Source rock characterisation and thermal maturity of the Rupelian Fish Shale (Bodenheim Fm./Hochberg Subfm.) in the central Upper Rhine Graben

The Rupelian Fish Shale acts as important source rock for several oil reservoirs in the central and northern Upper Rhine Graben (URG). Here, we present the first detailed organic geochemical characterisation of this unit and a comparison of molecular characteristics with those published on oils from the graben. In the central URG the Fish Shale can be classified as oil-prone source rock with a very good hydrocarbon generation potential characterised by high TOC contents of 4.0-5.5 % and hydrogen-rich type II kerogens with HI values of up to 550 mg HC/g TOC (e.g. in the clay pit Unterfeld). The highly branched isoprenoid C-25 originating from diatoms occurs in significant quantities in the Fish Shale and can be used as highly specific tool for oil-source rock correlations in the URG. Thermal maturity has been analysed using T-max and vitrinite reflectance (VRr) data. A detailed VRr map has been created using VRr values from 63 locations and a structure map of the Fish Shale as reference map for a depth dependent gridding operation. The Fish Shale in surface outcrops is immature (0.30 % VRr). Highest maturity levels occur in the area of Huttenheim (0.88 % VRr) and in the Heidelberg-Mannheim Graben (0.75-0.8 % VRr). The latter region represents the most significant kitchen area for the Fish Shale. Increased VRr gradients occur in the central parts of the URG, e.g. Huttenheim, Rulzheim and Landau, whereas lower gradients characterise the Heidelberg-Mannheim Graben indicating lower basal heat flows there. Two exploration targets exist within the Tertiary succession. First, in the central URG several minor oil reservoirs are located in thin, discontinuous sands in the Meletta Beds, Cyrena Marls and Bunte Niederrodern Beds overlying the Fish Shale and Upper Rupelian Clay. Secondly, in the Northern URG several reservoirs within heterogeneous, but more continuous sand units occur in the Upper Pechelbronn Beds underlying the Bodenheim Fm. For oil reservoirs in the Meletta Beds, Cyrena Marls and Bunte Niederrodern Beds overlying the mature Fish Shale relatively short lateral and vertical migration distances are postulated, whereas longer lateral migration distances from the Heidelberg-Mannheim Graben into reservoirs located in the Pechelbronn Beds in the Northern URG are likely.