Variation in organic matter quality and maturity in Draupne formation source rocks from the Oseberg field region, offshore Norway

Abstract

Detailed characterization of the kerogens from closely spaced samples from the Draupne Formation penetrated by two wells, A and B, in the Oseberg Field region has been carried out by geochemical and petrographic methods. For selected samples the C 15-hydrocarbon fraction has been examined by GC and GC/MS. Petrographic analyses by transmitted light and UV excitation fluorescence methods demonstrate that the organic matter in all samples from both wells consists predominantly of amorphous material. Total organic carbon contents and hydrogen indices indicate an overall trend of decreasing kerogen quality with increasing depth for the Draupne Formation in well A. Similar data for samples from well B demonstrated generally inferior kerogen quality. Maturation level for the organic matter of the Draupne Formation in well A is slightly higher than that in well B, while both Draupne sections are within the limits for an immature source rock with a type II kerogen. GC- and GC-MS-analysis showed that the samples from well B were enriched in lighter hydrocarbons as compared to samples from well A. It is suggested that this may be due to minor staining of the former samples by migrated light hydrocarbons from the underlying Upper Jurassic sandstone reservoir. A correlation between the two sections of Draupne Formation based on organic facies characteristics indicate a general correspondance between the lowermost part of the section in well A and the section in well B.