Comparative Study on the Primary Control Factors of Nanopores in Transitional and Deep Marine Shale in the Southeastern Sichuan Basin, China

Abstract

The deep marine shale from the Silurian Longmaxi Formation and transitional shale from the Permian Longtan Formation in the Sichuan Basin are two promising targets for shale gas exploration in China. In this study, nanopore characteristics for deep marine and transitional shale samples collected from the same well in the southeastern Sichuan Basin are investigated and compared via field emission scanning electron microscopy imaging, low-pressure gas adsorption, total organic carbon measurement, and X-ray diffraction. Results show that clay-related pores are well developed, pores within organic matter (OM) are rare, and shrinkage cracks along the OM-edge are widely prevalent in transitional shales. The deep marine shale is rich in bubble-like OM pores. The transitional shale has a larger pore volume but a lower specific surface area compared to the deep marine shale. The nanoporosity of the transitional shales is primarily controlled by clay and OM content. In deep marine shale, OM is the primary contributor to nanoporosity. Higher nanoporosity in the deep marine shales than in the middle-shallow ones was observed in this study. This occurrence is attributed to the high fluid pressure and the resulting superior preservation conditions for nanopores in deep marine shale formations.

Comparative Study on Flow Characteristics in Deep Marine and Marine–Continental Transitional Shale in the Southeastern Sichuan Basin, China

“…In the deep shale gas reservoirs of the Sichuan Basin, high fluid pressure provides excellent preservation conditions for shale porosity, resulting in higher porosity. Compared to the transitional shale, deep marine shale exhibits lower TOC content and higher quartz content, indicating that the porosity of deep marine shale is primarily associated with quartz, consistent with previous studies . Quartz can act as a rigid framework to inhibit the compaction of deep shale, providing favorable conditions for gas transport in nanopores.…”

“…On a macroscopic level, the porosity and gas permeability of the deep marine shale are greater than those of the marine–continental transitional shale, providing favorable conditions for gas transport in the nanoscale pores of the deep marine shale. On a microscopic level, compared to the marine–continental transitional shale, the deep marine shale has a smaller average pore size and a greater number of micropores as determined by nitrogen adsorption tests . These micropores are closer in size to the diameter of N 2 molecules (0.364 nm).…”

“…Both the deep marine shale and the marine–continental transitional shale are mainly composed of mesopores (2–50 nm), with developed micropores (<2 nm) in the deep marine shale and numerous macropores (>50 nm) in the marine–continental transitional shale. According to the nitrogen adsorption results, the average pore sizes of the deep marine shale and the marine–continental transitional shale are 5.79 and 9.38 nm, respectively . For a detailed analysis of the mineral, organic matter, and pore characteristics of the experimental samples, please refer to the literature …”

“…According to the nitrogen adsorption results, the average pore sizes of the deep marine shale and the marine−continental transitional shale are 5.79 and 9.38 nm, respectively. 47 For a detailed analysis of the mineral, organic matter, and pore characteristics of the experimental samples, please refer to the literature. 47 2.2.…”

“…47 For a detailed analysis of the mineral, organic matter, and pore characteristics of the experimental samples, please refer to the literature. 47 2.2. Experimental Setup and Method.…”