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Eogenetic karst in interbedded carbonates and evaporites and its impact on hydrocarbon reservoir: the Middle Triassic Leikoupo Formation in the Sichuan Basin of southwestern China

Hong Liu1, 2, Xiucheng Tan1, 2*, Ling Li2, Xiong Ding2, Jian Cao3, Guang Yang4, Teng Ma1
1. State Key Laboratory of Oil and Gas Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China;2. School of Earth Sciences and Technology, Branch of Sedimentology and Hydrocarbon Accumulation, CNPC Key Laboratory of Carbonate Reservoir, Southwest Petroleum University, Chengdu, Sichuan 610500, China;3. Department of Earth Sciences, Nanjing University, Nanjing, Jiangsu 210023, China;4. Institute of Petroleum Exploration and Exploitation, PetroChina Southwest Oil and Gas Field Company, Chengdu, Sichuan 610051, China


AbstractKarst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir and is critical to understand. This is significant for exploration and karst geology. Here, we report a new such case from the Middle Triassic Leikoupo Formation of the Sichuan Basin in southwestern China. Stratigraphic incompleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccias, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites.

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Keywords:karst; carbonate reservoir; evaporite; Middle Triassic Leikoupo Formation; Sichuan Basin

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