Advantageous shale lithofacies of Wufeng Formation-Longmaxi Formation in Fuling gas field of Sichuan Basin, SW China

Abstract

The lithofacies types of Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation shale, the main producing layer in Fuling gas field, are classified in detail using the modified ternary diagram of siliceous minerals-carbonates minerals-clay minerals. There develop eight lithofacies in the Wufeng-Longmaxi shale: siliceous shale lithofacies (S), mixed siliceous shale lithofacies (S-2), clay-rich siliceous shale lithofacies (S-3), calcareous/siliceous mixed shale lithofacies (M-1), argillaceous/siliceous mixed shale lithofacies (M-2), mixed shale lithofacies (M), silica-rich argillaceous shale lithofacies (CM-1), and argillaceous/calcareous mixed shale lithofacies (M-3). The advantageous shale lithofacies is defined as lithofacies with gas content reaching a specific industrial standard. Based on the current development status of the study area, advantageous shale lithofacies is divided into two classes, namely, Class I with gas content of more than 4.0 m3/t (also known as extra superior), Class II with gas content of 2.0−4.0 m3/t (also known as superior). The correlation between the abundance of organic matter, the content of siliceous mineral, clay content and gas content has been analyzed to establish the classification criteria for advantageous shale lithofacies in the Wufeng-Longmaxi shale. The mixed siliceous shale lithofacies (S-2) and clay-rich siliceous shale lithofacies (S-3) have been identified as Class I advantageous shale lithofacies, and argillaceous/siliceous mixed shale lithofacies (M-2) as Class II. The classification criteria of advantageous shale lithofacies can provide reference for shale gas evaluation in other exploration areas Key words: shale, advantageous shale lithofacies, Ordovician Wufeng Formation, Silurian Longmaxi Formation, Fuling gas field, Sichuan Basin