Characteristics of desorption gas of lacustrine shale in the Ordos Basin, China

Abstract

Abstract Desorption gases of four lacustrine shale cores from the Triassic Yanchang formation of the Ordos Basin, China were collected, and their molecular and carbon isotope compositions were analyzed in this study. Results show that hydrocarbon gases of desorption gas are dominated by methane with relatively low dryness coefficient ranging from 0.45 to 0.82. The 513C values for hydrocarbon gases increase with increasing carbon number, and which for methane ranges from -57.15%« to -53.71%«. The hydrocarbon gases generally derived from a mixed origin with primary contribution from thermogenic processes. Desorption gases of sample YK-340.7 could experience biodegradation for its relatively high dryness coefficient and (iC4H10 + iC5H12) / (nC4H10 + nC5H12) value. A significant depletion of13C for methane was observed during desorption process, the reason for which could be differences in molecular mobility of the12CH4 and13CH4. Changes in the 513Cethane and 513Cpropane during desorption process are week.