The origin and evolution of thermogenic gases in organic-rich marine shales

• Highly mature gases are derived from bitumen and kerogen cracking.
• Kerogen at >3.0% EasyRo still has methane generation potential.
• Hydrocarbon expulsion can affect the amount and δ13C values of late-generated gas.

In order to better understand the generation and primary source of mature thermogenic gas in shale, and to evaluate the residual gas generation potential of the shale at different maturity levels, we performed pyrolysis experiments on an organic-rich marine shale and its kerogens prepared by artificial maturation. The results indicate that the thermal maturation of organic matter in the shale can be divided into four stages: oil generation (<0.6–1.0% EasyRo), condensate generation (1.0–1.5% EasyRo), wet gas generation (1.5–2.2% EasyRo), and dry gas generation (2.2–4.5% EasyRo). Thermogenic methane is produced mainly during wet gas and dry gas generation, while most of the C2+ hydrocarbon gases are produced during condensate and wet gas generation. The kerogen at a thermal maturity of >3.0% EasyRo still has methane generation potential. Whether or not gas generation potential of a highly mature kerogen has a commercial significance depends on its organic matter richness, thermal maturity internal and some other geological factors, such as caprock sealing property, reservoir physical property, and tectonic movement. In addition to the gas produced from kerogen cracking, gas is also generated from the secondary cracking of residual bitumen as maturation progresses. Early hydrocarbon expulsion during oil generation likely has a considerable effect on the amount and δ13C values of the late-generated shale gas. The lower the oil expulsion efficiency of a shale, i.e., the more retained bitumen, then the higher the productivity of post-mature shale gas and comparative enrichment of the latter in 12C.