--> Charging and Connectivity of Fractured Basement Reservoirs in the West of Shetland (North Atlantic, UK) - Insights From Oil Geochemistry Studies

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Charging and Connectivity of Fractured Basement Reservoirs in the West of Shetland (North Atlantic, UK) - Insights From Oil Geochemistry Studies

Abstract

Three discoveries and a proven oil field have been discovered to date since 2009 by Hurricane Energy, plc in the weathered and fractured basement of the Rona Ridge, a prominent NE-SW-trending basement high acting as a structural feature separating basins in the West of Shetland, North Atlantic Ocean (UK). Basement oil had previously been observed in the large Clair field, in which Devonian sands act as reservoirs, further NE on the Rona Ridge. Jurassic and Triassic sand reservoirs are also explored in the Solan and Strathmore fields, respectively on the margins of the Rona Ridge, in its SW section near the Hurricane licences. A main objective of the exploration of the basement plays was to identify hydrocarbon sources and charging mechanisms (Trice, 2014), calling for a comprehensive organic geochemistry study. A preliminary geochemical interpretation of the connectivity of the basement reservoirs has also been performed. Here we present, for the first time, the results of these geochemical investigations. Oils, gas condensates and basement oil stain extracts originate from the proven Lancaster oil field and from nearby discoveries. The dataset encompasses bulk and molecular (biomarker) composition of the fluids. It also includes stable isotope compositions of oils, condensates and hydrocarbon gases (C1-C5). Our results show that the Upper Jurassic Kimmeridge Clay Formation (KCF) is the source of all fluids. However, the basement has received charges from several sub-basins in which the KCF shows subtle biomarker composition differences (e.g. more open marine vs. more restricted and anoxic depositional environments), with implications for the onset of hydrocarbon generation and charging of the basement. Consistently, the maturity of the fluids reflects charges from distinct kitchens. However, some fluids appear to have mixed sources and to have received charges of different maturities, showing that different areas of the fractured basement reservoir are connected. Our preliminary analysis of the composition of gasoline-range hydrocarbons reveals that the fluids have, in fact, experienced moderate evaporative fractionation implying several charging occurrences. This geochemical study thus provides early insights into charging and connectivity of the basement reservoirs. Trice, R., 2014. Basement exploration, West of Shetlands: progress in opening a new play on the UKCS. Geological Society, London, Special Publications 397, 81-105.