Fracture characteristics and evolution of Wufeng-Longmaxi Formation shale in Lu203 well area in southern Sichuan Basin
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TE122.2+21

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    Abstract:

    The deep shale gas in Luzhou in the southern Sichuan Basin is the focus of shale gas exploration at this stage.The development of natural fractures has an important impact on shale gas enrichment,fracturing development and productivity. This paper explores the fracture characteristics,phases,formation and evolution of Wufeng-Longmaxi Formation shale in Well Lu203 area with the data such as core,logging,inclusion homogenization temperature and conducting rock acoustic emission experiments. The results show that the structural genesis of Wufeng-Longmaxi Formation shale is dominated by shear fractures,with a few horizontal slip fractures and vertical tension fractures. The filling degree of the fractures is high and the fillings are complicated. The shear fractures are characterized by the vertical and horizontal fractures and short extension distances. Vertically,the fractures mainly develop in Wufeng Formation and the middle-upper parts of the layers 2 and 4 of the Submember I of the first Member of Longmaxi Formation,and the vertical and horizontal fractures develop at the bottom. Additionally,horizontal fractures mainly develop in the upward direction. Wufeng-Longmaxi Formation shale fractures were formed during tectonic movement in three phases. In the first phase,tectonic fractures were formed during the Indosinian movement(253.4-250.0 Ma),which were mainly filled by calcite and pyrite. The inclusion homogenization temperature was 130.4-150.6 ℃,and the tectonic stress was in the NNW-SSE(335°±5°)direction,forming near-NS- and NWW-direction plane shear fractures and NEE-direction section shear fractures. In the second phase,fractures were formed in the late Yanshanian-early Himalayan(70.58-42.64 Ma). They were filled with calcite and had the inclusion homogenization temperature of 194.8-210 ℃. The tectonic stress was in the SEE-NWW direction(110°±5°),which formed NW- and NEE-direction plane shear fractures and NNE-direction section shear fractures and also strengthened and reformed early fractures. The fractures of the third phase were formed in the middle to late Himalayan(42.64-0 Ma).Most of them were mainly semi-filled or unfilled,and the inclusion homogenization temperature was 163.3-190 ℃. The tectonic stress in the NEE(75°±5°)direction formed NNE- and WE-irection plane fractures and NW-direction section fractures. The structure became stabilized in this phase,and early fractures underwent a superimposed transformation.

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ZHAO Shengxian, XIA Ziqiang, LIU Wenping, ZHANG Chenglin, ZHANG Jian, WEN Li, HE Yuanhan, WANG Chongshan. Fracture characteristics and evolution of Wufeng-Longmaxi Formation shale in Lu203 well area in southern Sichuan Basin[J]. Petroleum Geology and Recovery Efficiency,2022,29(5):28~38

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  • Received:
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  • Online: January 13,2023
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