Temporal organic facies variations of Upper Jurassic - lowermost Cretaceous source rocks in the Danish Central Graben, North Sea

The Upper Jurassic – lowermost Cretaceous marine mudstones of the Farsund Formation constitute the principal source rocks in the Danish North Sea sector with the uppermost Bo Member considered most prolific. However, the areal distribution of the Bo Member is limited and since it is thermally immature over large areas it is unlikely that this interval has generated sufficient quantities of petroleum to fill the reservoirs in the Danish sector. In search for additional source rocks potential we have investigated the lower part of the Farsund Formation using a sequence stratigraphic framework. Organic geochemical screening and petrographic analyses of core and cuttings samples have been performed to define the source potential and map the regional distribution of organic-rich sequences in the Upper Jurassic succession. The study outlines variations of both maturity, source rock quality, and kerogen quality at sequence level with an upward improvement from poor gas-prone Type II kerogen in the Kimm-1 and Kimm-3 sequences to highly oil-prone Type II kerogen in the Volg-1, Volg-4 and Ryaz-1 sequences. The kerogen is dominated by fluorescent AOM and liptinite and structural alginite in form of Leiosphaerida- and Tasmanites-type telalginites throughout the entire succession, with vertical- and regional variations in the amount of larger Tasmanites. The study shows that the shales in Volg-1 and -3 sequences are organic-rich and oil prone with TOC > 4 wt% and HI > 340 mg HC/g TOC. Particularly the Volg-1 sequence, being the thickest and laterally most widespread forms a significant supplementary source for oil in the Danish sector.