Well logging evaluation of the shale gas potential in the lower Silurian Longmaxi Formation, Weiyuan, southern Sichuan Basin, China

Abstract

Recent observations of shale gas breakthroughs have in the Weiyuan marine shale gas play in the Sichuan Basin have attracted great interest. To better understand these breakthroughs, we use core description, FIB-SEM data, XRD data, organic geochemistry, and well logging data, to better understand the reservoir characteristics carbonaceous shale, calcareous shale, and siliceous shale lithology, with a focus on the organic-rich shale units. We find conventional well log methods are effective in mapping the spatial distribution of the organic-rich shale in the Weiyuan area where the. total organic carbon content in the Longmaxi Formation ranges from 1.35%-6.95%, averaging 4.42%. The kerogen is Type I-II and the vitrinite reflectance (Ro) is greater than 2.57%, which indicates that the formation is susceptible to shale gas accumulation. The clay mineral content ranges from 48 wt.% to 63 wt.% (avg. 51 wt.%).with illite and chlorite averaging 73.8% and 25.7%, respectively. The brittle mineral quartz and plagioclase content ranges from 32 wt.% to 61 wt.% (avg. 47 wt.%). Compared to the surrounding litholgic units, the marine shale exhibits relatively high GR, CNL, AC, RT, K, and U values and relatively low DEN, PE and Th/U values, allowing us to construct. Cross-plots to define the units of interest. Using the same process, we quantify the TOC content providing a spatial distribution of organic-rich shale using conventional well logging.