An extensive regional stratigraphic hiatus has been proposed in the Asturian to Stephanian foreland basin successions lying to the north of the main Variscan deformation belt, stretching from north west Europe to eastern North America. The hiatus is inferred on megafloral grounds and involves the absence of most if not all of Stephanian Substages B and C. If present it would have great significance for the understanding of regional basin dynamics in the later stages of the Variscan orogeny. However, its existence has been strongly contested. One of the keys to this hypothesis is the correct identification and interpretation of plant fossils from the Warwickshire Coalfield (Central England). The published identifications are here shown to be incorrect, and revised determinations taken in conjunction with other stratigraphic data from Central England suggest that a regional hiatus is not present in the early-mid Stephanian, and that any stratigraphic hiatuses at other horizons in the succession are of purely local extent. A review of known occurrences of the Asturian and Stephanian successions in basins stretching from eastern Canada to Germany suggests that, while facies developments may be similar, there is little uniformity in stratigraphic style and little evidence for a major Stephanian hiatus. (C) 2020 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.

Local seismic and borehole-based mapping of the Carboniferous Pennine Coal Measures and Warwickshire Group successions in the Canonbie Coalfield (SW Scotland) provides evidence of repeated episodes of positive inversion, syn-depositional folding and unconformities. A Duckmantian (Westphalian B) episode of NE–SW transpression is recognized, based on onlapping seismic reflector geometries against NE-trending positive inversion structures and contemporaneous NNE-trending syn-depositional growth folding. The basin history thus revealed at Canonbie is at variance with generally accepted models in neighbouring northern England that imply subsidence was due to post-rift thermal subsidence during late Carboniferous times. A late Westphalian–Stephanian unconformity recognized within the Warwickshire Group succession signifies NW–SE, c . 10% local basin shortening during a time of major shortening in the late Carboniferous Variscan foreland, contradicting suggestions that maximum Variscan shortening had negligible impact on Carboniferous basins in northern Britain. Local inversion structures appear to have strongly influenced local late Westphalian–Stephanian depocentres. In this respect, the Variscan foreland at Canonbie may have resembled a ‘broken’ foreland system. Variations in crustal rheology, fault strength and orientation, and mid-crustal detachments are suggested to have played important roles in determining strain localization and the nature of Westphalian–Stephanian depocentres in the Canonbie Coalfield.

Abstract The Pelican Field lies in the East Shetland Basin, in Block 211/26, roughly 150 km NE of the Shetland Islands. It was discovered in 1975 by exploration well 211/26-4. Development was delayed until 1995 when economic development became feasible as a subsea tie back to the Cormorant Alpha Platform. The reservoir is the Middle Jurassic Brent Group, comprising sands deposited in a fluvio-deltaic, shallow-marine, wave-dominated system. The reservoir interval has an average thickness of around 300 ft, ranging from 220 ft on the crest to 400 ft in down-flank areas. The crest of the field lies at around 10 500 ft true vertical depth subsea. Current estimate of oil in place for the field is c. 500 MMbbl. The Pelican Field suffers from significant deterioration of reservoir properties with depth, leading to low recovery factors of 15–20%. To date, 21 production and injection wells have been drilled recovering a total of 76 MMbbl. Oil production started in 1996 and peak oil production was achieved at 50 000 bopd in the same year. Rates declined due to water-cut development in most of the wells and current production rates are around 2000 bopd.

As a consequence of 19th and 20th century coal mining, a vast quantity of subsurface data has been accumulated on Britain’s late Carboniferous Coal Measures and the basins in which they have accumulated. Here we discuss current understanding of the geological evolution of the British Isles during this period, as well as how these data can be repurposed as the UK seeks to reduce its greenhouse gas emissions. It is widely assumed that Britain’s Coal Measures accumulated during a period of tectonically quiescent, thermally induced post-rift subsidence and that the lateral extent of the Variscan foreland in southern England and Wales was restricted. Burial history curves constructed from stratigraphic successions across the UK suggest, however, that during the late Carboniferous the UK was characterised by accelerated subsidence rates as far north as northern England and Scotland, beyond any perceivable flexure-induced foredeep. On local scales, seismic and borehole-based mapping shows that many individual depocentres were strongly influenced by syn-depositional faulting, folding and positive inversion. This influence is illustrated by repeated local unconformities within the late Carboniferous succession across England and Scotland as well as variable isochore thickness trends. We propose that this succession was influenced by a combination of both local tectonic controls and regional controls such as supralithospheric orogenic loading and sublithospheric dynamic loading. In this sense, we believe that the British Variscan foreland system, as the British Isles should be referred to as during the late Carboniferous, resembled a ‘broken’ foreland system such as that of Patagonia, South America. Understanding the nature of Britain’s Coal Measures has gained renewed importance given the need to reduce carbon emissions and seek alternative sources of energy. Across the UK, there are a number of active projects aiming to harness latent heat from abandoned underground coalmines. In addition, coupled CO2 sequestration and enhanced coal bed methane recovery offers a further, if riskier, low carbon subsurface energy prospect. To aid deep geothermal exploration, subsurface data from northern England is being compiled in order to construct regional 3D geothermal models. Our models highlight hot areas within the subsurface more realistically than equivalent maps based on contouring around individual borehole temperature measurements that are skewed by sparsely distributed data points and, potentially, inaccurate measurements. Deep heat-producing granite bodies and the variable thickness of thermally resistive rock units, such as the Pennine Coal Measures Group, are highlighted as dominant controls on the distribution of deep geothermal energy in northern England.

The Alma Field (formerly Argyll and then Ardmore) is located within Blocks 30/24 and 30/25 on the western margin of the Central Graben. Hamilton drilled the first discovery well 30/24-1 in 1969 and the field, named 'Argyll', became the first UK offshore oilfield when production commenced in 1975. Oil was produced from the Devonian Buchan Formation, Permian Rotliegend and Zechstein groups, and Jurassic Fulmar Formation from 1976 until 1992, when the field was abandoned for economic reasons. In 2002, Tuscan Energy and Acorn Oil & Gas redeveloped the field and renamed it as 'Ardmore'. A further 5 MMbbl were produced until 2005, when the field was again abandoned due to commercial considerations. In 2011, EnQuest was awarded the licence to redevelop the field and renamed it as 'Alma'. The field came on stream in October 2015 and has produced oil at an average c. 6000 bopd since start-up. Total ultimate recovery was expected to be about 100 MMbbl. As of end 2005, the field had produced 72.6 MMbbl as Argyll and 5 MMbbl as Ardmore. A further 4.3 MMbbl has been produced from the Alma Field to September 2017 (which includes about 0.5 MMbbl from a long-reach well drilled into the Duncan/Galia Field immediately west of Alma). In January 2020 EnQuest announced that the Alma Field would cease production early. The total production from the three phases of field development will be about 85 MMbbl of oil.

A review is presented of the progress of exploration for, and development of, gas fields in the Carboniferous of the UK Southern North Sea in the period since the first significant discoveries were made in 1984. The outcomes of such exploration have generally failed to live up to high initial expectations and exploration targeting of the Carboniferous has declin ed, the objective having come to be seen by many as difficult and risky. This review includes a summary of the published consensus regarding elements of the Carboniferous petroleum system and discusses the reasons for the decline in interest, which encompass geological complexity, interpretational and operational problems and other non-technical factors. Five areas of Carboniferous petroleum geology are identified in which the currently accepted status quo is open to challenge. More detailed discussion of these leads to the following general conclusions: (1) the distribution of source rocks and their maturation history remains poorly understood, largely as a result of the hitherto unquestioned acceptance that Westphalian coals have acted as the dominant gas source; (2) in many early wells the combination of formation damage and shortcomings in petrophysical data acquisition and evaluation has resulted in a failure to identify potential pay in low permeability formations and an overemphasis on the importance of channel sand bodies as reservoir objectives; (3) the controls on seal capacity and integrity within the Carboniferous succession have been little studied and, as a result, an unduly pessimistic view of intra-Carboniferous sealing potential has prevailed; (4) the distribution of sub-basin depocentres, and thus of basinal shale source rocks and potential hydrocarbon migration paths, remains poorly understood; and (5) conceptual models of the large-scale tectonic history of the Carboniferous basin complex have failed to evolve from early and simplistic rift and sag models, which do not adequately explain the observed distribution of stratigraphic thicknesses and are inconsistent with some published burial histories.

New data from three shale gas exploration wells in the Bowland Basin of NW England contribute to the understanding of the stratigraphy, tectonic history and unconventional hydrocarbon resource potential of Lower Carboniferous strata. Three main prospective shales dominate the identified unconventional reservoirs: the Upper Bowland and Lower Bowland shales and the Hodder Mudstone, which are recognized by their distinctive lithology, corresponding log signatures and key zonal ammonoids. With a combined thickness of over 5000 ft ( c. 1500 m), this sequence of shales is one of thickest known potential self-sourced, unconventional hydrocarbon resources. The strata are organic rich with total organic carbon (TOC) values of between 1 and 7%, with an average of 2.65%, and organic maturity that ranges from the upper oil window (pyrolysis T max c . 450°C) in the higher part of the section to dry gas (R o = 2.4%; pyrolysis T max >470°C) in the Lower Bowland Shale. The sequence is strongly heterolithic, and up to 60% free gas is stored in thinly bedded carbonate and clastic silty turbidites. Adsorbed gas is concentrated in more organic-rich, hemipelagic shales which are distributed throughout the sequence. Near maximum burial temperatures of c. 130°C are inferred from vitrinite reflectance (R o ) and are consistent with fluid-inclusion microthermometry of carbonate-filled fractures. This indicates oil generation in the Late Carboniferous, prior to Variscan uplift. Renewed subsidence through the early Mesozoic resulted in increased maturity and gas generation. In the Bowland Shale the gas per unit volume of rock ranges from about 0.6 to 1.5 Bcf (billion cubic ft) per metre per square mile. The thick interval of gas-charged strata provides the opportunity to exploit these major hydrocarbon resources by using stacked multilateral wells from a common, strategically located and environmentally optimized surface pad.

A series of well logs and cores penetrating the predominantly aeolian Auk Formation, Permian Rotliegend Group, Central North Sea, UK, have been evaluated to determine the morphology and style of migratory behaviour of the original dune bedforms, the overall depositional environment, and to assess implications for reservoir heterogeneity. This has been achieved by detailed facies analysis of subsurface datasets and by comparison of the observed sedimentary styles of accumulation to analogous modern aeolian dune fields. Aeolian bedform type, morphology, detailed migratory behaviour, and the nature of the accumulation surface have been interpreted. Analysis of the facies architecture of preserved cross-bedded sets and cosets indicates accumulation on a dry substrate via the migration and climb of large linear bedforms that possessed low-angle inclined lower plinths, up to 15 m thick. Dune plinth elements are dominated by wind-ripple and reworked wind-ripple strata, and were preferentially preserved as successive bedforms migrated over one another at low angles. Packages of grainflow-dominated strata representative of accumulation on the higher part of the bedform lee slope are less common and tend to be preserved mostly in the upper parts of large cosets of strata (similar to 30 m thick). Large linear bedforms were separated by dry interdune areas. Although the primary direction of sand transport was along the elongated crests of the bedforms, a secondary component of transverse motion enabled the lateral migration and preferential preservation of lee-slope deposits that arose from a minor oblique component of bedform migration. In places, the architecture records the preservation of small barchanoid dune deposits, either within interdune depressions or superimposed on the lower flanks of the large linear bedforms. The preserved aeolian facies types exert a primary control on reservoir quality. Few previous studies have documented linear dunes in ancient successions; these findings represent a valuable case example.

The report revises and expands upon the 1976 and 1978 publications for the Dinantian and Silesian, respectively, combining them into a single account of British and Irish Carboniferous stratigraphy. The need to update the two Special Reports reflects the considerable advances in Carboniferous geology over the last 30 years. The report covers developments in international chronostratigraphy and incorporates wholesale reassessments of British lithostratigraphy. A huge volume of biostratigraphical information has been published over recent decades and the report summarizes the key information.Carboniferous rocks have long been of economic importance, but it is the search for hydrocarbons, in its infancy at the time of the previous reports, which has greatly increased our understanding of Carboniferous successions offshore and at depth, particularly in southern and eastern England.

The report revises and expands upon the 1976 and 1978 publications for the Dinantian and Silesian, respectively, combining them into a single account of British and Irish Carboniferous stratigraphy. The need to update the two Special Reports reflects the considerable advances in Carboniferous geology over the last 30 years. The report covers developments in international chronostratigraphy and incorporates wholesale reassessments of British lithostratigraphy. A huge volume of biostratigraphical information has been published over recent decades and the report summarizes the key information.Carboniferous rocks have long been of economic importance, but it is the search for hydrocarbons, in its infancy at the time of the previous reports, which has greatly increased our understanding of Carboniferous successions offshore and at depth, particularly in southern and eastern England.

The report revises and expands upon the 1976 and 1978 publications for the Dinantian and Silesian, respectively, combining them into a single account of British and Irish Carboniferous stratigraphy. The need to update the two Special Reports reflects the considerable advances in Carboniferous geology over the last 30 years. The report covers developments in international chronostratigraphy and incorporates wholesale reassessments of British lithostratigraphy. A huge volume of biostratigraphical information has been published over recent decades and the report summarizes the key information.Carboniferous rocks have long been of economic importance, but it is the search for hydrocarbons, in its infancy at the time of the previous reports, which has greatly increased our understanding of Carboniferous successions offshore and at depth, particularly in southern and eastern England.

The report revises and expands upon the 1976 and 1978 publications for the Dinantian and Silesian, respectively, combining them into a single account of British and Irish Carboniferous stratigraphy. The need to update the two Special Reports reflects the considerable advances in Carboniferous geology over the last 30 years. The report covers developments in international chronostratigraphy and incorporates wholesale reassessments of British lithostratigraphy. A huge volume of biostratigraphical information has been published over recent decades and the report summarizes the key information.Carboniferous rocks have long been of economic importance, but it is the search for hydrocarbons, in its infancy at the time of the previous reports, which has greatly increased our understanding of Carboniferous successions offshore and at depth, particularly in southern and eastern England.

The Miqrat Formation (middle Cambrian, Haima Supergroup) of North Oman is an identified deep, tight (low permeability) gas reservoir that was deposited in an arid to semi-arid continental setting, consequently it is biostratigraphically barren. It is dominated by finely interbedded, red-brown shales intercalated with argillaceous and feldspathic/micaceous very fine- to fine-grained sandstones and siltstones. These were deposited in alluvial and playa/lacustrine to sabkha environments with minor aeolian intervals. The formation offers a variety of geological and well engineering challenges, largely related to its depositional setting and age. Facies relationships were controlled by a spectrum of sedimentary processes operating at different scales. Important constraints, at a basin scale, are wet/dry climate cycles within a framework of varying sedimentation rates and accommodation space. Developing accurate subsurface depositional models is hampered by poor seismic resolution and a lack of reliable correlation events to constrain palaeogeographic reconstructions and tie these to field-scale depositional models. With limited well control, absence of biostratigraphic markers and non-unique wireline log characteristics, correlation of similar-appearing sheetflood sands may be erroneous as these may correlate with time equivalent flood margin or muddy playa deposits. The identification of correlatable markers is, therefore, significant. Pilot studies applying chemostratigraphy suggest that this technique may prove to be a useful tool for subdividing the Miqrat Formation. Regional and in-field Miqrat well data, integrated with outcrop studies, provide the main input into geological models and the basis for play maps and static reservoir modelling. However, many challenges exist, especially for early appraisal activities. These include reservoir productivity identification and “sweet-spotting”; assessment of gas mobility during drilling; petrophysical evaluation (saturation/mobile phase identification). Additionally, the role of fractures needs to be understood in achieving commercial rates, whilst fraccing and testing require innovative solutions to provide optimal stimulation and reservoir assessment, respectively. Project attractiveness may rely on improvements in seismic imaging, improved play models, better prediction of reservoir quality/fracture networks, together with a better understanding of charge history and improved offtake rates.

I011 The Ketch Field – From Core Description to Development Drilling in 1 Year I. Cloke* (Tullow Oil Plc) K. Beattie (Tullow Oil Plc) J. Boucher (Tullow Oil Plc) A. Kirsch (Tullow Oil Plc) B. Besly (Ichron Ltd) M. Bentley (TRACS International) & the S&K Development Team SUMMARY EAGE 69 th Conference & Exhibition — London UK 11 - 14 June 2007 The Ketch gas Field was discovered by Shell/Esso in 1984 with first production in 1999. The field is reservoired in the Carboniferous age Ketch and Cleaver Formations of the Westphalian C/D groups and sealed by the Silverpit shales

Chemostratigraphy has been applied to onshore Duckmantian/Stephanian successions encountered in outcrop and penetrated by two boreholes from the West Midlands (U.K.). These successions represent the onshore equivalents of the `Barren Red Measures' which are important hydrocarbon-bearing sequences in the Southern North Sea. Much is known about the onshore successions in terms of sedimentology, mineralogy and provenance and thus they provide the ideal test for the validity of chemostratigraphy as a stratigraphic tool. Reliable inorganic geochemical data have been acquired from geochemical analyses of core, sidewall core and cuttings samples, with 19 elements being determined. Stratigraphic variations in elemental concentrations are compared with known variations in the mineralogical data. The established lithostratigraphic units of the Duckmantian/Stephanian intervals can also be recognized from the geochemical data and by using these data can be subdivided further. This results in an independent chemostratigraphic correlation being established for the two boreholes, which has been assessed statistically by discriminant function analysis. From the geochemical and mineralogical data, distinct changes in provenance are identified within the Upper Carboniferous successions. The sediments of the Coal Measures were derived from a north westerly and westerly source (?Caledonian), whereas the Etruria Formation sediments came from the Wales Brabant Massif, the sediments having mixed Caledonian and Cadomian characteristics. Eventually these sediments were replaced by sediments from a southern Hercynian source (Halesowen and Salop Formations).

Interpretation of commercial seismic reflection data from the west of Banbury, south-central England, has provided information on the late Carboniferous structural evolution of the Oxfordshire Syncline. End-Carboniferous structures are preserved in the footwall of the main basin-bounding faults of the Permo-Triassic Worcester Graben. Restoration of major pre-Triassic normal faults has allowed reconstruction of earlier compressional structures. In contrast to the WNW-ESE trend of structures to the south, in the Variscan foreland fold-thrust bell, major faults and folds in Oxfordshire are approximately N-S trending. The structures seen to deform Westphalian D-Stephanian-aged sequences on the seismic profiles are a combination of steep (30-40 degrees dip) Fault-bounded uplifts and less dominant thin-skinned tectonics, apparently showing transport to the east. In the south, some thin-skinned structures are present, and in the north cover sequences are uplifted in a narrow zone of deformation on reactivated basement Faults. The amount of shortening also changes along strike, from c. 1600 m in the south to c. 950 m in the north. Reactivation of major basement faults with differential basement shortening accounts for the main structures. Localized detachment in the young and weak sediments close to the palaeo-land surface explains the thin-skinned structures seen on some of the seismic lines. These end-Carboniferous structures in Oxfordshire and structures in the Malvern-Abberley Hills represent the structural limits of a large N-S-striking intra-Stephanian uplift in the late Variscan foreland. It is suggested that the structures formed during a discrete phase of intra-Stephanian E-W-directed regional shortening prior to the final phase of movement in the Variscan belt.