Assessment of unconventional shale gas potential of organic-rich Mississippian and Lower Pennsylvanian sediments in western Germany

Abstract

The assessment of unconventional shale gas potential for the Mississippian Alum shales, the Kulm-Plattenkalk and the Lower Pennsylvanian Ziegelschiefer Formation in western Germany provides insight on how fluctuation in depositional environments has a significant role on lithofacies and shale gas potential. This paper uses a standard evaluation workflow for unconventional shale gas potential taking into account thickness, TOC content, mineral composition and brittleness, porosity and permeability data as well as thermal maturity. The average organic carbon content of the Lower and Upper Alum Shales reaches 2.4 wt% and 2.6 wt%, respectively and deposition took place in an oxygen-depleted marine environment, where hydrogen-rich type II kerogen was preserved in the sediments. In this study, the Kulm-Plattenkalk is investigated for the first time with regard to unconventional shale gas potential. The TOC content ranges between 0.22% and 6.85% (average: 2.45%) and carbonate content is 21% on average. In contrast, the Lower Pennsylvanian Ziegelschiefer Fm was deposited in a shallow marine suboxic to oxic environment with stronger terrestrial influence. TOC is 1.2% on average and the kerogen rich in terrestrial material has only a moderate original hydrocarbon generation potential. It should be noted that all samples are in the gas generation window and may hence have already lost about half of their original organic carbon content. A simple control of mineralogy on porosity and permeability was not evident, but a weak correlation was found between permeability and the average effective pore radii or slit widths. The Mississippian black shales have great similarity in terms of organic richness, thickness and quartz content with known gas shales of similar stratigraphic age from the US. However, differences with respect to burial and uplift history might have important influence on gas in place.