Distribution and evolution of sedimentary facies of the Lower Silurian Longmaxi Formation in southern Sichuan and northern Guizhou area

Organic-rich black shale,which is formed in shallow shelf sedimentary environment, is widely developed in the Lower Silurian Longmaxi Formation in southern Sichuan and northern Guizhou area.Based on the data of field measured profiles, drill core observation, testing of organic carbon content (TOC), and logging results, the sedimentary facies of Longmaxi Formation has been studied in detail. According to lithology markers, logging facies, and characteristics of sedimentary tectonics,paleotology and geochemistry,we identified neritic shelf facies composed of shallow shelf subfacies and deepwater shelf subfacies. The neritic shelf facies can be divided into storm layer, shallow sandy and muddy shelf, shallow muddy shelf, shallow gray and muddy shelf, shallow gray shelf, turbidite deposit, deep sandy and muddy shelf, deep muddy shelf and deepgray and muddy shelffurtherly. The Longmaxi Formation can be divided into SSQ1 and SSQ2 sedimentary cycles, which indicate a sea-level falling and affect the distribution of organic-rich rservoir of black shale. The deepwater muddy shelf microfacies in TST of SSQ1 are the best shale reservoir.