Abstract The Hewett Field has been in production for some 50 years. Unusually for a Southern North Sea field in the UK Sector, there has been production from several different reservoirs and almost entirely from intervals younger than the principal Leman Sandstone Formation (LSF) reservoir in the basin. Some of these reservoirs are particular to the Hewett area. This reflects the location of the field at the basin margin bound by the Dowsing Fault Zone, which has influenced structural evolution, deposition and the migration of hydrocarbons. The principal reservoirs are the Permo-Triassic Hewett Sandstone (Lower Bunter), Triassic Bunter Sandstone Formation (BSF) (Upper Bunter) and Permian Zechsteinkalk Formation. There has also been minor production from the Permian Plattendolomit Formation and the LSF. Sour gas is present in the BSF only. Several phases of field development are recognized, ultimately comprising three wellhead platforms with production from 35 wells. Gas is exported onshore to Bacton, where the sour gas was also processed. Peak production was in 1976 and c. 3.5 tcf of gas has been recovered. Hewett has also provided the hub for six satellite fields which have produced a further 0.9 tcf of gas. It is expected that the asset will cease production in 2020.

Abstract The Mid North Sea High (MNSH) is located on the UKCS in quadrants 35–38 and 41–43. It is a large structural high that is flanked by the mature hydrocarbon provinces of the Central North Sea (CNS) to the NE and the Southern North Sea (SNS) to the SE. In the MNSH region, the source and reservoir intervals that characterize the SNS (Westphalian, Lower Permian) are absent and therefore the area is relatively underexplored compared to the SNS Basin ( c . one well per 1000 km 2 ). Nevertheless, two discoveries in Dinantian reservoirs (Breagh and Crosgan) prove that a working petroleum system is present, potentially charged either via lateral migration from the SNS or from within the lower Carboniferous itself. Additionally, gas was found in the Z2 carbonate (lower Zechstein Group) in Crosgan, with numerous other wells in the area reporting hydrocarbon shows in this unit. The results of the interpretation of recently acquired 2D and 3D seismic reflection datasets over parts of UKCS quadrants 36, 37 and 42 are presented and provide insight into both the geology and prospectivity of this frontier area. This study suggests that intra-Zechstein clinoform foresets represent an attractive, hitherto overlooked, exploration target. The Zechstein Group sits on a major unconformity, probably reflecting Variscan-related inversion and structural uplift. Below it, fault blocks and faulted folds occur, containing pre-Westphalian Carboniferous and Devonian sediments, both of which contain potential reservoirs. In the lower Zechstein, a large build-up is observed, covering a total area of 2284 km 2 . This is bounded on its margins by seismically defined clinoforms, with maximum thicknesses of 0.12 s two-way time ( c. 240–330 m). This rigid, near-tabular unit is clearly distinguished from the overlying deformed upper Zechstein evaporites. In map-view, a series of embayments and promontories are observed at the build-up margins. Borehole data and comparisons with nearby discoveries (e.g. Crosgan) suggest this build-up to represent a Z1–Z2 sulphate–carbonate platform, capped by a minor Z3 carbonate platform. Interpreted smaller pinnacle build-ups are observed away from the main bank. The seismic character, geometry, size and inferred composition of this newly described Zechstein platform are similar to those of platforms hosting notable hydrocarbon discoveries in other parts of the Southern Permian Basin. The closest of these discoveries to the study area is Crosgan, which is characterized by the Z2 carbonate clinothem (Hauptdolomit Formation) as a proven reservoir.

Abstract Six satellite fields have been developed through the Hewett Field facilities: Big Dotty, Little Dotty, Deborah, Della, Dawn and Delilah. Little Dotty has produced from both the Leman Sandstone Formation (LSF) and Bunter Sandstone Formation (BSF) whilst the other satellites are exclusively LSF developments. The LSF reservoir quality exhibits a marked contrast across the Dowsing Fault Zone, which separates the inboard satellites to the SW from the outboard satellites to the NE. The inboard satellites, Big Dotty, Little Dotty and Dawn, display the best reservoir quality, reflecting their lesser depth of maximum burial. These fields share a strong aquifer, exhibited a rapid water-cut development and are now shut-in. The greater depth of maximum burial experienced by the outboard satellites, Deborah, Della and Delilah, is reflected in poorer reservoir quality along with weaker aquifers that are also more compartmentalized. These remain in production and will achieve higher recovery factors. Big Dotty was developed from a wellhead platform whereas the other fields were developed as subsea tie-backs. Collectively, these satellite fields have produced some 0.9 tcf of gas, playing an important strategic role in offsetting the production decline in the Hewett Field and extending the life of the asset.

Abstract The abandoned Juliet gas field is a small, highly compartmentalized, accumulation situated south and east of the Amethyst Gas Field. It was discovered in 2008 by well 47/14b-10 and flowed first gas on 5 January 2014. The field consists of at least two culminations within a very low-relief east–west-orientated fault-bounded anticline. The reservoir comprises aeolian sandstones of the Permian, Rotliegend Group, Leman Sandstone Formation. Reservoir quality varies from good to moderate, with a high production rate achieved from horizontal wells. Seismic time-to-depth conversion is affected by Quaternary seabed channels, chalk burial history and a rapid thickening in the Basalanhydrit Formation located over the east of the field, associated with the edge of the Zechstein Basin. Gas-in-place at pre-development was expected to be 105 bcf, with reserves of 67 bcf. The field was developed using two horizontal wells and a subsea tie-back to the Pickerill Field, 22 km to the east. Since development, the field appears to be more compartmentalized than initially expected.

Abstract The Hewett Fields complex, discovered in 1966, is located in the southwestern part of the Southern North Sea Basin. Gas is currently being produced from four horizons: Upper and Lower Bunter sandstones in the Triassic, Rotliegendes sandstones, and Zechstein dolomites in the Permian. Seven fields, containing a total of ten reservoirs, lie within the Hewett Unit and all have a NW-SE orientation. Gas flows, via two 30-inch (0.76 m) pipelines, to the ConocoPhillips-operated Bacton terminal, where onshore compression facilities are located. There are also compressors located on two platforms. Over 4 TCF (113 billion cubic metres) of gas has so far been produced in over 30 years of production.

Abstract The Carrack Field, located in the Southern North Sea Blocks 49/14b and 49/15a, has of the order or 15 bcm (530 bcf) gas initially in place and is operated by Shell UK Ltd. The field consists of a pop-up structure in the south of the field and extends to the north with a gently-dipping monoclinal structure. The reservoir comprises sandstones of the Permian Silverpit and Leman Sandstone formations, which contain c. 85% of the in-place resources. The quality of the reservoir decreases rapidly to the north. Gas is also produced from Carboniferous sandstones of late Duckmantian (Westphalian B)–Bolsovian (Westphalian C) age. Initially, the field was in pressure communication both laterally and vertically with a single gas–water contact. During production time, however, the three main fault blocks behaved independently, and decimetre-thick shale intervals acted as vertical baffles between the sandstone units. The Carrack Field has been in production since 2003 and is developed by a single platform with seven mainly deviated wells. The current production rate is c. 0.7 MMm 3 /day (25 MMscfgd). Until the end of field life in the 2030s, the field is expected to produce gas of the order of a few bcm. The main remaining opportunity is the undeveloped Carrack West compartment.

Abstract Corvette is a small prolific gas reservoir with reserves of 211 BSCF located on the Indefatigable Shelf in the Southern North Sea. The reservoir is the Permian Rotliegend aeolian sandstone, capped by Zechstein evaporites and sourced from the Carboniferous Coal Measures. The structure is a 'pop up' between the Gawain and Baird Fields. The field was discovered in 1996 and brought on production in 1999, with gas being evacuated via the Leman Field to the Shell/Esso Bacton gas terminal.