No abstract

The Trent Field gas accumulation in the UKCS (United Kingdom Continental Shelf) Southern Gas Basin is reservoired in Carboniferous age sediments. It is contained within a NW–SE trending base Permian structural feature that straddles Blocks 43/24a, 43/23 and 43/25. The proven reserves within the core area lie solely within the 43/24a block. Field appraisal and development encountered three main areas of complexity: structure, reservoir continuity and reservoir property variations.The 20 km long base Permian feature is controlled by NW–SE trending post-Rotliegendes reverse faults which produced a positive inversion. Antithetic faults to these reverse faults also occur within the structure. A second WSW–ENE normal fault trend comparmentalizes the field, and is also thought to have influenced sedimentation during Carboniferous times. Similar fault-influenced deposition is documented at equivalent times in the UK onshore. Ultimately this structural complexity controls the Carboniferous subcrop, which is seen to increase in age from late to early Westphalian A, in a southeasterly direction, although dramatic changes can occur on a more local scale in some fault compartments.The Trent Field accumulation is contained within an interval ranging in age from Marsdenian to early Westphalian A. It encompasses a number of facies associations in delta top and lower delta plain environments, which vary rapidly both vertically and laterally. Reservoir facies include incised valley-fills, transgressive reworked sandstones and distributary channels, which vary markedly in their lateral extent. Reservoir quality of these facies ranges from very poor to excellent and is controlled primarily by the depositional environment in which grain size and sediment maturity are the most important factors.

SUMMARY Palaeogeographic reconstructions, marine band deposition and the variations of uranium enrichment in sediments enable marine bands to be divided into four types: (1) Namurian marine bands, which represent marine anoxic black shale events, with thick ammonoid acme phases that concentrate uranium; (2) Vanderbeckei marine bands have thin ammonoid acme zones and abundant benthos, resulting from shallower water depths and poorly developed anoxic events with negligible uranium enrichment; (3) Westphalian B/C marine bands are more marginal and have an abundance of land-derived plant fragments with adsorbed uranium, as well as uranium entrapped within phosphatic tests of Lingula; (4) brackish water Lingula beds with abundant terrigenous matter and negligible uranium response. This classification scheme provides a means of predicting the uranium response of individual marine bands which is attributed primarily to the type and amount of organic matter, and the salinity of the waters responsible for deposition. This approach allows marine bands to be recognized and identified in the subsurface from their gamma ray and spectral gamma response, and together with palynological analysis allows the marine band to be placed more accurately within the regional stratigraphic framework.

The Tyne Fields, operated by ARCO British Limited, are located within UKCS Block 44/18a, situated 180 km off the Yorkshire coast in a water depth of 65 ft. The fields comprise three separate accumulations, each with a different gas-water contact and varying gas compositions, which are known as Tyne North, Tyne South and Tyne West. The three accumulations are combined structural and stratigraphical traps. The reservoir sandstones are of the Lower Ketch member of the Schooner Formation and are Carboniferous Westphalian C/D in age. The fields have been developed through long reach development wells. LocationThe Tyne Fields, operated by ARCO British Limited are located within UKCS Block 44/18a (Fig.

The Trent gas field lies within the UKCS Southern Gas Basin (Block 43/24a) located 120 km off the Yorkshire coast in average water depths of 160 ft. The accumulation is contained within a NW-SE trending Base Permian closure, which straddles blocks 43/24, 43/23 and 43/25. The Carboniferous subcrop beneath the Base Permian unconformity varies in age from Westphalian A in the east to Namurian in the west. Although the Base Permian closure covers an area of 75 km2 the producible reserves are only located in the central core area of 43/24a. The main reservoir horizon is the Trent Sandstone of Marsdenian age, equivalent to the Chatsworth Grit Sandstone, UK onshore. Additional reservoir zones are within the lowermost Westphalian A. The field has been developed through the application of fracture stimulation of deviated wells.