--> Fluid flow during early compartmentalisation of extensional rafts: A North Sea analogue resulting from >40 years of gas exploration and production

European Regional Conference and Exhibition

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Fluid flow during early compartmentalisation of extensional rafts: A North Sea analogue resulting from >40 years of gas exploration and production

Abstract

The Southern Sector of the North Sea has witnessed >40 years of gas exploration and production in offshore concession blocks. In this talk, we will present the geological setting of new exploration regions in the Mediterranean, NW Africa and Southern Atlantic, highlighting the similarities between the Southern North Sea and these regions. In this talk the emphasis is given to petroleum-rich basins in the Gulf of Cadiz, NW Morocco, Southern Crete, Cyprus and SE Brazil, where early rafting structural styles are ubiquitous.

High-quality 3D seismic data tied to Industry boreholes illustrate the styles of faulting and associated fluid flow features in Triassic–early Jurassic rafts of the Broad Fourteens Basin, in the Southern North Sea. The geological processes leading to flow of gas from Carboniferous and Permian units into Mesozoic strata of the Broad Fourteens Basin, crossing thick Zechstein salt, are described in detail. Importantly, the study area is shown to be an analogue for continental margins experiencing early stage gravitational gliding, i.e. prior to complete separation and downslope translation of individual rafts.

An important result of our study is that, for present-day stress conditions, fault slip data indicate that chasms and faults separating rafts can comprise structures subject to dip slip and strike-slip reactivation. These structures are the most significant bypass areas for fluid sourced from pre-salt strata in the Broad Fourteens Basin, as shown by fluid-flow models. In turn, faults sub-parallel to main chasms show limited propagation into Early Cretaceous and Cenozoic strata draping the rafts. The moderate propagation style is further stressed by the depth of occurrence of fluid pipes and dim spots, mostly oriented NNW-SSE along faults and fractures. This is an important observation, and leads us to postulate that faults formed during early stage rafting control fluid flow in regions where petroleum exploration is equated at the moment, but gravitational gliding is limited, such as Northwest Africa, Gulf of Cadiz and the Mediterranean Sea.