Generation and geochemical signatures of natural gas from solid bitumen: Implications from closed and stepwise semi-open pyrolysis experiments

Abstract

Solid bitumen has been acknowledged as an important source of overmature shale gas. However, the gas generation potential, chemical signatures and carbon isotopic compositions of natural gas derived from solid bitumen in an overmature hydrocarbon system are not well-understood with respect to various geological conditions. In this study, a relatively low-maturity solid bitumen from Cambrian reservoirs in the northwest Sichuan Basin was pyrolyzed in both closed and stepwise, semi-open systems. In the closed system, methane generated in the overmature thermal stage is dominated by the secondary cracking of wet gas and cumulative methane carbon isotopic values that are 4.4‰–14.8‰ lighter than the carbon isotope composition of the original solid bitumen. In semi-open systems, the gas is predominantly generated from residual solid bitumen and has wetness (i.e., C2–C5/C1–C5 by volume) values of generally less than 1%. At the same thermal maturity levels, the carbon isotope composition of methane in various semi-open systems is 4.4‰–10.0‰ heavier than those in the closed system and may also be isotopically heavier than the carbon isotope value of the original solid bitumen. The present study also illustrates that the calculated gas yields and methane carbon isotopes within a certain thermal maturity range in the closed system cannot be compared with those from residual solid bitumen as measured in the corresponding semi-open systems. This highlights the need to conduct specific semi-open pyrolysis experiments to investigate the late gas generation potential and geochemical signatures of natural gas from solid bitumen when the geological system is not a closed system. By comparing the above-mentioned results with the geochemical characteristics of Wufeng-Longmaxi shale gas from the Changning and Jiaoshiba gas fields of the Sichuan Basin, southwest China, it is suggested that the shale gas examined in this study is most likely generated in semi-open systems rather than in a closed system. In other words, the early-generated gas has been expelled from the shales, leaving the late-generated gas from residual solid bitumen and/or kerogen as the main gas source in the present hydrocarbon system.