Assessment of Shale Gas Potential of Marine-Continental Transitional Longtan Formation from Southwestern Guizhou Province, China

In the past 2 decades, the shale gas industry and academia in China have mainly focused on marine shales due to the commercial success of marine shale plays in the United States. Marine-continental transitional shales have long been overlooked for high clay content, thin thickness, and poor lateral continuity. However, recent exploration activities and on-site canister desorption data indicate a high exploitation potential for these shales. Therefore, we comprehensively studied transitional Longtan shale and coal samples from southwestern Guizhou Province using petrophysical, mineralogical, petrographic, and geochemical methods. The shale samples have total organic carbon (TOC) contents between 2.9 and 11.6%, porosity of 3.12%, vitrinite reflectances between 1.2 and 1.6%, and Langmuir sorption capacity between 0.198 and 0.469 mmol/g. Thus, Longtan shale shows similar or even superior properties compared to other commercially exploited gas shale formations. Compared to the interbedded coals, Longtan shales exhibit higher porosity, larger N-2 mesopore volume, and smaller CO2 micropore volume. For Longtan coals, both the maximum and Langmuir sorption capacities correlate with TOC contents. However, no such relationship exists in Longtan shales, which is attributed to the high average contribution (approximately 52%) of clay minerals to the total Langmuir sorption capacity. While the sorption capacity of Longtan shale depends on the micropore and fine mesopore amounts of clay minerals and organic matter, it is mainly controlled by the micropores of organic matter in Longtan coal. Based on our mathematic model, maximal gas-in-place of Longtan coal and shale occurs at depths between 700 and 800 m and in a much wider depth range between 1100 and 2100 m, respectively.