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Early Transgressive Versus Late Transgressive Oil Shale Deposits — New Insights From Optical Kerogen Analysis

Abstract

The lower Jurassic (Liassic) was a time of widespread deposition of organic-rich mudstones, partially oil shales, which are a major source of many hydrocarbon systems in Europe and elsewhere, and also in SW-Germany. The study is focused on the two major black shale intervals in the Liassic in SW-Germany: the early transgressive Psilonoten shale (Hettangian-Sinemurian), which is mostly overlooked as source rock, and the late transgressive Posidonia shale (Toarcian), a well known highly prospective source rock all over Europe and elsewhere. To get a better understanding about the similarities and differences of both oil shales, a detailed source rock study is done based on geochemical and optical kerogen analysis. The focus is on optical analysis of the kerogen composition, preservation and maturation. Optical Kerogen Analysis quantifies the productive versus unproductive proportions of the total kerogen and the oil-prone versus gas-prone kerogen proportions within the productive kerogen of each sample, leading to significantly improved resolution and reliability of kerogen analysis. Analysis of organic matter degradation (microporosity) gives informations on hydrocarbon generation. The integrated optical analysis of organic maturation provides highly reliable information on the maturity of the play. Additional in-situ analysis of microporosity and distribution of organic matter in the Liassic shales is done, based on thin sections and SEM. This interdisciplinary workflow leads to new improved characterization and definition of different types of source rocks and their regional and stratigraphical distribution, providing new detailed insights into the variations of the hydrocarbon potential between early transgressive and late transgressive black shale deposits in the Liassic in SW-Germany, beyond the geochemical data available from previous exploration. Results of the optical based source rock analysis will lead to the definition and characterization of different types of source rocks in the pre-Tertiary of SW-Germany.