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Petroleum Correlation in the Tampen Spur Area, North Sea: A New Approach from Molecular and Stable Carbon Isotopic Data of Light Hydrocarbons

Abstract

The Tampen Spur (TS) is a structural high located in the northern North Sea, a prolific oil-producing region whose petroleum system comprises mainly Jurassic source rocks. The current study focuses on the molecular and carbon isotopic compositions of light hydrocarbons (LHCs) to differentiate mixed oils from the TS area, especially in the Gullfaks field and its surroundings. In the TS, the d13C pattern of n-alkanes (>C11) showed little differences among oils, however, wide variations in d13C values of gasoline-range branched and cyclic hydrocarbons were useful to distinguish oils. In the case of biodegraded oils from the Goliat field, the d13C of cycloalkanes were used to correlate these oils with other oils of comparable thermal maturity. The marine-sourced oils in the Gullfaks West field, characterized by the lowest Pr/Ph ratio and a predominance of even n-alkanes typical of calcareous source facies, exhibited the lightest d13C values of branched and cyclic alkanes, while oils from the Gullfaks South field, which show a high contribution from more terrestrially-influenced source facies, were the most 13C-enriched. Intermediate d13C values were found in oils from the Gullfaks field, however, the d13C values alone was not enough to differentiate Gullfaks oils hosted in the Brent group from those hosted in the Cook Fm., in spite of having been filled from two different source basin. Differences between these two oil families were detected using Mango's parent-daughter ratio plots and the isoheptane ratio (K1). These C7 hydrocarbon ratios were also able to distinguish other oil families in the TS, which are related to petroleum charging systems. Similarly, variations in the normalized distributions of three-, five- and six-ring compounds, which are consistent with terrigenous versus marine inputs, were recognized in TS oils. Oils from the Gullfaks South field were enriched in six-ring compounds whereas oils from the Gullfaks field, which are sourced mainly from marine organic matter, tended to show a preference for five-ring compounds. Both the molecular and compound-specific isotopic analyses of gasoline range hydrocarbons were useful in correlating genetically related oils in the TS. They allowed a very good discrimination of oils into four families whose geographical distribution agrees with the oil migration pathways in the area.