LITHOFACIES-BASED HIGH-FREQUENCY SEQUENCE STRATIGRAPHIC ANALYSIS OF ORGANIC-RICH SHALE OF FULING SHALE GAS FIELD IN THE SICHUAN BASIN IN CHINA

Lithofacies-based high-frequency sequence stratigraphic study is a useful tool for stratigraphy and reservoir prediction.

This study focuses on the Upper Ordovician Wufeng formation to the first member of Lower Silurian Longmaxi formation, which is the main producing shale interval of the first commercial shale gas field-Fuling Shale Gas Field in the Sichuan basin in China. The ternary diagram of mineralogy was employed to identify 8 shale lithofacies: 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).

Based on the lithofacies analysis combined with the analysis of logging curve, geochemical parameters and paleontology, sequence boundaries were identified for the Wufeng- Longmaxi shale. 2 third order depositional sequences from base to top were identified, each consisting of transgressive system tract (TST), early highstand system tract(EHST) and late highstand system tract(LEST).

5 lithofacies association patterns were recognized within high frequency sequence-stratigraphic framework. Type A: clay-rich siliceous shale lithofacies (S-3) with thin siliceous shale lithofacies (S), argillaceous/siliceous mixed shale lithofacies (M-2), and calcareous/siliceous mixed shale lithofacies (M-1). Type B: argillaceous/siliceous mixed shale lithofacies (M-2) with thin clay-rich siliceous shale lithofacies (S-3). Type C: argillaceous/siliceous mixed shale lithofacies (M-2) and mixed shale lithofacies (M). Type D: silica-rich argillaceous shale lithofacies(CM-1) with thin argillaceous/ calcareous shale lithofacies (M-3) and argillaceous/siliceous mixed shale lithofacies (M-2). Type E: silica-rich argillaceous shale lithofacies (CM-1). Different system tracts develop particular lithofacies association patterns. Transgressive system tract consists of Type A. Early highstand system tract is composed of Type B-C, indicating a sea level starts to fall. Late highstand system tract is Type D-E, indicating sea level falls rapidly and lots of clay supply.