Effects of hydrocarbon expulsion from shale source rocks of high maturity in upper carboniferous strata of the Ruhr area, Federal Republic of Germany

Abstract

Based on detailed geochemical analysis of closely spaced core samples from a sequence of interbedded source rock-type shales and reservoir sandstones of Upper Carboniferous Westphalian-age certain effects of primary migration were recognized qualitatively. Expulsion of hydrocarbons from two high mature shale source rock units (1.1% R m) has occured preferentially from their edges and is indicated by the following effects: An overall decrease of the concentrations of total extract, saturated and aromatic hydrocarbons and decreasing values for the “production index” (from Rock-Eval measurements) from the centre portions of the shale units towards their outer edges. These depletion trends are paralleled by systematic changes in the composition of the C 15+-saturated hydrocarbons. Also, evidence was obtained to suggest that preferential depletion has occurred into a local fracture zone. The effects of hydrocarbon expulsion could be quantified on a molecular level for two thin source rock-type shale layers interbedded in a reservoir sandstone. For the n-alkanes less than C 26 the expulsion efficiencies range between 70 and 80%. Despite the high maturity level and despite the resulting advanced migration stage some evidence for chromatography-type compositional fractionation in the shale could still be observed in the C 27+-range n-alkanes and between pristane and its n-alkane isomer: pristane is preferentially retained in the source rock.