Biomarker and Petrographic Evidence for the Origin and Maturity of Abundant Palaeocene Oil-Prone Type III Kerogen Deposits, Spitsbergen
Chris Marshall1, David J. Large1, Colin E. Snape1, William Meredith1, Clement Uguna1, Julius Babatunde1, Baruch F. Spiro2, and Alv Orheim3
1Department of Chemical and Environmental Engineering, University of Nottingham, Nottingham, United Kingdom.
2Department of Mineralogy, Natural History Museum, London, United Kingdom.
3GeoArktis AS, Stavanger, Norway.
Oil-prone perhydrous (>5.6% H dmmf basis) coals are economically important as source rocks. Abundant oil-prone coal (Type III Kerogen) deposits are preserved within the high latitude, middle Palaeocene, Todalen Member of the Central Tertiary Basin, Svalbard. These coals provide a significant opportunity to understand the processes which control Arctic oil-prone Type III kerogen production. This paper presents initial findings from the Svea & Longyear coals and four coals sampled from borehole 05/2009 in the Longyearbyen region. Samples were separated into kerogen, free maltene and asphaltene fractions by solvent extraction and n-hexane precipitation. Hydropyrolysis-GCMS was then used to compare the source, maturity and biodegradation parameters for the free, kerogen and asphaltene-bound biomarkers to determine the origins of the perhydrous coal and associated bitumen. The principal findings of this study are;
Coals in the Longyearbyen region are of high volatile bituminous rank (% Ro 0.65-0.69) and contain significant amounts of fluorescent vitrinite and detrovitrinite peaking in the upper Todalen member coals and low liptinite content (3-6%). Coals from the Svea region are more mature (% Ro 0.77) and exhibit variable maceral compositions. Generally, the Svea coals contain increased inertinite content (50-70%) at the expense of oil-prone macerals (perhydrous vitrinite & liptinite) limiting oil potential. Extracts from the Longyear and Svea coals produce significant bitumen with ca. 10% & 5% extracted bitumen respectively. Bitumen is composed mostly of maltene fractions (57.5% and 60-68% of extract respectively) with bound asphaltenes making up the remainder of the bitumen.
Comparison of hopane, sterane, n-alkane and tricyclic diterpane source and maturity biomarker ratios for the asphaltene and kerogen phases indicates a terrestrial source for the bitumen with no evidence of in-migration from elsewhere within the basin. The Pr/nC17 ratio of the maltene fraction is higher than that of the bound asphaltene fraction (4.5 vs. 0.1 in the Longyear seam) which is a general trend for coals reflecting the weaker covalent bonding of the iso-prenoids compared to the n-alkanes. A key feature of the Svalbard coals and bitumen is the extremely high concentrations of hopanes (up to 72 μg/g C). Examination of the m/z 177 mass chromatogram indicates the absence of norhopanes, which suggests that the hopanes are derived from the coal kerogen rather than from the mixing of bitumen generated in situ with more degraded bitumen from elsewhere in the basin. Consequently, high hopane concentrations are attributed to rapid microbial biodegradation rates during peatland deposition caused by the unique climatic position of Svalbard in the Palaeocene and preservation of hydrogen rich organic matter by organo-sulphur compound formation.
AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.����������������������������������������������������