiagou Formation （Ma 1 Member） was in parallel， unconformable contact with the Cambrian Sanshanzi Formation as the result of Huaiyuan Movement， and on either side of the unconformity were carbonate strata with great differences in terms of electrical logging properties， mineral， elemental and isotopic compositions. The Sanshanzi Formation was of fine-crystalline dolomites with irregular dissolved pores and karst reservoirs. The well logging response shows low gamma value and high resistivity at the interval. The reservoirs of the formation were mostly deposited in relatively high-energy shoals near the paleo-uplift with normal seawater， and experienced reflux dolomitization at shallow burial stages. The Ma 1 Member is the first interval of sediments after the Huaiyuan movement， featuring complex lithology and mixed deposition of anhydrite， dolomite， mud and sand due to its proximity to the Wushenqi paleo-land. In terms of the depositional setting， the Ma 1 member was of marine type， that is， mixed tidal flat near the paleo-land in the early transgression period， though with great difference from the Sanshanzi Formation. The climate was relatively arid during its deposition， and the deposits were dominated by marine carbonates with intermittent terrigenous clastic supply. Its dolomite was a result of enecontemporaneous sabkha dolomitization. Controlled by both the primary facies belt with relatively high energy and the dolomitization as well as karstification related to the Huaiyuan movement， the reservoirs in the Sanshanzi Formation were mainly formed by a combination of dolomitization and epigenetic karstification and are widely seen in the vicinity of Wushenqi， Zhenyuan and Lyuliang paleo-uplifts.

Sequence stratigraphy models of carbonate-evaporite successions and their controls on source rocks and reservoirs in the Ordovician Majiagou Formation， Ordos Basin

Tao ZHANG, Yaxiong ZHANG, Xiaohui JIN, Yan ZHOU, Juntao ZHANG, Ning GU, Wei ZHANG, Ruyue WANG, Kai LU

2023, 44(1):  110-124.  doi:10.11743/ogg20230109

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Based on the existing research results and the classical sequence stratigraphy theory of Vail P. R.， this study takes into account the sedimentological principle of carbonate-evaporite， platform morphology， and lithological association characteristics of basin interior and margins to propose a sequence stratigraphic division model for the Majiagou Formation in Ordos Basin. According to the model， the sequence， which consists of evaporites mainly at the lower part and shallow water carbonates in the upper part， can be divided into five third-order sequences with boundaries respectively at the bottom of members of Ma1， Ma3， Ma5¹⁰， Ma5⁶， and Ma5⁴. The bottom and top of Majiagou Formation are regional unconformity surfaces. Within the duration of lowstand systems tract of these third-order sequences， the carbonates and evaporite interfinger each other. The number of the evaporite layers is related to the number of hiatus （unconformity） in the adjacent carbonate sequences. The existence of these 4-5^th order hiatus （unconformities） is the prerequisite for the development of interlayer dissolution and determines the location of source rock layers of inter-evaporite. Potential hydrocarbon source rocks are likely to be developed in three evaporative sequences of Ma1， Ma3 and Ma5⁶ members in the lowstand systems tract， which are mainly distributed in the lows surrounding the salt lake and the slope areas of the basin center； moreover， are characterized by thin layers and large accumulated thickness. The sedimentary facies and multi-order sequence boundaries control the development of reservoirs， and two types of reservoirs are mainly formed： （1） The porous dolomitic reservoirs of beach facies are mainly developed in the middle and upper parts of highstand systems tract （HST） of third-to-fourth-order sequence， and the superimposed areas of the sedimentary hiatus and beach facies are the favorable zones for the porous reservoirs. （2） The moldic pore-type gypsodolomite reservoirs are mainly developed during the secondary transgression of the lowstand systems tract， and the tidal flat facies zone in the basin margins surrounding the paleo-uplift are the favorable horizons for the development of reservoirs of moldic pore type.

Coupling relationship between formation of calcite veins and hydrocarbon charging in Middle-Lower Ordovician reservoirs in strike-slip fault zones within craton in Shunbei area， Tarim Basin

Jianzhang LIU, Zhongxian CAI, Changyu TENG, Heng ZHANG, Chen CHEN

2023, 44(1):  125-137.  doi:10.11743/ogg20230110

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Shunbei oil/gas field is of the ultra-deep carbonate type controlled by multi-stage strike-slip faulting within the craton. The significant differences on the oil/gas property and abundance are shown in different fault zones even different segments in a fault zone. An integration of the core and thin section observation， cathodoluminescence， in-situ trace and rare earth element （REE） analysis， systematic analysis of fluid inclusions， is applied to study the sequence of fracture vein formation， sources of vein-forming fluids and their coupling relationships with hydrocarbon charging in the Middle-Lower Ordovician reservoir of the No.1 and 5 strike-slip fault zones in Shunbei area. We also investigate the factors controlling differential hydrocarbon accumulation in Shunbei area while understanding the evolution of strike-slip faults and the hydrocarbon accumulation pattern. The results show that there are at least four phases of calcite vein development， that is， Cal-1， Cal-2， Cal-3， and Cal-4， respectively， in the Middle-Lower Ordovician reservoir of the No. 1 and 5 strike-slip fault zones in Shunbei area. The vein-forming fluids of Cal-1 are mainly sea-sourced fluids， while that of Cal-2， Cal-3 and Cal-4 are derived from diagenetic fluids in the reservoir where the veins occur. The formation of Cal-2， Cal-3 and Cal-4 calcite veins is associated with tectonic activities and hydrocarbon charging in the Late Caledonian to Early Hercynian， the Late Hercynian to Indosinian， and the Yanshan-Himalayan， respectively. The strike-slip fault zones are different in hydrocarbon charging， as shown by that the main charging phase of the No. 5 fault zone was during the Late Hercynian to Indosinian， while the main charging period of No. 1 fault zone was during the Late Hercynian to Indosinian， and Yanshan-Himalayan. The differential hydrocarbon charging controlled by strike-slip fault structural style and evolution history is the key to differential hydrocarbon accumulation in Shunbei area.

Numerical simulation and prediction of hydrocarbon phase evolution of wells Shunnan 1 and 4， Tarim Basin

Yueyi HUANG, Yuhong LIAO, Chengsheng CHEN, Shuyong SHI, Yunpeng WANG, Ping’an PENG

2023, 44(1):  138-149.  doi:10.11743/ogg20230111

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The wells Shunnan 1 and 4 in the Tarim Basin are located close to each other and have similar geological backgrounds. The reservoirs therein are similar in burial history， thermal history and pressure history， with their crude oils having undergone severe thermal cracking. However， the two wells are significantly different in hydrocarbon phase： the reservoir in well Shunnan 1 is of gas condensate type， while that in the well Shunnan 4 is typical of dry gas type. The hydrocarbon phase evolution processes of the two wells and the reasons for the phase difference between the two wells are yet to disclose. In this study， the thermal simulation data from the gold tube closed system experiment of crude oils in combination with the actual geological setting information are applied to the numerical simulation and prediction of hydrocarbon phase evolution of wells Shunnan 1 and 4 in the Tarim Basin via PetroMod and PVTsim， and the simulation results are compared with the composition and phase of present hydrocarbon fluids. The results show that the hydrocarbon reservoir in the Yingshan Formation in well Shunnan 1 varied from liquid phase to gas condensate phase at 34 Ma and remains in gas condensate phase till now. The molar concentration of n-alkane in the gas condensates has a very good linear relationship with the carbon number， indicating that it was not affected by gas invasion， evaporative fractionation or multi-stage charging. The hydrocarbon reservoir in the Yingshan Formation in well Shunnan 4 transformed from liquid phase to gas condensate phase at 49 Ma， and probably suffered gas invasion in the Miocene （22?10 Ma） by over-mature dry gas from the Cambrian source rocks through channels of multiple NE-striking faults developed in Shunnan area. The gas invasion got weakened from east to west and disappeared in well Shunnan 1. This is the main reason for the marked difference in composition and phase between wells Shunnan 1 and 4.

Infrared spectra evolution of crude oil under pyrolysis and its controlling factors

Botong LIU, Peng CHENG, Haifeng GAI, Qin ZHOU, Tengfei LI, Hui TIAN

2023, 44(1):  150-163.  doi:10.11743/ogg20230112

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Infrared spectra of crude oils can indicate information of various molecular groups， which is of great potential to identifying the genetic type and thermal evolution degree of crude oil. In this study， pyrolysis experiments are performed on two immature crude oil samples from different organic facies， to preliminarily investigate the infrared spectra evolution of crude oils under pyrolysis and its controlling factors. The results indicate that the infrared spectra evolutions are similar for the two oil samples and can be approximately divided into two stages with the increase in oil cracking extent. At the early stage with experiment temperatures of lower than 370 ℃， the A_{\mathrm{C}{\mathrm{H}}_{\mathrm{3}}}/A_{\mathrm{C}{\mathrm{H}}_{\mathrm{2}}} value of cracking oil varies slightly， while the A_aro and A_aro/A_sat values progressively decrease. This is due to some compounds with a weak thermal stability undergoing debranching during this stage， which results in an increase in saturated hydrocarbon content and decrease in the ratio of aromatic/saturated hydrocarbons. At the later stage with experiment temperatures of over 370 ℃， the A_{\mathrm{C}{\mathrm{H}}_{\mathrm{3}}}/A_{\mathrm{C}{\mathrm{H}}_{\mathrm{2}}}， A_aro and A_aro/A_sat values of oil samples significantly increase mainly due to the compounds with long molecular chain structures significantly cracked into compounds with short molecular chain structures， causing an increase in the A_{\mathrm{C}{\mathrm{H}}_{\mathrm{3}}} content. Meanwhile， the aromatization degree of the compounds is significantly enhanced at this stage， which results in an increase in aromatic hydrocarbon content as well as the ratio of aromatic/saturated hydrocarbons. There are still some differences in the infrared spectral parameters between the two oil samples during the oil cracking process， especially at the early stage. Therefore， cross plots of infrared spectral parameters， such as the plot of A_CH3/A_CH2-A_aro/A_sat， can be used to classify the types of crude oils and identify their thermal maturity.

Exploring the mineral dissolution-precipitation processes in fracture-fluid-rock systems based on simulation experiments

Qian DING, Jingbin WANG, Leilei YANG, Dongya ZHU, Wenbin JIANG, Zhiliang HE

2023, 44(1):  164-177.  doi:10.11743/ogg20230113

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Water-rock interactions in fracture systems and their significance to reservoir formation have always been a hot topic of interest for scholars around the world. Fluid may flow and transport along the fractures， dissolve surrounding rocks， precipitate new minerals， and change the morphology of storage space， all playing critical roles in the formation and distribution of carbonate reservoirs as well as hydrocarbon migration and accumulation. It is therefore of great theoretical and practical significance to identify the genetic mechanism of deep and ultra-deep fractured carbonate reservoirs. In this study， we carried out high-temperature and high-pressure dissolution simulation experiments on samples from the Ordovician Yijianfang Formation in the Shunbei area of Tarim Basin and performed numerical simulation with tools such as TOUGHREACT to identify the interaction mechanism between brine with dissolved CO₂ and carbonate rocks， to investigate the influence of temperature， pressure， fluid property and physical heterogeneity， and to calculate the Ca²⁺ diffusion properties and mineral dissolution/precipitation trends. The results show that the overall reaction is dominated by calcite dissolution with an increase in fracture width， number and volume， as well as sample permeability and porosity， indicating improvement of reservoir quality. This study clarifies that the physical heterogeneity and fluid hydraulic properties promote the main fractures as the main flow channels. The flow and reaction processes promote each other and together determine that the main fractures will not only be the dominant channels for fluid flow and the main place where water-rock reactions occur， but will also be the dominant reservoir space for oil and gas.

Geochemical characteristics of crude oils under alteration of deep Pb-Zn-bearing hydrothermal fluids

Huiyuan XU, Quanyou LIU, Dongya ZHU, Qingqiang MENG, Zhijun JIN

2023, 44(1):  178-185.  doi:10.11743/ogg20230114

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Active deep hydrothermal fluids of different types are widely distributed in many areas of the basins in South China. The Lanping area， northwest Yunnan Province， is one of such places where mantle-sourced Pb-Zn-bearing hydrothermal fluids are quite typical and had led to the formation of ultra-large terrestrial sedimentary epithermal deposits with residual paleo oil and gas reservoirs showing hydrocarbon generation and reservoir alteration obviously dented by the deep Pb-Zn-bearing hydrothermal fluid activities. By analyzing the geochemical characteristics of biomarkers， hydrocarbons and heterocyclic compounds in crude oils and solid bitumen after hydrothermal alteration in Pb-Zn ore from the area， the study shows that， with increasing hydrothermal alteration， the relative abundance of biomarkers （e.g.， n-alkanes， steranes， hopanes， etc.） gradually decrease， the relative content of UCM increases， the weak even carbon number predominance gets more obvious （CPI ≤ 1.0）， the relative content of polycyclic aromatic hydrocarbons （PAHs） gradually decreases， and the relative abundance of phenyl derivatives （e.g.， biphenyl） and S-containing compounds （e.g.， thiophenes） progressively increase. The samples show no signs of influence of secondary effects such as biodegradation， evaporative fractionation， thermochemical sulfate reduction （TSR） and oil cracking. The change of geochemical characteristics of molecular compounds in the associated crude oils and bitumen of Pb-Zn ore samples is possibly due to the hydrothermal catalytic reaction of sedimentary organic matter or paleo-oil reservoirs with catalyst-rich mantle-sourced Pb-Zn-bearing hydrothermal fluids. This study reveals the coupling relationship between the deep hydrothermal alteration and the molecular geochemical characteristics， identifies the genetic mechanisms of heterocyclic compounds and proposes indicators for hydrothermal alteration of crude oils.

Catalytic role of MoS₂ in hydrocarbon generation during thermal evolution of low-maturity kerogen

Kang LI, Hong LU, Hongliang MA, Zhongfeng ZHAO, Huamei HUANG, Ping’an PENG

2023, 44(1):  186-194.  doi:10.11743/ogg20230115

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MoS₂ has been commonly used as an efficient catalyst for the kerogen hydropyrolysis to improve the yield with activated high-pressure and mobile phase of H₂. However， there is little document on whether MoS₂ has a certain effect on the thermal maturation of organic matter and hydrocarbon generation. In this study， the effects of MoS₂ on the thermal evolution of organic matter and hydrocarbon generation are investigated by comparative thermal simulation experiments on kerogen of low maturity from the Alum Shale with and without MoS₂ added. The results show that low-mature kerogen has greater potential for hydrocarbon generation in presence of MoS₂. MoS₂ acts to increase the peak yield of light hydrocarbons （C_6-14）， while decrease the peak yield of heavy hydrocarbons （C₁₄₊）， and lowering the temperature of peak hydrocarbon generation by about 24 ℃ at the same time， indicating that MoS₂ serves to accelerate the thermal cracking of heavy hydrocarbons to light hydrocarbons. The adding of MoS₂ also can slightly increase the yield of wet gases （C_2-5） before the peak hydrocarbon generation temperature （456 ℃） is reached， but significantly decrease it after the temperature surpasses its peak hydrocarbon generation value， indicating that MoS₂ can promote the generation and later cracking of wet gases. In addition， the existence of MoS₂ also remarkably raise the yield of methane （C₁）， especially at a temperature higher than 528 ℃ when the methane yield increases by nearly 50 %， a possible result of MoS₂ promoting the hydrogenation of kerogen at the high-to-over-mature stage. The difference of catalytic effect between pyrite （FeS₂） and MoS₂ is systematically compared， which is mainly controlled by the structure， stability and sulfur activity of catalysts.

Methods and Technologies

Concept， method and prospect of molecular oil recovery

Bingyu JI, Jichao FANG, Shu YANG, Yong HU

2023, 44(1):  195-202.  doi:10.11743/ogg20230116

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The understanding and control at molecular level has become an inevitable trend in the development of modern science and technology， but it has not attracted enough attention in the field of oil and gas development. Based on some achievements and understandings of molecular simulation， this paper puts forward the concept of molecular oil recovery （molecular gas recovery） for the first time， and analyzes its connotation， current situation and development trend. Relevant research achievements in surfactant flooding， polymer flooding and CO₂-EOR have been documented and enriched the connotation of molecular oil recovery to a certain extent. Relying on molecular simulation technology represented by molecular dynamics and quantum chemical calculation combined with modern experimental technology represented by chromatography， spectroscopy， mass spectrometry， and nuclear magnetic resonance， etc.， it is possible to deepen the understanding of the mechanism of viscosity formation and viscosity reduction of heavy oil， and the characteristics of oil and gas adsorption， desorption and slippage on rock surface from the molecular level. In this way， a new mechanism based on molecular oil recovery is proposed， and the molecular structures of more targeted viscosity reducer， oil displacement agent， plugging agent and other auxiliary materials are designed. Molecular oil recovery technology based on molecular simulation methods is expected to bring revolutionary advances to the research of chemical oil displacement mechanism， thermal recovery mechanism， CO₂ displacement mechanism， low salinity water displacement mechanism， shale oil and gas exploitation mechanism and natural gas hydrate exploitation mechanism， etc.， providing theoretical support for efficient oil and gas recovery methods.

Deep learning-based geological modeling of ultra-deep fault-karst reservoirs in Shunbei oilfield， Tarim Basin

Taizhong DUAN, Wenbiao ZHANG, Zhiliang HE, Yanfeng LIU, Qiqi MA, Meng LI, Peiqing LIAN, Yuan HUANG

2023, 44(1):  203-212.  doi:10.11743/ogg20230117

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Fault-karst reservoir is of a special type distributed in the Ordovician strata in the Tarim Basin， China. It’s characterized by deep burial， complex genesis and strong heterogeneity. Due to sparse well data and low seismic quality and other adverse conditions， its accurate characterization and fine modeling are faced with great challenges. In the study， an integration of drilling， core， outcrop and 3D seismic data is applied to build a deep learning-based training dataset for the fault-karst reservoir with the guidance of architecture mode of fault-controlled fractured-vuggy reservoir. Based on the comprehensive analysis of deep learning network， we propose a deep learning-based modeling method suitable for fault-karst reservoirs. The results show that the “in-situ， equal-scale” training dataset established based on multi-source data is the basis for deep learning-based modeling of fault-karst reservoirs. The selected pix 2 pix （P2P） neural network could realize the 3D model prediction of fault-karst reservoirs by seismic data. A 3D fault-karst reservoir model is then established for the south segment of the No. 5 fault zone in Shunbei area following the built of training network. The model is conformed to the geological mode and distribution pattern of the reservoir type on all fronts， and also highly consistent with the reservoir prediction based on drilling data. One of the key research directions therefore lies in improving the accuracy and conditional degree of deep learning-based geological modeling of fault-karst reservoirs.

Classified-staged-grouped 3D modeling of multi-scale fractures constrained by genetic mechanisms and main controlling factors： A case study on biohermal carbonate reservoir of the Upper Permian Changxing Fm. in Yuanba area， Sichuan Basin

Xiangyuan ZHAO, Yuchun YOU, Xiangyang HU, Jingrong LI, Yu LI

2023, 44(1):  213-225.  doi:10.11743/ogg20230118

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Fractures of diverse types and multiple scales are widely distributed in oil/gas reservoirs. They feature multiple stages in terms of origin and different classes in terms of scale under the comprehensive control of various geological factors， posing great challenge for fine 3D geological modeling. The biohermal carbonate reservoir of the Upper Permian Changxing Fm. in Yuanba area， Sichuan Basin is taken as a case to establish a classified-staged-grouped 3D geological modeling method for multi-scale fractures constrained by genetic mechanisms and main controlling factors. It is proposed that the multi-scale fracture modeling should follow the principles of equivalent time constraint， hierarchical constraint and genetic control. According to the actual situation of the study area， geological factors for fracture generation should be fully considered in the modeling process， and a reasonable division scheme for fracture classes and developmental stages should be set for 3D fracture modeling. The large-scale fractures are mainly modeled by the deterministic method， while the medium-to-small-scale fractures are modelled according to their genetic mechanisms and main factors controlling fracture development. Following the understanding of critical geological patterns acquired from the staged and grouped description of fractures with relevant parameters collected， we carry out staged prediction of the medium-to-small-scale fracture distribution by geomechanical method， and then modeling in line with different stages and groups of fractures. Finally， the large-scale fracture model and the medium-to-small-scale fracture model are integrated to obtain the multi-scale fracture network model. Its application to the multi-scale fracture modeling of Changxing Fm. reservoir in Yuanba area shows that the method can effectively make up for the shortcomings of traditional methods in the accuracy of establishing medium-to-small-scale fracture models.

Deep learning-based geological modeling driven by sedimentary process simulation

Yanfeng LIU, Taizhong DUAN, Yuan HUANG, Wenbiao ZHANG, Meng LI

2023, 44(1):  226-237.  doi:10.11743/ogg20230119

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Problems including insufficient quantity and low resolution of data face exploration and development of deep and complex reservoir targets， and the traditional geological modeling methods have been inadequate in terms of technical needs. The intelligent geological modeling represented by deep learning capable of fully integrating multi-scale and multi-dimensional data as well as expert knowledge， is a key research and development direction of geological modeling technology. The study discusses the deep learning-based geological modeling driven by sedimentary process simulation following the comprehensive analysis of the advantages and disadvantages of stratigraphic forward modeling and deep learning-based geological modeling technology. First， forward modeling of sedimentation is carried out based on comprehensive geological analysis， parameter uncertainty is analyzed， and a large amount of geological models are established through parameter disturbance as a training dataset； Second， the geological patterns contained in the learning dataset are learned with the conditional Generative Adversarial Nets （cGAN）， in which the Generative Adversarial Networks （GAN） takes the conditional data such as well and seismic data as the input， and the geological model as the output； Finally， the trained GAN is applied to the real conditional data to obtain the geological model of the target block. The feasibility of this method is verified through testing on the typical geological profiles of the main block of Puguang gas reservoir， and the impact of the training dataset scale on simulation results is analyzed. The combination of sedimentary simulation and deep learning could make up for the shortage of training data and indirectly realize knowledge-driven deep learning-based geological modeling. The method is therefore of great significance to popularization.

Facies sequence-based MPS reservoir facies modeling algorithm and its application

Mingchuan WANG, Taizhong DUAN

2023, 44(1):  238-246.  doi:10.11743/ogg20230120

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The current multiple point geostatistical （MPS） modeling methods take the probability or geometric distance， which is inferred from training image based on a certain dimension of data template， as the criterion for determining the simulated value in the unknown area of the simulation grid. The relation between the criterion and geological meaning is weak in geological modeling. In view of the shortcomings of geological connection in the previous MPS modeling methods， returning to the essence of geological modeling， considering the sedimentological significance and geological implication of the similarity comparison between data events and training patterns during the facies modeling process， a facies sequence-based MPS modeling method is proposed. The method takes the facies sequence with sedimentological significance as the basic similarity comparison unit， creates a new variable data template suitable for facies sequence similarity comparison， utilizes the semantic recognition technology， directly calculates the similarity between the known facies sequence in the data event and the corresponding facies sequence in the training pattern， and then simulates the facies of unknown area in the simulation grid， through which realizes MPS modeling of strongly heterogeneous reservoirs from the sedimentological significance. The sedimentary facies simulation results of ideal models and the delta front reservoir in Tahe X area show that the proposed method can effectively reproduce the geometric shape displayed in the training image and well reflect the deposition and distribution of different facies in the reservoir， and greatly improve the accuracy of facies modeling. The method effectively combines sedimentology with MPS， and puts forward a novel similarity comparison means and modeling framework of MPS， which provides a new method for facies modeling of complex reservoirs.