THE GEOLOGY AND HYDROCARBON HABITAT OF THE BRISTOL CHANNEL BASIN

Abstract

The Bristol Channel Basin forms an early Mesozoic ((?Permo‐) Triassic‐Jurassic) basin development with a relatively thin cover of Cretaceous and Tertiary sediments. On the basis of structural trends and stratigraphy, the Basin can be divided into two sub‐basins: the ENE‐WSW trending Main Bristol Channel Basin and the E‐W trending East Bristol Channel Basin. In between them there is an Intermediate Area which incorporates features of the sub‐basins on either side. This subdivision appears to be the result of the presence of major NW‐SE basement faults which intersect the Bristol Channel area in several places.As a result of intermittent periods of tectonic activity three main stages of basin development can be recognised: (1) a (?Permo‐) Triassic‐ Middle Jurassic stage terminated by mid‐Kimmerian epeirogenetic movements, (2) an Upper Jurassic—Lower Cretaceous stage terminated by a Lower Cretaceous (late Berriasian‐pre‐Aptian) period of deformation, (3) an Upper Cretaceous—Tertiary stage.The bulk of the preserved sedimentary fill in the main Bristol Channel Basin consists of up to 11,000 ft of (?Permo‐) Triassic‐Jurassic sediments. The section is severely truncated by the overlying late Lower Cretaceoussediments or, in theirabsence, by Upper Cretaceous strata. Below the unconformity, the Upper Jurassic and, depending on their structural position, parts or the whole of the Middle and Lower Jurassic are missing.The East Bristol Channel Basin hasapreserved fill of some 7,500 ft of Triassic‐Jurassic sediments. The Middle and Upper Jurassic are completely preserved in the centre of this Eastern Basin, contrasting with the situation found in the main Basin.The basin fill of both sub‐basins consists largely of Triassic continental red‐beds, mainly silty claystones and evaporites and Jurassic marine calcareous siltstonesand claystones.Structures are essentiully the result of the Lower Cretaceous tectonic phase(s) and are therefore mainly confined to the Triassic—Jurassic—early Lower Cretaceous sections. A number of structural traps of interest to exploration have been delineated and (unsuccessfully) tested in the Main Bristol Channel and Intermediate Area.Evaluation of the currently available data indicates that the absence of significant hydrocarbon indications in these wells is probably due to insufficient and untimely hydrocarbon generation. The possible generation is thought to have occurred prior to the main phases(s) of structural deformation, during a period of temporarily increased heatflow (Middle Jurassic—Lower Cretaceous). An additional unfavourable aspect for hydrocarbon prospects is the lack of good quality reservoir developments.From a megateetonic point of view the Bristol Channel Basin formspart of a regional rift basin development which also includes the Celtic Sea and Western Approaches Basins. The different basins and blocks of this area can, according to their behaviour during the Mesozoic, be grouped as follows: (a) the East Bristol Channel, E. of the zone of majorNWSE faults (e.g. Sticklepath fault), (b) a Central Fault Block, comprising the Cornubian Platform, the Main Bristol Channel and Haig Fras Basins, (c) Flanking Basins, such as the Intermediate Area, the Celtic Sea and Western Approaches Basins.