Total shale oil inventory from an extended Rock-Eval approach on non-extracted and extracted source rocks from Germany

Abstract

Rock-Eval pyrolysis data are used for assessments of the quality and maturity of potential hydrocarbon source rock, but is also often applied among shale oil studies to estimate the so-called “total oil” for Oil In Place (OIP) calculations. However, for these approaches Rock-Eval may underestimate total oil due to a “carryover effect”, a bias which is particularly crucial for shale oil investigations.In this study we examined the total oil of marine to lacustrine source rocks from German shale oil formations using data from Rock-Eval combined with solvent extraction. Each source rock was analysed by Rock-Eval both non-extracted and extracted to obtain values for total oil (= [S1wholerock−S1extracted rock]+[S2wholerock−S2extracted rock]). Comparison of these two data sets revealed that for the individual formations in average 17–23% of the S2 fraction is extractable, herein termed “S2 oil”. This “S2 oil” increases the total oil amount compared to the conventional free oil (S1) by average factors of 2.2–3.6. For selected source rocks the molecular composition of the “S2 oil” was obtained by a specific approach: Rock-Eval analysis was terminated after S1-detection at 301°C, samples were retrieved, solvent extracted and analysed by gas chromatography. Depending on the type of organic matter and maturity, the “S2 oil” contains significant amounts of higher boiling hydrocarbons such as n-alkanes but also contributes to the unresolved complex mixture. It is to date unclear, how much of this “S2 oil” should be finally considered as technically mobilized shale oil.