Carboniferous geology of the Southern North Sea Basin and controls on hydrocarbon prospectivity

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Abstract
The thick sequences of Upper Palaeozoic strata present in the Southern North Sea Basin were probably deposited on a basement of Caledonian low-grade metamorphic rocks intruded by numerous late-Caledonian granitoids which form part of the subsurface extension of the Mid-European Caledonides linking the English Lake District and Northern Pennines to the Ardennes. This basement, along with the better known onshore areas of the British Isles and Ireland was intensely fragmented during late Devonian to mid-Carboniferous times by a major phase of extensional tectonics. Active fault-bounded tilted blocks developed in the Dinantian with deposition of fluviatile redbeds, marine carbonate/deltaic clastic cycles and deltaic coastal plain facies. These are penetrated by a number of wells on the southern flanks of the Mid-North Sea High, but details of basin geometries and facies distributions remain unclear. Crustal stretching appears to have given way to thermal subsidence by early to mid-Namurian times. This was coincident with a major climatic change, probably forced by glacial expansion in Gondwanaland. The major drainage system that resulted dominated sedimentation in the SNSCB, punctuated by numerous marine transgressions of undoubted glacio-eustatic origins. Within this broad depositional framework, Namurian facies were closely controlled by basement tilt block topography, with good evidence for a gradual southerly migration of the active fairways of fluviodeltaic sandbodies. Westphalian facies were dominated by the interplay between glacio-eustatic and sedimentary processes on a vast low-gradient alluvial plain with much less pronounced tectonic control. By late-Westphalian B times a phase of basin inversion caused the gradual elevation of the area as a retro-arc foreland basin. Vadose diagenesis and pedogenesis occurred in the fluviatile facies of the so-called Barren Red Measures (BRM). With continued compressive deformation a series of over 50 NW-SE orientated folds, some associated with thrusts, developed. Significant erosion and truncation of these inversion anticlines was accompanied by the deposition of the BRM facies in characteristic growth fold basins, with marked onlap around the basin margins. The Carboniferous play in the SNSCB has been greatly influenced by the subsequent Mesozoic/Cenozoic geological history of the basin, including periods of subsidence and uplift. We demonstrate that variations in original detrital mineralogy and various diagenetic processes have exerted significant controls upon Carboniferous reservoir quality.