OIL: SOURCE ROCK EVALUATION AND PETROLEUM GEOCHEMISTRY OF THE LULITA OILFIELD,
DANISH NORTH SEA
H. I. Petersen*+, J. Andsbjerg*, J. A. Bojesen-Koefoed* and H. P. Nytoft*
*Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmark.
+corresponding author (email: firstname.lastname@example.org)
Controversy still exists as to whether coals can source commercial accumulations of oil. The Harald and Lulita fields, Danish North Sea, are excellent examples of coal-sourced petroleum accumulations, the coals being assigned to the Middle Jurassic Bryne Formation. Although the same source rock is present at both fields, Lulita primarily contains waxy crude oil in contrast to Harald which contains large quantities of gas together with secondary oil/condensate. A compositional study of the coal seams at well Lulita-1Xc (Lulita field) was therefore undertaken in order to investigate the generation there of liquid petroleum.
Lulita-1Xc encountered six coal seams (0.150.25 m thick) which are associated with reservoir sandstones. The coals have a complex petrography dominated by vitrinite, with prominent proportions of inertinite and only small amounts of liptinite. Peat formation occurred in coastal-plain mires; the coal seams at Lulita-1Xc represent the waterlogged, oxygen-deficient and occasionally marine-influenced coastal reaches of these mires. Vitrinite reflectance values (mostly 0.820.84 %Ro) indicate that the coals are thermally mature. Most of the coal samples have Rock-Eval Hydrogen Index values above 220 mg HC/g TOC, although the HI values may be increased due to the presence of extractable organic matter. Oilsource rock correlations indicate that there are similarities between crude oil samples (and an oil-stained sandstone extract) from the Lulita field, and extracts from the Bryne Formation coals immediately associated with the reservoir sandstones; from this, we infer that the coals have generated the crude oil at Lulita. The presence in the coals of oil-droplets, exsudatinite and micrinite is further evidence that they have generated liquid petroleum. The generation of aliphatic-rich crude oil by the coals in the Lulita field area, and the coals high expulsion efficiency, may have been facilitated by a combination of the coals favourable petrographic composition and their capability to generate long-chain n-alkanes (C22+). Moreover, all the Lulita coal seams are relatively thin and this may have facilitated oil saturation to the expulsion threshold. We suggest that during further maturation of the coals, 1922% of the organic carbon will potentially participate in petroleum-generation, of which about 4253% will be in the gas-range and 4758% in the oil-range.