Behavior and controlling factors of methane adsorption in Jurassic continental shale, northeastern Sichuan Basin

Abstract

The behavior and controlling factors of natural gas adsorption in the Jurassic continental shale in the northeastern Sichuan Basin are studied based on the organic geochemical features, mineral compositions and pore structure parameters through a series of experiments on samples from the shale. Results show that the total gas content of the shale measured on-site is 0.1–5.3 cm3/g, with an average of 0.7 cm3/g. The methane isothermal adsorption curves show a trend of increasing first and then decreasing, indicating an obvious excessive adsorption. The shale has a maximum adsorption capacity (VL) of 0.44–3.59 cm3/g, with an average of 1.64 cm3/g, lower than that of marine shale in the same basin. The organic matter content and pore structure characteristics are identified as the two main factors controlling the adsorption capacity of the shale. Micropores in the shale are the main storage space for gas to be adsorbed. Due to well developed shell laminae and interlayers in the shale, calcite plays a more important role than clay minerals in affecting the adsorption of gas to the rock. The formation temperature and water content also significantly inhibit the gas adsorption to the shale. Compared with marine shale in the basin, the Jurassic continental shale is more heterogeneous and lower in TOC values. Furthermore, with a more widely developed clayey shale lithofacies and shell limy shale lithofacies as well as relatively less developed organic pores and micropores, the continental shale is inferior to marine shale in terms of gas adsorption capacity.