Abstract

Discovery of the Breagh gas field in the Southern North Sea has demonstrated the potential that the Lower Carboniferous (Visean, 330.9-346.7 Ma) Farne Group reservoirs have to contribute to the UK's future energy mix. New biostratigraphic correlations provide a basis to compare Asbian and Brigantian sedimentary cores from the Breagh Field and age-equivalent sediments exposed on the Northumberland Coast, which has proved critical in gaining an understanding of exploration and development opportunities. Thirteen facies associations characterise the mixed carbonate-siliciclastic system, grouped into: marine, delta front, delta shoreface, lower delta plain and upper delta plain gross depositional environments. The facies associations are interpreted as depositing in a mixed carbonate and siliciclastic fluvio-deltaic environment and are arranged into coarsening and cleaning upwards cycles (parasequences) bounded by flooding surfaces. Most cycles are characterised by mouth bars, distributary channels, interdistributary bays and common braided rivers interpreted as river-dominated deltaic deposits. Some cycles include rare shoreface and tidally-influenced deposits, interpreted as river-dominated and wave- or tide-influenced deltaic deposits. The depositional processes that formed each cycle have important implications for reservoir net/gross, thickness and lateral extent. The deltaic deposits were controlled by a combination of tectonic and eustatic (allocyclic) events and delta avulsion (autocyclic) processes and likely reflect a changing tectonic regime, from extension within elongate fault-bounded basins (syn-rift), to passive regional thermal subsidence (post-rift). Deep incision by the Base Permian Unconformity across the Breagh Field has removed the Westphalian, Namurian and Upper Visean to leave the more prospective thicker clastic reservoirs within closure.

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