Geochemical evidence for differential accumulation of natural gas in Shiwu fault depression, Songliao Basin, China

The carbon, helium and argon isotopes and composition data of natural gas sampled from the Shiwu fault depression indicated complex origin of natural gas. Carbon isotope reversal was possibly caused by mixing of sapropelic and humic sourced gases and leakage from gas reservoir. The shallow natural gas in both Bawu and Houwujiahu gas fields came from deep source rocks. In the Bawu gas reservoir, it was kerogen pyrolysis gas with a low degree of maturity and originated from type II kerogen in deep formation, whereas the high maturity shallow gas in the Houwujiahu reservoir was oil cracking gas and originated from type II kerogen and type III kerogen. The deep primary gas reservoir was destroyed, and natural gas migrated through the deep faults at the early stage and was accumulated in the shallow reservoir. The fault activities, source rock types, degree of thermal evolution and leakage from gas reservoir resulted in the differential accumulation of natural gas vertically and areally.