THE POTENTIAL APPLICATION OF STABLE ISOTOPES FOR DISTINGUISHING INDIGENOUS VERSUS MIGRATED HYDROCARBONS IN A MATURE SHALE SANDSTONE SEQUENCE

Bitumens isolated from Kokelsumer Heide No. 1 corehole (Ruhr area, F.R.G.) are indicative of the problem of resolving contributions of indigenous versus migrated fluids. Within a depth interval of approximately 50 m, thermally mature shales containing Types II and III organic matter and a coal seam (1.2% R(m)) are all encountered, in addition to interbeded sands that are present in the upper portion of this interval. Presented here is an approach based on the unique isotopic compositions of indigenous bitumens, kerogens and migrating fluids which appears to enable the evaluation of the proportion of extracts which migrated from a source rock. Stable carbon isotope values of kerogens and of saturate and aromatic fractions of residual bitumens isolated from Kokelsumer Heide No. 1 and Reckelsumer Heide No. 2 corehole samples are used to estimate the relative contributions of the various sources to the bitumens. Preliminary results indicate that from 10% (shale sequences) to 100% (sandstone sequences) of the extractable materials (i.e. bitumens) could be characterized as a migrating fluid. Also, stable carbon and nitrogen isotope values for kerogens and coal samples from the Kokelsumer Heide corehole can be used to verify previous geochemical and microscopic evidence for the existence of distinct kerogen Types (I and II) in this corehole.