Print this pageA Gravity Collapse Origin for the Hampshire Basin, and Its Effect from the Petroleum System of the Wytch Farm and Other Oilfields in Southern England
The Hampshire Basin is a topographic depression in southern England, some 3500 km2 in area. It extends about 140 km from west to east and up to 40 km from north to south. Geologically it is a half-graben, with normal faulting along its north edge, and the Purbeck monocline forming a hangingwall rollover fold along the south edge. Starting in the 19th century, the structural history of the region had been described in terms of Mid-Cretaceous rifting followed by Tertiary (Alpine) compression. The 1973 discovery of the Wytch Farm oilfield in Mesozoic sediments provided well and seismic data leading to a new model for the evolution of the area through the mechanism of inversion tectonics. That model postulated Cretaceous rifting with normal faults that became inverted during the Tertiary to form supposedly Alpine-related folds, reverse, and thrust faults.
Our study supports neither the original interpretation nor the inversion tectonic model. There is no evidence for compressive folding and repetition in the shallow basin section, nor evidence that Cretaceous normal faults in the deeper reservoir section became reactivated and inverted in the Tertiary to form “Alpine” compressional structures in the shallow section. Instead, we propose that the present-day structure is a result of large-scale sub-horizontal southward movements along blind bedding-plane detachments in over-pressured and mobile Upper Cretaceous and Jurassic clays and Triassic salt, with emphasis, on the vital role of the Upper Cretaceous Gault Clay as a lubricant for large-scale bedding-plane slippage. This mechanism is consistent with a structural model of Miocene or later extension, driven by southward gravity collapse along blind detachment faults, a process proved through marine seismic profiles of huge scale gravity collapse structures along the continental margins of the equatorial Atlantic Ocean.
Removal of “Alpine” deformation implicit in older versions of the structural evolution of the Hampshire basin suggests that its development can be related to the catastrophic opening of the English Channel, some 450,000 years ago. The conclusions of this study radically alter earlier views of the Wytch Farm and regional petroleum system, suggesting the need for detailed review of burial history, hydrocarbon maturation and migration, leading to renewed exploration.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009