Metagenetic methane generation in gas shales I. Screening protocols using immature samples

Abstract

The gas generative potential of organic matter is one key parameter for the calculation of total gas in place (GIP) when evaluating thermogenic shale gas plays. Having first demonstrated that late gas-forming structures are present in coals of anthracite rank (>2% R0) we go on to examine other rocks at the immature stage of maturity and report on how to recognise which might generate significant amounts of late dry gas at geologic temperatures well in excess of 200 °C in the zone of metagenesis (R0 > 2.0%), i.e. subsequent to primary and secondary gas generation by thermal cracking of kerogen or retained oil. Such a distinction could clearly be of major value when assessing risks and pinning down “sweet spots”. A large selection (51 samples) of source rocks, i.e. shales and coals, stemming from different depositional environments and containing various types of organic matter which contribute to the formation of petroleum in putative gas shales were investigated using open- and closed-system pyrolysis methods for the characterisation of kerogen type, molecular structure, and late gas generative behaviour. A novel, rapid closed-system pyrolysis method, which consists of heating crushed whole rock samples in MSSV-tubes from 200 °C to 2 different end temperatures (560 °C; 700 °C) at 2 °C/min, provides the basis for a newly proposed approach to discriminate between source rocks with low, high, or intermediate late gas potential. It is noteworthy that late gas potential goes largely unnoticed when only open-system pyrolysis screening-methods are used. High late gas potentials seem to be mainly associated with heterogeneous admixtures or structures in terrestrially influenced, in some cases marine, Type III and Type II/III coals and shales. Aromatic and/or phenolic signatures are therefore indicative of the possible presence of elevated late gas potential at high maturities. High temperature methane was calculated to potentially contribute an additional 10–40 mg/g TOC, which would equal up to 30% of the total initial primary petroleum potential in many cases. Low late gas potentials are associated with homogeneous, paraffinic organic matter of aquatic lacustrine and marine origin. Source rocks exhibiting intermediate late gas potentials might generate up to 20 mg/g TOC late dry gas and seem to be associated with heterogeneous marine source rocks containing algal or bacterial derived precursor structures of high aromaticity, or with aquatic organic matter containing only minor amounts of aromatic/phenolic higher land plant material.