261 / 2019-07-01 00:25:59

The control of eogenetic karst reservoir by the shoals in the Middle Permian Maokou Formation, Southern Sichuan Basin

Eogenetic karst reservoir, Shoals, Maokou Formation, Southern Sichuan Basin

The shoals control the eogenetic karst reservoir of Maokou Formation in Southern Sichuan Basin. However, researches on the characteristics of carbonate deposition and diagenesis in different sedimentary environments, especially the control on physical properties of carbonate rocks after the eogenetic diagenesis to karstification, are inadequate. Based on outcrop, core, thin section and cathode luminescence data, 8 rock types can be identified. And the open platform facies in the study area can be divided into shoals (including high-energy shoals and low-energy shoals), intershoal sea and open subtidal sedimentary subfacies. Syngenetic and eogenetic diagenesis determines that carbonate rocks in different sedimentary environments have different porosity and permeability before karstification. Bioclastic grainstone of high-energy shoals has high porosity and permeability, and it is affected easily by karstification to exhibit multiple layers of cavities and flower-porphyritic karstitic zones with characteristics of eogenetic karst. The porosity and permeability in bioclastic packstone of low-energy shoals is worse than bioclastic grainstone of high-energy shoals and the degree of karstification is poor. Before karstification, the bioclastic wackstone and argillaceous limestone of intershoal sea and open subtidal was compacted and karst was not developed. In the middle-late diagenetic stage, the bedrock was compacted, and only karst caves and dissolution joints were retained as good spaces, forming the facies-controlled eogenetic karst reservoirs by the shoals. High-energy shoals, low-energy shoals and intershoal sea are characterized by unequal thickness interbedding vertically and developed in the P2m2 and P2m3. The distribution of shoals on plane has a large thickness in the Luzhou ancient uplift area, which is characterized by the thick center of high-energy shoals and thin center of low-energy shoals. The distribution of shoals are consistent with the distribution of karst reservoirs and high-yielding Wells, which indicates that shoals have a controlling effect on eogenetic karst reservoirs.