Source rock analysis and petroleum geochemistry of the Trym discovery, Norwegian North Sea: a Middle Jurassic coal-sourced petroleum system

Abstract

The 3/7-4 well (Trym) in the Norwegian part of the Søgne Basin, North Sea, discovered condensates in Middle Jurassic reservoir sandstones and in addition penetrated four Middle Jurassic coal seams, 0.14–0.56 m thick. The precursor peats of the coals, which have a complex vitrinite- and inertinite-rich composition, accumulated in coastal plain mires, and the depositional conditions of the peats influenced the source rock potential of the resulting coals. Vitrinite reflectance values from 0.86–0.87% R o and an average T max of 438°C indicate that the coals are thermally mature, but they may be interpreted only to be in the early phase of petroleum generation. The Hydrogen Indices of the coals are generally below 200, and crude oil saturation of the coals may have been hindered by an uneven in-seam distribution of favourable Hydrogen Indices (>200) coupled with the comparatively thick seams. Condensate-source rock correlations indicate a similarity between the Trym condensates and extracts from the coals in the 3/7-4 well, and stable carbon isotope data infer a common source for the Trym condensates and the condensates and oils from the Harald and Lulita Fields in the Danish part of the Søgne Basin. The Harald and Lulita Fields have recently been proven to represent a Middle Jurassic coal-sourced petroleum system (the Lulu-Bryne/Lulu(!) Petroleum System), and the Trym condensate-discovery is thus another example of this petroleum system, which may be more widespread in the Central Graben.