Geochemical. characterization of lower toarcian source rocks from NW Germany. Interpretation of aromatic and saturated hydrocarbons in relation to depositional environment and maturation effects

The characterization of crude oils in terms of source rock facies and depos itional environment, as well as their maturity and alteration stage, is a c rucial element in exploration studies. The present contribution has implica tions for oil-oil and oil-source rock correlations. In the past, numerous p arameters have been used for this purpose most of which are based on the an alysis 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 methylphenan threne, as well as pristane (PRI) and phytane (PHY), have also been propose d (Hughes et al., 1995). These studies have attempted to infer a crude oil source rock facies and lithology and to classify, the source rock depositio nal environment. In the present study, the above compounds have been quanti fied by solvent extraction, liquid chromatography and capillary gas chromat ography 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 7 0 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 calcare ous shales. The rocks contained mainly marine organic matter(Type II keroge n), the thermal maturity of which ranged from early mature to postmature (c orresponding to 0.48-1.44% mean vitrinite reflectance), therefore encompass ing the range over which effective petroleum generation had occurred. We fo und that the influences of organic matter type and maturity on the molecula r distributions of the above compounds were not obvious when interpreted in terms of a DBT/PHE vs PRI/PHY diagram. However, Principal Component Analys is (PCA) of our data-set showed that alkylphenanthrene concentrations are s trongly controlled by maturity, while the concentrations of PRI, PHY and 1- MDBT display a distinct source effect.