Origin of marine petroleum source rocks from the Late Jurassic to Early Cretaceous Norwegian Greenland Seaway—evidence for stagnation and upwelling

Abstract

Forty samples were selected from Upper Jurassic to Lower Cretaceous black shales of IKU sites 6307/07-U-02 and 6814/04-U-02, located on the mid-Norwegian shelf, for a detailed maceral analysis. The penetrated rocks include the Spekk and Hekkingen Formations, which represent major potential petroleum source rocks in the region. It was our first objective to reveal the type of organic material that is responsible for the source rock potential of these sediments. The results suggest that black shale formation has occurred in two different paleoceanographic settings; (1) in a ‘high-productive’ and (2) an ‘anoxic/stagnant’ environment. This conclusion is supported by inorganic and sedimentological data. In addition, sedimentation rates (SR) from recent biostratigraphic and sedimentological work on these sequences gave impulse for using accumulation rates to estimate the original organic carbon flux to the sediment. Organic carbon accumulation rates are relatively low but similar to mid-Cretaceous black shales from other ocean areas (average 10–300 mg/cm 2/ka). Supported by redox-sensitive Re/Mo ratios, SR/TOC relationships, and paleoproductivity estimates we suggest that the formation and preservation of organic carbon during black shale formation in the Spekk Formation has followed largely the conditions of the ‘stagnation model’, whereas the Hekkingen Formation is likely one possible example for the ‘productivity model’.