Effect of Lithofacies on Gas Storage Capacity of Marine and Continental Shales in the Sichuan Basin, China

Abstract

Lithofacies types of the marine shale of the Lower Cambrian Qiongzhusi Formation in the southwestern Sichuan Basin and the continental shale of the fifth member of Upper Triassic Xujiahe Formation in the western Sichuan Basin were classified based on a modified three-end diagram concerning the contents of siliceous minerals, carbonate minerals and clay minerals. Various experiments including X-ray diffraction, low pressure nitrogen adsorption, high pressure methane adsorption, and gas content measurement were designed for comparative analyses among different lithofacies in terms of core, pore structure, methane adsorption and gas-bearing characteristics. Then, the discrepancies between marine and continental shales lithofacies were analyzed, and the effects of lithofacies on gas adsorption and storage capacities were investigated. It is demonstrated that there are mainly five types of lithofacies developed in the marine shale of the study area, namely siliceous shale lithofacies (S), mixed siliceous shale lithofacies (S-2), clay-rich siliceous shale lithofacies (S-3), silica-rich argillaceous shale lithofacies (CM-1), and argillaceous/siliceous mixed shale lithofacies (M-2), while there are also mainly five types of lithofacies developed in the continental shale of the study area, which are carbonate-rich siliceous shale lithofacies (S-1), mixed siliceous shale lithofacies (S-2), clay-rich siliceous shale lithofacies (S-3), silica-rich argillaceous shale lithofacies (CM-1), and argillaceous/siliceous mixed shale lithofacies (M-2). Geological characteristics significantly vary among different lithofacies in terms of core, pore structure, methane adsorption and gas-bearing characteristics. Compared to the continental shale lithofacies, the marine shale lithofacies was characterized by a higher proportion of siliceous shale lithofacies group and a lower proportion of argillaceous shale lithofacies group, which could be attributed to different sedimentary environments and sediment provenances. On the condition that the organic matter content keeps constant, the silica-rich argillaceous shale lithofacies (CM-1) is favorable for adsorbed gas storage due to its strong methane adsorption capacity resulting from the highest clay minerals content, while the siliceous shale lithofacies group is favorable for gas storage due to its well-preserved primary and organic pores resulting from the highest siliceous mineral content.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018