Occurrence of thermogenic gas in the immature zone—implications from the Bakken in-source reservoir system

The Bakken petroleum system has been the subject of intense research. Nevertheless, results of the present organic geochemical study revealed new and previously unreported findings. Most interestingly, immature (<0.7% R0) Bakken Shales are charged with high concentrations of indigenous gas as revealed by thermovaporization-gas chromatography. Both closed and open system pyrolysis experiments elucidated that generation of thermogenic gaseous hydrocarbons early on in maturation seems to be a feasible process in the case of the Bakken. Interestingly, a considerable portion of the immature kerogen macromolecule is made up of diaromatic carotenoid structures which might be related to the unusual high gas generative capacity. A good correlation of gas yield with organic richness (TOC) for the immature Bakken might indicate that the light hydrocarbons are adsorbed to the organic matter and are at the same time indigenous. Alternatively to the hypothesis of early gas generation, the catagenetic/metagenetic formation of gas and its intraformational migration into the immature zone has to be considered as well. However, the likelihood of intraformational migration within the overpressured Bakken Formation is rather low.