ORGANIC GEOCHEMISTRY OF BARENTS SEA PETROLEUM: THERMAL MATURITY AND ALTERATION AND MIXING PROCESSES IN OILS AND CONDENSATES

B. Lerch^{1 *}, D. A. Karlsen¹, R. Seland² and K. Backer-Owe¹

¹ Department of Geosciences, University of Oslo, PO Box 1047 Blindern, N-0316 Oslo, Norway.

² Noreco, Postboks 550 Sentrum, 4003 Stavanger, Norway.

^* Corresponding author, email: eduard.albert@ub.edu

The petroleum system in the Barents Sea is complex with numerous source rocks and multiple uplift events resulting in the remigration and mixing of petroleum. In order to investigate the degree of mixing, 50 oil and condensate samples from 30 wells in the SW Barents Sea were geochemically analysed by GC-FID and GC-MS to evaluate their thermal maturity and secondary alteration signatures. Saturated and aromatic compounds from C₁₄–C₁₈ and biomarker range (C₂₀₊) hydrocarbons were compared with light (C₄-C₈) hydrocarbon alteration and maturity signatures from a previous study. The geochemical data demonstrate that petroleum generation occurred from the early- to late-oil/condensate window, correlating to calculated vitrinite reflection values of between 0.7% R_C and 1.9% R_C. Two maturation traits are in general present in the oil samples analysed and indicate mixing of petroleum phases: a C₂₀₊ fraction which represents a possible "black-oil -related" signature; and a C_20- fraction, which is probably a more recent oil charge. However, maturity variations are less pronounced in condensates, which in general exhibit higher generation temperatures than oils but are influenced by severe phase fractionation effects. The samples are characterised by diverse biodegradation signatures including depletion of C_15- saturated compounds, almost complete removal of n-alkanes, elevated Pr/n-C₁₇ values, high 17α(H), 25-norhopane content, and a reverse trend in methylated naphthalene distribution. However, the presence of the more recent, unaltered light hydrocarbon charge together with the oil with a palaeo-biodegraded signature is clear evidence that mixing has occurred. A cross-plot of C₂₄-tetracyclic terpane/C₃₀αβ-hopane versus C₂₃-C₂₉-tricyclic terpane/C₃₀αβ-hopane can be used to discriminate between Palaeozoic/Triassic and Jurassic-generated petroleums in the Barents Sea region, since it appears to be maturity independent.

Keywords: Barents Sea, Arctic Norway, petroleum systems, petroleum geochemistry, biomarkers, alteration, in-reservoir mixing, oil-source rock correlation

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