Stable isotope geochemistry of coals, humic kerogens and related natural gases

Abstract

Isotope systematics are well defined for conventional sapropelic, Type I/II kerogens and their associated bacterial and thermogenic natural-gas products. These geochemical tools are used to estimate source type, maturity and depositional environment, and as a correlation technique. In many cases the natural gas signatures in near-surface samples and drill cuttings can be used to classify or predict a deeper lying source rock or reservoir. Corresponding interpretative schemes for coals, Type III kerogens and their associated hydrocarbons are progressing quickly. The shift in attention to humic sources is driven primarily by depletion of conventional oil and gas resources and the economic and societal requirements of coal and coal-bed methane. Carbon, hydrogen and nitrogen stable isotope variations can be large between different coals and humic kerogens. These differences can often be recognized in their bulk δ 13C org, δD org and δ 15N org values. Isotope signatures of coals can be diagnostic of several factors, including deposit age, type, geographic location, maturity and generation history. However, these characteristic isotopic variations are substantially better defined by the C-, H- and N-isotope ratios of the separate maceral groups, such as vitrinite, exinite and inertinite. This new application of stable isotopes, at the maceral and compound levels, have great potential to improve the interpretative precision over conventional whole coal or bulk techniques. Hydrocarbon gases, including coal gases, derived from coals and humic kerogens can be distinguished from Type I/II sources, based on their molecular rations, i.e., C 1 (C 2 + C 3) and by comparing their stable isotope compositions, especially δ 13C CH 4 and δD CH 4 . The δ 13C C 2H 6 can also be valuable, but ethane is generally present in small amount (<1 vol. %) and requires