Polyphase tectonic inversion and its role in controlling hydrocarbon prospectivity in the Greater East Shetland Platform and Mid North Sea High, UK

Thick Paleozoic successions are buried under the Greater East Shetland Platform (ESP) and Mid North Sea High (MNSH), two large underexplored platform regions flanking the structural depocentres of the North Sea. Here, newly acquired broadband seismic data are interpreted to provide a novel assessment of the regional tectonostratigraphic evolution and its influence on hydrocarbon prospectivity. Numerous working reservoir units are present over these two frontier areas, together with large Paleozoic traps. Hydrocarbon charge occurs either via a likely maximum 30-40 km lateral migration from the Jurassic/Carboniferous basinal source kitchens or, possibly, via vertical/lateral migration from deeper Devono-Carboniferous source intervals. The two regions underwent a largely similar evolution, consisting of at least eight successive switch-overs between regional compression/uplift and extension/subsidence in the last 420 myr. However, on the Greater MNSH, the lack of significant Permo-Triassic rifting probably resulted in too little subsidence for the lower Carboniferous interval to reach sufficient burial depth for gas maturation. Seep and fluid escape data suggest a working 'deep' source in the Greater ESP. Here, the presence of localized Permo-Triassic intra-platform grabens and half-grabens provided sufficient subsidence for the oil-prone middle Devonian unit to eventually enter the oil maturation window and faults provide easy conduits for the upwards migration of oil.