Characteristics of eogenetic karst and its influence on reservoir of the Member 2 of Dengying Formation from Wellblock GS1, central Sichuan Basin
ZENG Jianjun1,2, TAN Xiucheng1,2, ZHAO Dongfang1,2, HE Ruyi1,2, LUO Wenjun3, LIU Yun3, XU Wei3, LI Shaorui1,2, DENG Yu1,2, WU Hao1,2
1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,Chengdu 610500,China; 2 Southwest Petroleum University Division of Key Laboratory of Carbonate Reservoirs,CNPC,Chengdu 610500,China; 3 Research Institute of Exploration and Development,PetroChina Southwest Oil & Gas Field Company,Chengdu 610041,China
Abstract：The genesis of reservoirs in Member 2 of the Sinian Dengying Formation in the Sichuan Basin has long been a subject of debate,including weathered crust karst,pene-sedimentary karst and buried hydrothermal karst,which significantly impedes the establishment of reservoir prediction model and the exploration and development benefit. This paper uses Member 2 of the Dengying Formation from Wellblock Gaoshi 1 as a case study. By analyzing drilling core samples,thin sections,well logging data,etc.,the karst characteristics of Member 2 of the Dengying Formation is identified and categorized. Furthermore,the variations in karst features and their impact on reservoir development and maintenance have been discussed. The results indicate that: (1)Member 2 of the Dengying Formation exhibits features such as high-frequency corrosion surface,fabric-selective corrosion cavity,geopetal structure,semi-dissociation zone,graniphyric textures,spongy corrosion structures,and karst breccias;(2)The dissolution characteristics are closely associated with the high-frequency shallower-upward sedimentary sequence,predominantly developing in the middle and upper parts of this sequence;(3)Based on the karst strength,three types of karst sequences can be identified: fabric selective corrosion,karst zonation,and integral brecciation;(4)Additionally,a negative bias in δ13C and δ18O as well as a positive bias in87Sr/86Sr,are observed at the high-frequency cycle interface. Further analysis indicates that the thin reservoir of Member 2 of the Dengying Formation is also developed in the middle and upper parts of the high-frequency shallower-upward sedimentary sequence. A significant number of residual karst caverns are filled with micritic envelopes and fibrous isopachous cements. Additionally,some of these caverns exhibit geopetal structures,indicating that they re-entered the submarine diagenetic environment after the formation of karst caverns. Considering the characteristics of negative carbon and oxygen isotope biases and positive strontium isotope biases at high-frequency cycle interfaces,it is proposed that the development of this kind of porous thin reservoir is primarily influenced by eogenetic karst processes driven by the high-frequency sea level changes. The vertical superposition exhibits a distribution pattern characterized by “multiple thin layers”,with distinct facies-controlling features. The karstification with different intensities change the connectivity of pores. Relatively isolated small dissolved pores are primarily formed during the fabric selective solution stage,where they remain more isolated and preserved for extended periods due to seabed cements. In contrast,during the karst zonation and integral brecciation stage,these dissolved pores become increasingly interconnected. In the later stages,they are more prone to mechanical filling and chemical bonding,which enhances reservoir heterogeneity and can significantly deteriorate reservoir performance. Furthermore,it has been revealed that the connectivity of the karst system controls the maintenance pathway of dissolution pores during the early diagenetic stage,under different karst strength transformation. This finding offers a new perspective for understanding the formation and preservation of ancient deep carbonate reservoirs.
ZENG Jianjun,TAN Xiucheng,ZHAO Dongfang et al. Characteristics of eogenetic karst and its influence on reservoir of the Member 2 of Dengying Formation from Wellblock GS1, central Sichuan Basin[J]. JOPC, 2024, 26(5): 1221-1234.
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