Reservoir Characteristics and Exploration Potential of Lower Carboniferous Shale Gas in Lianyuan Sag, Central Hunan

There are few studies on the reservoirs and hydrocarbon accumulation characteristics of the marine-continental transitional shale.Lower Carboniferous Ceshui Formation transitional shale is one of the most important shale gas exploration beds in Lianyuan Sag of Xiangzhong Depression.In this paper, the accumulation characteristics of marine-terrigenous facies shale of the study area are discussed on the basis of 2D seismic data, drilling core and logging data.Firstly, the structure of Well 2015H-D6, and the lithology of Ceshui Formation are described.Then, testing and analysis of the geochemical, physical properties and gas content of shale reservoir are carried out.Finally, the burial and thermal history of the single well is simulated.Reuslts illustrate that Ceshui shale in Lianyuan Sag deposited in coastal lagoon or swamp environment with a deposition thickness of 71.8 m and a buried depth of more than 2 500 m.The shale with type Ⅱ kerogen remains in over-mature stage, with the average organic carbon content of 1.68%, and the maturity ranging from 2.30%~3.32%(average value of 2.96%).The brittle mineral content exceeds 60%, and clay mineral content is less than 30%.The shale is characterized by low porosity and permeability, with microspore dominated reservoir porosity, which shows better shale gas reservoir conditions.It is revealed that the Ceshui shale has a good content of shale gas by well logging interpretation, isothermal adsorption and in-situ gas quantity analysis.Combining with single-well burial, thermal history simulation, and the evolution of shale gas reservoir, it is concluded that the stable tectonic and sustaining subsidence are the key factors of shale gas accumulation formed in Lianyuan Sag.