Upper Jurassic to Lower Cretaceous source rocks in the Norwegian Barents Sea, part I: Organic geochemical, petrographic, and paleogeographic investigations.

Abstract

This study provides a subregional to regional characterization of organofacies changes within the two members of the Upper Jurassic to Lower Cretaceous Hekkingen Formation (i.e. Alge and Krill) by integrating geochemical and petrographic analysis with paleogeographic models.

The gross kerogen composition of the Hekkingen Formation is dominated by terrestrial macerals. This preponderance of land-derived particles is more pronounced in the Krill Member than in the underlying Alge Member. There is a greater proportion of marine macerals within distal areas of the Hammerfest Basin and well 7218/1-S in the southern Bjørnøyrenna Fault Complex. This shift in the relative proportion of marine and terrestrial macerals is ascribed to changes in the location of the depositional sites with respect to the sources of the terrestrial materials.

The Alge Member features the highest levels of total organic carbon (TOC ≥7 wt %), but the more discrete and organically poorer beds of the Krill Member still remain sufficiently rich to be considered petroleum source rocks (TOC ≥2 wt %). Hydrogen indices (HI) between 50 and 400 mg HC/g TOC recorded throughout the entire formation indicate that the kerogen within both members has similar oil and gas generation capabilities. These low to moderate HI values are indicative of immature Type III to II-III kerogens and are generally consistent with the high proportions of terrestrial macerals. Prior to thermal maturation, marine type II kerogens (i.e. ≤ 400 mgHC/gTOC) probably existed in the marine liptinite-rich rocks in the distal Hammerfest Basin and Bjørnøyrenna Fault Complex. At least three factors controlled the detected variability in geochemical parameters: dilution rates of organic matter, varying inputs of terrestrial versus marine organic matter, and the degree of preservation. The documented variability in organic-rich facies assists in reducing source rock risk in the study area, but also helps explorationists understand source rock distribution across other shelfal areas.