Main controlling factors of enrichment and accumulation pattern of Carboniferous Ceshui Formation shale gas in Lianyuan sag, Central Hunan

The central Hunan depression is an important area for shale gas exploration in the periphery of the Sichuan Basin, and the Carboniferous Ceshui Formation is the key shale gas layer in this area.Taking the secondary structure Lianyuan sag in the area as the object, based on the field stratigraphic section and drilling data, experimental analysis and field gas-bearing monitoring data, the geological conditions and differential distribution characteristics of shale gas accumulation of the Ceshui Formation were studied. The main controlling factors of enrichment and accumulation pattern of shale gas were discussed above. And the following research results were obtained. (1) Dark shale are mainly developed in the lagoon sedimentary facies area. The shale with a thickness of 30-80m, high content of organic matter(TOC>1.5%), mainly type II organic matter and moderate thermal evolution degree(2.0o<3.0%)has good hydrocarbon generation conditions. (2) The Ceshui Formation shale is a type of siliceous shale and mixed shale with high content of brittle minerals. The shale reservoir space includes organic pores, inorganic pores and fractures, and has the physical characteristics of ultra-low porosity and ultra-low permeability. (3) The shale gas reservoir of Ceshui Formation is a residual normal pressure gas reservoir. It mainly experienced three stages of forming and evolution: early in-situ accumulation, medium-term adjustment and transformation, and late dissipation-residue. The Middle Jurassic-early Cretaceous is the most important period of accumulation and transformation. (4) Organic-rich shale formed in the reduction environment of the lagoon facies is the material basis for gas enrichment and accumulation in Ceshui Formation. The suitable thermal evolution degree is an important factor affecting the enrichment and accumulation. Good preservation conditions are the key to enrichment and accumulation. The Intraformational detachment structure controls the enrichment degree of gas. Based on the comprehensive analysis, the shale gas accumulation pattern of ' sedimentary facies hydrocarbon supply control area, tectonic-decollement double control Baoding rich ' in Ceshui Formation is established. The wide and gentle syncline structure in the lagoon facies area is a favorable region for gas enrichment, and the footwall of the closed reverse fault on both wings of the syncline is a favorable location for gas enrichment.