Differential characteristics of lithological assemblages and gas-bearing of the Permian Longtan Formation mudstone in well L3, southeastern Sichuan Basin

Abstract As the main reservoir of coalbed gas in southeastern Sichuan, the mudstone of the Permian Longtan Formation has been drilled to obtain industrial gas, but the level of exploration and development is low. The researches on the types of lithological assemblages, reservoir characteristics, and gas-bearing properties are poor, which limits the evaluation and selection of the sweet point area for the marine-continental transitional shale gas. In this paper, by comparing the differential of different lithological distribution in the well L3, multiple discriminant functions and logging interpretation models for different lithology are established to determine the classification criteria of lithological assemblage types of shale formations. Based on the experimental results of high-temperature and high-pressure isothermal adsorption, the reservoir space distribution and gas-bearing characteristics of mudstone in different lithological assemblages are compared and analyzed. It is indicated that the four lithological assemblage types are found in the Permian Longtan Formation, including thick mudstone with the interlayer of coalbed (Type I), rich mudstone with the interlayer of sandstone and thin coalbed (Type II), sandstone interbedded with mudstone with the interlayer of coalbed (Type III), and limestone interbedded with sandstone with the interlayer of mudstone (Type IV), which are superimposed with each other. The different pore structure characteristics of mudstone in different lithological assemblages is the main influencing factor of differential gas-bearing property. The dominant lithological assemblages are Type I and Type II. Coalbed and carbonaceous mudstone are the source rock and primary storage space of adsorbed gas. Moreover, with low porosity and permeability, high breakthrough pressure and the strong sealing capacity of regional mudstone, it is easy to form the “microtrap” to store the natural gas. The sealing capacity of mudstone provides a favorable condition for gas preserve. Under the dynamic condition of hydrocarbon generation, the pressure storage box is formed, accompanied with the fine reservoir spaces and gas-bearing contents.