GEOCHEMICAL CHARACTERIZATION OF LOWER TOARCIAN SOURCE ROCKS FROM NW GERMANY: INTERPRETATION OF AROMATIC AND SATURATED HYDROCARBONS IN RELATION TO DEPOSITIONAL ENVIRONMENT AND MATURATION EFFECTS

Abstract

The characterization of crude oils in terms of source rock facies and depositional environment, as well as their maturity and alteration stage, is a crucial element in exploration studies. The present contribution has implications for oil‐oil and oil‐source rock correlations. In the past, numerous parameters have been used for this purpose most of which are based on the analysis of saturated and aromatic hydrocarbons (including sulphur aromatics) and also on stable isotope signatures and elemental compositions. Recently, molecular indicators based on dibenzothiophene (DBT), phenanthrene (PHE) and their methyl derivatives methyldibenzothiophene (MDBT) and methylphenanthrene, as well as pristane (PRI) and phytane (PHY), have also been proposed (Hughes et al., 1995). These studies have attempted to infer a crude oil's source rock facies and lithology, and to classify the source rock's depositional environment. In the present study, the above compounds have been quantified by solvent extraction, liquid chromatography and capillary gas chromatography in 98 core samples of the Lower Toarcian Posidonia Shale Formation, a source rock in NW Germany. Most samples, cored between depths of 7m and 70 m, came from the Hils Half‐Graben in the Lower Saxony Basin. With a few exceptions from one borehole, the samples were unweathered marls or calcareous shales. The rocks contained mainly marine organic matter (Type II kerogen), the thermal maturity of which ranged from early mature to postmature (corresponding to 0.48‐1.44% mean vitrinite reflectance), therefore encompassing the range over which effective petroleum generation had occurred. We found that the influences of organic matter type and maturity on the molecular distributions of the above compounds were not obvious when interpreted in terms of a DBT/PHE vs PRI/PHY diagram. However, Principal Component Analysis (PCA) of our data‐set showed that alkylphenanthrene concentrations are strongly controlled by maturity, while the concentrations of PRI, PHY, and 1‐MDBT display a distinct source effect.