Shale gas accumulation conditions and resource potential evaluation of member 1 of the Lower Carboniferous Luzhai Formation in the northern Guizhong depression

In order to clarify shale gas accumulation conditions of the first member of the Lower Carboniferous Luzhai Formation (C₁lz¹) in the northern Guizhong depression and evaluate their resource potential, geological characteristics of shale gas accumulations were systematically analyzed based on well logging, seismic data and experimental analyses. Furthermore, shale gas resources in the study area were also revealed using a static method. The results show that the C₁lz¹ shale is characterized by moderate organic matter abundance (TOC content between 0.49% and 5.15%), well organic matter type (type Ⅱ₁~Ⅱ₂), and suitable thermal evolution maturity (thermal maturity between 2.2% and 2.9%). The C₁lz¹ shale reservoirs generally have a relatively high brittleness index at 51%~94%, well porosity and permeability of 3.12%~5.02% and 0.0005~0.161 mD, respectively, and pores with diameters below 400 nm. The shale reservoirs are primarily composed of organic matter related and dissolved pores, and their specific surface area is mainly contributed by micropores, while their pore volume is commonly derived from micropores and mesopores. The development of micropores is simultaneously controlled by TOC and clay mineral contents, whereas that of mesopores is only controlled by clay minerals. The total gas content is 1.73 m³/t on average, with methane proportion of up to 95% (dry gas with high calorific value). The absolute methane adsorption capacity of shale reservoirs primarily ranges from 1.07 m³/t to 3.67 m³/t, indicating a relatively strong adsorption capacity. The burial depth of shale gas reservoirs in the study area is generally 1000~3000 m, and their roof and floor are the Huangjin and Yaoyunling Formations with tight (argillaceous) limestone, corresponding to a cumulative thickness of 720 m and 490m, respectively. The degree of tectonic deformation in the study area is low, and the blocking ability of the thrust fault was strong. Magmatic activity is very weak in the study area. The circulation depth of hot spring water is primarily between 800 m and 1000 m, corresponding to a low degree of hydrogeological opening. The geological resources of shale gas in the Shaping synclinorium of the study area are 255×10⁸ m³, with technically recoverable resources of 38×10⁸ m³ and technically recoverable resource abundance of 0.45×10⁸ m³/km². In summary, the C₁lz¹ shale in the residual synclinal core of the northern Guizhong depression is a favorable target for shale gas exploration and development, due to its superior hydrocarbon generation basis, well-developed storage space, great gas-bearing capacity, good preservation conditions, moderate burial depth, and relatively high resource abundance.