DEPOSITIONAL ENVIRONMENTS, ORGANIC MATURUTY AND PETROLEUM POTENTIAL OF THE CRETACEOUS COAL-BEARING ATANE FORMATION AT QULLISSAT, NUUSSUAQ BASIN, WEST GREENLAND
G. K. Pedersen1*, L. A. Andersen1, E. B. Lundsteen1, H. I. Petersen2, J. A. Bojesen-Koefoed2 and H. P. Nytoft2
1 University of Copenhagen, Geological Institute, 10 Øster Voldgade, DK-1350K Copenhagen, Denmark.
2 Geological Survey of Denmark and Greenland (GEUS), 10 Øster Voldgade, DK-1350K Copenhagen, Denmark.
*Corresponding author, email: email@example.com
Coals and coaly mudstones of the Cretaceous Atane Formation are exposed along the north coast of the island of Disko and the south coast of Nuussuaq peninsula, West Greenland. Numerous oil seepages have been found in the region, but the so-called Kuugannguaq oil type only occurs at the north coast of Disko. The oil is presumed to have been generated from coaly (Type III) source rocks in the Vaigat strait where the Atane Formation is thermally mature due to deep burial. The exposed coals and coaly mudstones may thus be thermally immature equivalents to the active source rocks. The exposed section at Qullissat on the island of Disko is composed of four sedimentary facies associations: delta plain, distributary channel, delta front, and transgressive sand sheet. Samples of coals and coaly mudstones from the delta plain association were analysed for their total organic carbon (wt % TOC) and total sulphur (wt % TS) contents, and their source rock potential was determined by Rock-Eval pyrolysis. The organic matter composition was analysed by reflected light microscopy and the thermal maturity was established by vitrinite reflectance measurements. The Qullissat samples were supplemented with source rock screening data from coals and coaly mudstones from the Atane Formation at Paatuut at the south coast of Nuussuaq. The coals and coaly mudstones from Qullissat are dominated by huminite, but several samples have a considerable content of inertinite. The mineral content is high in some samples. Inundations of the peat-mires may have been quite frequent resulting in the formation of the coaly mudstones. TS contents (0.13–8.97 wt %) and the presence of framboidal pyrite suggest that the precursor peats were influenced by seawater, and that peat formation probably occurred during rises in relative sea-level. The organic matter is thermally immature, and a constructed vitrinite reflectance gradient for the region suggests that the Qullissat section prior to exhumation was buried to 1,500–1,600 m depth. Hydrogen Index (HI) values from both Qullissat and Paatuut are generally low; estimated maximum HI values for three Qullissat coals yield values of 140–190 mg HC/g TOC. The coals are gas-prone and only marginally oil-prone, and may in addition possess a limited oil expulsion efficiency. The effective oil window extends from approximately 1.0–1.6%Ro and the start of the effective oil window is located at about 3,000 m depth. Very thick sedimentary successions in the Vaigat strait indicate that such burial depths have been reached for the Atane Formation offshore, and up-dip migration of hydrocarbons from these source rocks may have generated the Kuugannguaq oil seepage.
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