The differences in reservoir property between marine and transitional facies shale gas: a case study of the Longtan Formation and the Longmaxi Formation in Nanchuan, Sichuan

The shale strata in Nanchuan area, Sichuan, SW China, include sedimentary types of transitional facies and marine facies. The gas in marine shale represented by the Longmaxi Formation has been commercially developed, while that in the transitional facies shale represented by the Longtan Formation remain poorly understood. Based on data from field outcrops, drilling cores, thin sections, argon ion polishing scanning electron microscope, and Formation MicroScanner Image (FMI) imaging logging data, the differences in the characteristics of the two shale reservoirs were analyzed in terms of sedimentary facies, lithological combinations, reservoir conditions, mineral components, compressibility, and gas content. Results show that there are great differences in those aspects. ① The lithology of Longtan Formation is complex, including silty shale, carbonaceous shale, gray shale, limestone and coal seam, while the Longmaxi Formation is dominated by siliceous shale. ② Influenced by lithology, organic matter type, and thermal evolution degree, the reservoir space of Longtan Formation is dominated by micro-fractures (grain margin fractures and shrinkage fractures) and inorganic pores, while that of Longmaxi Formation is dominated by organic pores and micro-fractures. ③ The shale of Longtan Formation has high clay mineral content (47.6%), low brittle mineral content (35.8%), low Young's modulus (32.3 GPa), high Poisson ratio (0.3), strong plasticity, and lower compressibility than those of Longmaxi Formation. ④ The average gas content in Longtan Formation shale is 2.2 m³/t, in which 54.5% is adsorption gas; and the average gas content of Longmaxi shale is 5.2 m³/t, in which 43.3% is adsorption gas. Through the difference analysis of the two types of shale reservoirs, the characteristics of Longtan shale reservoir are defined, which are characterized by complex lithology, high clay mineral content, poor compressibility and high adsorption gas proportion. Therefore, the shale of the first member of the Longtan Formation has large continuous thickness, high gas content, good reservoir physical properties. In overall, it should be the "dessert" section of Longtan Formation. However, due to high clay mineral content, the coupling between geological "sweet spot" and engineering "sweet spot" is worse than that of Longmaxi shale. It is recommended to improve the fracturing technology, increase the volume of fracture network transformation and the output of a single well, so as to achieve exploration breakthroughs.