The Orwell gas field, located in part-block 50/26a of the UK Southern North Sea, has long been known to contain a Triassic Bunter Sandstone Formation (BSF) reservoir in what is otherwise a Permian, Leman Sandstone Formation play fairway. Results of a new, well-calibrated interpretation of a seismic 3-D volume provide important new insights on the structural and stratigraphic controls on hydrocarbon prospectivity of the gas field.
It can now be shown that Late Cretaceous contractional reactivation (structural inversion) of pre-existing Permo-Triassic and Jurassic extensional faults creates a fault propagation fold and the structural trap in the hangingwall of a WNW-ESE-striking fault. Sequential restoration for the effects of dip-slip contraction permits the precursor fault geometry and full tectonic-stratigraphic history of the structure to be determined.
Furthermore, integration with electrical well log analysis show that the original normal fault geometry controlled deposition and subsequent preservation of the BSF reservoir. Sedimentological studies show that the middle parts of the reservoir section have the best quality, with the upper and lower parts having the poorest. The inherited structural geometries during extensional fault growth contribute to the preferential preservation of the thicker poor quality reservoir after a major Late Jurassic erosional event. During inversion, the thicker part of the reservoir becomes uplifted such that it now occurs at the crest of the closure.
The possibility of insufficient gas charge and/or the mismatch between gas migration and trap formation is suggested by the free-water-level being above the deepest closing contour of the reservoir as there is also no evidence for cross-fault juxtaposition or seal-breach having caused leakage.
The determination of the pre- and post-inversion geometries combined with the understanding of the stratigraphic evolution in inverted basins helps predict reservoir quality and minimize pre-drill exploration and production risk.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California