Late gas generation potential for different types of shale source rocks: Implications from pyrolysis experiments

Abstract

Gas generation from shale source rocks typically occurs via cracking of both kerogen and retained oils, such that it is difficult to predict and compare gas generation potentials of different shales because they are related not only to the kerogen type but also to the oil expulsion efficiency. In this study, five different shale kerogen samples were pyrolyzed in sealed gold tubes to investigate how kerogen type and oil expulsion efficiency affect their gas generation after oil-window maturity. The results illustrate that the maximum extractable organic matter (EOM) and gas generation potentials of different original shale kerogens (O-kerogen) in a closed system vary widely in the range of 229–790 mg/g TOCOK and 308–594 mL/g TOCOK, respectively. However, the gas yields of different residual shale kerogens (R-kerogen) with a starting equivalent vitrinite reflectance (EqVRo) value of approximately 1.22% are quite similar and vary between 131 and 145 mL/g TOCRK. Pyrolysis experiments also reveal that the late gas generation potential (EqVRo > 1.22%) of shale is mainly controlled by the amount of retained EOMs rather than kerogen type. When the shale source rocks containing types I and II kerogens have the same amount of retained EOMs, their gas generation potentials are quite similar. Under most geological conditions, the late gas generation potentials of shale source rocks, normalized to the TOC of matured shale at 1.22% EqVRo (TOCShale), vary approximately in the range of 180–300 mL/g TOCShale. To reach a gas content of 3 m3/ton shale for the present-day overmature shale gas exploration in the Lower Palaeozoic shales of South China, a conservative present-day TOC (TOCpd) value of 2.0% is proposed as a screening parameter that can eliminate the risk of insufficient gas generation potential as much as possible.