Organic matter characterization of the Lower Cretaceous tight reservoirs in the Danish North Sea

This paper presents the first detailed organic geochemistry and petrography of the Lower Cretaceous tight reservoir units from the Valdemar Field, Danish Central Graben. The Tuxen and Sola formation chalks and the interbedded argillaceous Munk Marl Bed and Fischschiefer Member, respectively, contain thermally immature marine Type II kerogen, mainly algal liptinites, degraded bituminite and late-oil solid bitumen. The more clay-rich Fischschiefer Member and Munk Marl Bed constitute a relatively efficient seal for vertical migration and have significantly higher kerogen content as compared to the more calcareous Sola and Tuxen reservoirs. Evidence for significant oil migration can be observed across all the lithounits, highest in the Tuxen Fm which also constitute the main reservoir. Microscopic observations show a widespread distribution of granular degraded bituminite, formed by microbial degradation and diagenetic alteration of liptinites, and late-oil solid bitumen precipitated from the migrated oils. Biomarkers from rock extracts indicate open-marine depositional conditions of the source rocks that charged the hydrocarbons, having a geochemical composition similar to oils charged from the Upper Jurassic–lowermost Cretaceous Farsund Formation. The solid bitumen within such heterogeneous reservoir units might influence both reservoir quality and petrophysical properties.