The reservoir geochemistry of the Eldfisk Field, Norwegian North Sea

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Abstract
A detailed geochemical study has been performed on the North Sea Eldfisk Field chalk reservoir, which forms part of a larger regional evaluation of the Greater Ekofisk area. Utilizing rapid screening methods (Rock-Eval, Iatroscan), conventional biomarker maturity parameters, stable carbon isotope data (bulk and individual n-alkanes) and pyrrolic nitrogen distributions, it is shown that the Eldfisk Field petroleum is heterogeneous, both laterally on a kilometre scale and vertically on a metre scale. The chemical heterogeneities throughout the field led to a better understanding of the charging of the reservoir, the nature of communication between the Upper Jurassic source rocks and trap, and the identification of geological barriers to petroleum mixing within the field itself. The biomarker maturity and pyrrolic nitrogen compound data suggest that charging of the Eldfisk Field ocurred from the south, petroleum moving northwards up and along the Skrubbe fault zone charging the Eldfisk Alpha and Bravo structures. Although filling has largely taken place since c. 10 Ma BP , initial charging of the Eldfisk structure started c. 20 to 25 Ma BP . The data suggest that petroleum emplacement was via faults and fractures, with communication between the source rocks and trap being discontinuous. The distribution of the different petroleum populations within the field was controlled significantly by the internal field plumbing which is sedimentologically influenced. Sharp, step-like biomarker-parameter maturity variations downhole in several wells have been identified and verified statistically. These geochemial discontinuities can be correlated with specific geological features (hard-grounds and tight zones) which have prevented mixing of the petroleum column. In one case, the presence of a barrier in the reservoir has been confirmed by repeat formation test (RFT) pressure data. The identification of lithofacies variations laterally across the field and barriers vertically in the petroleum column may become important when assessing production scenarios.