Research paperExperimental study on the gas generation processes of lacustrine and marine shales in North China: Source implications for shale gas

- A new semi-closed system was used to investigate the gas generation processes.
- Kerogen-degraded and retained-bitumen cracking gas in shale can be differentiated.
- Retained-bitumen cracking can contribute 30% of shale gas at the stage of 1.5-2.0% Ro.

Using a high pressure, semi-closed thermal simulation system, two organic-rich lacustrine and marine immature shales from North China were investigated to probe the dynamic processes and potential for the gas generation throughout the oil-gas window. The results identified at least two processes during gas generation, as revealed by changes of the gas dry coefficient (C1/ΣC1-5%) and carbon isotopic composition of methane (δ13Cmethane). The first process is associated with kerogen thermal degradation and occurs during the main phase of oil generation, showing an increasing trend of the dry coefficient and δ13Cmethane. The second process begins during the late phase of oil generation, accompanied by increasing yields of wet gas components and 13C depleted methane. Due to the lack of aliphatic carbon in kerogen in the highly to over mature stage, as demonstrated by FTIR analyses, the second process is mainly related to the cracking of kerogen-generated bitumen which is retained in the shale.The hydrogen content, a key parameter for petroleum generation, is then used to quantitatively evaluate the gas generation potential from kerogen thermal degradation and retained-bitumen cracking. Mass balance calculations show that kerogen-generated bitumen that is retained in lacustrine and marine shales is estimated to contribute 30% of the total gas yields at the stage of 1.5-2.0% Ro. This result is consistent with the theoretical calculations from Xia et al. (2013) and has important implications for the shale gas sources, and further the shale gas resource assessments before exploration in shale-gas petroleum systems.