Abstract Unconventional hydrocarbon exploration in the UK is in its infancy and Third Energy's activities represent a major step towards unlocking the potential contained within the Carboniferous section beneath North Yorkshire. In the United States, production from unconventional petroleum systems has transformed the country's domestic energy markets. If the UK can replicate this success, it would go some way to offsetting the decline of North Sea production and thereby reduce dependence on foreign imports. At the end of 2015, no other company had undertaken any unconventional exploration in North Yorkshire and great uncertainty remained over the region's prospectivity. To rectify this dearth of information, Third Energy carried out a detailed examination of vintage data from the area and also drilled a new well to acquire a modern, unconventionally focused dataset, including approximately 800 ft of core and 4000 ft of wireline log data. Analysis of the mineralogy, maturity, kerogen type, total organic content, brittleness, porosity and permeability of the Carboniferous Bowland sequence has enabled several intervals to be high graded. Given these results, Third Energy now plan to hydraulically fracture these intervals with the intent of producing the UK's first unconventionally sourced electricity at its Knapton Generating Station.

Abstract The Vale of Pickering gas fields were discovered over a 20-year period. The development scheme was aimed to deliver 9.3 MMscfd gas to the Knapton Power Station nearby. Cumulative production is 30.3 bcf from an estimated 172 bcf gas initially in place. The gas fields comprise a series of low relief structures at depths around 5000 ft true depth subsea. The primary reservoir is Zechstein Group dolomitized and fractured carbonates of the Permian Kirkham Abbey Formation with average reservoir quality ranges of 12–13% porosity and 0.5–1.5 mD permeability. Secondary reservoirs exist in Carboniferous sandstones directly below the Base Permian Unconformity. The gas is sourced from Lower Carboniferous shales. The fields were discovered using 2D seismic data and subsequent 3D seismic data have been merged to form a 260 km 2 dataset. Zechstein production has been limited by early water breakthrough. Artificial lift is planned to enhance the gas flow rate on the Pickering Field and anticipated water influx will be re-injected. If this enhanced gas recovery scheme is successful it can be applied to the other fields. Plans to hydraulically fracture a number of zones in the Carboniferous Lower Bowland Section are in progress.

Abstract Hydrocarbon exploration in North Yorkshire began in 1937, targeting Triassic and Permian reservoirs below the surface expression of the Cleveland Anticline. D’Arcy drilled the successful well Eskdale-2, marking the first gas discovery in the Zechstein carbonates in the UK. Since then approximately 100 wells have been drilled in the basin with exploration success relatively high. Out of the 25 pure exploration wells in the region, 13 have found hydrocarbon accumulations (flowed gas) and eight of the discoveries have been developed to date. The primary reservoir is the Permian-aged Zechstein carbonate sequence and, more specifically, the Z2, Kirkham Abbey Formation (KAF), which is a tight carbonate reservoir overprinted by a high permeability fracture system. Despite considerable investment and effort over the years, the historical development story of these fields has been very much one of repeated technical and investment failure, with approximately 39 Bcf (billion cubic feet) of the mapped gas initial in-place (GIIP) of c . 326 Bcf produced to date, an estimated recovery factor of 12%. Historical production data show that all the Zechstein reservoirs have experienced early water breakthrough, leading to impaired gas rates and low recoveries. The water influx is due to a highly mobile, but finite aquifer, which under field production conditions preferentially flows through the high permeability fracture system, bypassing the gas stored in the tighter matrix. Third Energy is aiming to resolve the issue of water influx by using artificial lift to encourage the gas to flow. A trial is currently being undertaken at the Pickering gas field and, if this programme is successful, this will provide sufficient confidence for a phased redevelopment programme of surrounding fields. Whilst North Yorkshire has experienced only limited exploration and production (E&P) activity in the last decade, solving the issue of premature water influx in the KAF fields, combined with the search for unconventional resources in the Bowland section of the Mid and Lower Carboniferous strata will herald a new and exciting phase of E&P activities for this province.

Abstract Spatially and temporally variable Tournaisian to Namurian Carboniferous fluvial, fluvio-deltaic, platform carbonate and shale-dominated basin sedimentary successions up to 3.5 km thick are preserved in a complex series of basins from the Outer Moray Firth (Quadrant 14) to the Silverpit Basin (Quadrant 44). Differences in stratigraphic nomenclature in the areas surrounding the Mid North Sea High and onshore, combined with sparse biostratigraphic data, have hindered the systematic regional understanding of the timing and controls on stacked source and reservoir rock intervals. Over 125 well reinterpretations, tied to seismic interpretations, provide evidence of the inception and extent of a delta system. Regional time slices highlight a long-lived laterally equivalent basinal, mud-rich succession across Quadrants 41–44. They also show that the area from the Outer Moray Firth to the Silverpit Basin was part of the same sedimentary system up to at least Namurian times. All of this is placed within a simplified stratigraphic framework. Supplementary material: Appendix A in the Supplementary Material contains the stratigraphic intervals interpreted on each well and highlights which intervals have biostratigraphic control. Supplemental Figures 1 and 2 are larger scale versions of Figures 6 – 8 . The Supplementary Material is available at https://doi.org/10.6084/m9.figshare.c.4087046

Abstract 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 declined, 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.

Appendix A contains a tabulation by field of approximately 900 published references and other publicly accessible sources covering around 500 UK fields, both developed and undeveloped, onshore and offshore. Where possible, hyperlinks to the source document are provided. Fields in production are current to the July 2020 OGA listing of consented fields. References listed include all relevant Geological Society (GS) publications including the Petroleum Geology Conference series 1–8 (only 4–8 published by GS), and PESGB DEVEX presentations up to and including DEVEX 2019, with content from multiple additional sources. Papers published by the Society of Petroleum Engineers are not routinely listed below but can be readily searched online via www.onepetro.org .

Abstract Having discussed the broad-scale tectono-stratigraphic subdivision of the north of England Carboniferous in the previous section, we now use the megasequences and tectono-stratigraphic sequences to determine the spatial and temporal evolution of depositional systems using sequence palaeogeo- graphies. By late Devonian times, rifting had begun in northern England with sedimentation occurring in incipient half graben under an arid climate. The remnant Caledonian mountain belt to the north acted as a major sediment source (e.g. Gilligan 1920 ; Leeder 1988 ; Gawthorpe et al. 1989) and, in the study area, Caledonian structures were reactivated and also acted as local sediment sources. The northward drift of European Pangaea during the Dinantian led to a change to humid climatic conditions by the late Dinantian ( Duff, 1980 ). This, together with regional transgression, caused a change from red-bed style deposition to fluvio-deltaic deposition in the north of the area, close to the major sediment source, and predominantly carbonate depositional systems in the south of the area, particularly on footwall highs starved of clastic sediment. The development of high-frequency cyclicity in late Dinantian times (e.g. Walkden 1987; Leeder & Strudwick 1987) signifies the growing importance of glacio-eustasy as a control on stratigraphic development; a control which became dominant in the Silesian. There is general agreement that northern Britain occupied an equatorial position during the Namurian ( Scotese et al. 1979 ; Smith et al. 1981 ), and the occurrence of coal and bauxitic soil horizons in Scotland indicates a humid, tropical

Abstract Despite successful production from Carboniferous and Permian reservoirs in the Southern North Sea and onshore Netherlands and Germany, Paleozoic hydrocarbon plays across parts of NW Europe remain relatively under-explored onshore and offshore. This volume brings together new and previously unpublished knowledge about the Paleozoic plays of NW Europe. Improvements in seismic data quality and availability tied to previously unpublished well datasets form the basis for improved understanding of local to regional structural interpretations, depositional environments and basin history. New interpretations move significantly away from generalized basin development models, with improved definition of structural traps and source rock basins feeding to better constrained, locally variable burial, uplift, maturation and migration models. Particularly notable are the significant mapped extents and thickness of Paleozoic source, reservoir and seal rocks. Areas previously dismissed as regional highs and platforms are dissected by Paleozoic basins with evidence for mature source rocks into basin centres. Numerous potential Paleozoic plays or play elements result within thick organic-rich and variably mature successions. Outside or below existing Jurassic and Southern North Sea to onshore Netherlands and German Permian-Carboniferous plays, Paleozoic plays in frontier areas offer significant additional exploration opportunities.

Abstract Hydrocarbon reservoirs hosted in Permian strata were some of the first to have been discovered in Europe. With discoveries in the Zechstein carbonates of Norway in recent years, and with exploration of Zechstein prospects both onshore and offshore UK, as well as in Dutch, Danish and Norwegian offshore sectors, understanding the architecture of the Zechstein carbonates remains very relevant. Here we study outcrops of Roker Formation carbonates (Z2, Ca2) in NE England to better understand geological processes associated with deformation following evaporite dissolution, with implications for exploration and production. Collapse of Z2 Roker Formation strata in NE England, following the dissolution of c. 100 m or more of the Z1 Hartlepool Anhydrite, resulted in fundamental changes to the architecture of the succession. Complete dissolution of the anhydrite removed an effective regional seal and dramatically enhanced matrix and fracture permeability of the overlying Roker Formation. The collapsed Roker Formation can be vertically divided into three zones, based upon the degree of deformation. The lower zone and vertical collapse-breccia pipes that can extend across all zones have the highest permeabilities. The process of collapse was gradual, with local variations in the degree of brecciation. We derive a schematic sequence of collapse, recognizing the impact of mechanical barriers within the succession in retarding deformation up-section and it is this that ultimately leads to the vertical zonation. Timing of evaporite dissolution is poorly constrained: it could have occurred soon after deposition, at the end of the Permian or during Tertiary uplift. It is known that evaporite dissolution has occurred offshore, with the oil fields Auk and Argyll (UK Central North Sea) given as examples of dissolution collapse-brecciated reservoirs. Reservoir quality is typically improved, with both matrix and fracture porosity and permeability enhanced. Complete evaporite dissolution could in some cases lead to the potential breach of the seal.

Abstract The history of the European oil and gas industry reflects local and global political events, economic constraints, and the personal endeavours of individual petroleum geoscientists, as much as it does the development of technologies and the underlying geology of the region. Europe and Europeans played a disproportionately large role in the development of the modern global oil and gas industry. From at least the Iron Age until the 1850s, the use of oil in Europe was limited, and the oil was obtained almost exclusively from surface seeps and mine workings. The use of oil increased in the 1860s with the introduction of new technologies in both production and refining. Shale oil was distilled on a commercial scale in various parts of Europe in the late eighteenth century and throughout the nineteenth century but, in the second half of the nineteenth century, the mineral oils and gas produced primarily from shale and coal could no longer satisfy demand, and oil produced directly from conventional oil fields began to dominate the European market. The first commercial oil wells in Europe were manually dug in Poland in 1853, Romania in 1857, Germany in 1859 and Italy in 1860, before the gradual introduction of mechanical cable drilling rigs started in the early 1860s. In the late nineteenth century, the northern part of the Carpathian Mountains in what is now Poland and Ukraine was one of the most prolific hydrocarbon provinces in the world. The Bóbrka Field in the Carpathian foothills of Poland, discovered in 1853, is still producing and is now the oldest industrial oil field in the world. The 1914–18 and 1939–45 world wars were both major drivers in exploration for and exploitation of Europe’s oil resources and in the development of technologies to produce synthetic fuels from the liquefaction of bituminous coal and the combination of carbon monoxide and hydrogen as the Allied and Axis governments struggled to maintain adequate supplies of fuel for their war efforts. In Britain, the first ‘accidental’ discovery of gas was made in 1875 in the Weald Basin, but it was not until 1919 that Britain’s first oil field was discovered at Hardstoft, in Derbyshire, as a result of a government-funded exploration drilling campaign, triggered by the need to find indigenous supplies of oil during World War I. The period of reconstruction after World War II was also critical for the European oil and gas industry with further successful exploration for oil and gas in the East Midlands of England resulting in Britain’s first ‘oil boom’, and the discovery and development of deep gas fields in the Po Valley in northern Italy fuelling the Italian economy for the next 50 years. Drilling technologies developed during Britain’s first oil boom, together with the extrapolation of the onshore geology of the East Midlands oil fields and of the Dutch gas fields, led to the discovery of the huge oil and gas resources beneath the North Sea in the 1960s and 1970s, which enabled Britain, Norway, Denmark and The Netherlands to be largely self-sufficient in oil and gas from the late 1970s until production began to decline rapidly in the early 2000s. Today, oil and gas production in most European countries is at an historical low. Exploration for new sources of oil and gas in Europe continues, although increasingly hampered by the maturity of many of the conventional oil and gas plays, but European companies and European citizens continue to play a major role in the global oil and gas industry.

Abstract The Carboniferous basin development of northern England is illustrated in a series of regional seismic lines presented in this chapter. Each of the major syn– rift basins will be described in turn, using representative seismic lines (Fig. 10 ) that have been tied to well and outcrop control. In addition to the seismic data, depth converted geological interpretations for each of the seismic lines are presented to illustrate the development of these tectono–stratigraphic sequences across the province. Particular attention is paid to the Widmerpool Gulf because the combination of seismic quality, well penetrations and the presence of exposure of the syn–rift along–strike in Derbyshire allows us to discuss the tectono–stratigraphic sequences in detail. The depth converted geological interpretations are based on corrected sonic logs taken from nearby boreholes, where available. Elsewhere, and for the deeper Dinantian section on most lines, seismic stacking velocities have been applied. The errors inherent in this latter method could result in errors in the depth section of as much as ±10%. Typical velocities used in the depth conversion of the Eakring and Welton sections in the East Midlands and the Northumberland Trough (Kimbell et al. 1989) are shown in Table 1 .

Abstract Seismic mapping of key Paleozoic surfaces in the East Irish Sea–North Channel region has been incorporated into a review of hydrocarbon prospectivity. The major Carboniferous basinal and inversion elements are identified, allowing an assessment of the principal kitchens for hydrocarbon generation and possible migration paths. A Carboniferous tilt-block is identified beneath the central part of the (Permian–Mesozoic) East Irish Sea Basin (EISB), bounded by carbonate platforms to the south and north. The importance of the Bowland Shale Formation as the key source rock is reaffirmed, the Pennine Coal Measures having been extensively excised following Variscan inversion and pre-Permian erosion. Peak generation from the Bowland source coincided with maximum burial of the system in late Jurassic–early Cretaceous time. Multiphase Variscan inversion generated numerous structural traps whose potential remains underexplored. Leakage of hydrocarbons from these into the overlying Triassic Ormskirk Sandstone reservoirs is likely to have occurred on a number of occasions, but currently unknown is how much resource remains in place below the Base Permian Unconformity. Poor permeability in the Pennsylvanian strata beneath the Triassic fields is a significant risk; the same may not be true in the less deeply buried marginal areas of the EISB, where additional potential plays are present in Mississippian carbonate platforms and latest Pennsylvanian clastic sedimentary rocks. Outside the EISB, the North Channel, Solway and Peel basins also contain Devonian and/or Carboniferous rocks. There have, however, been no discoveries, largely a consequence of the absence of a high-quality source rock and a regional seal comparable to the Mercia Mudstone Group and Permian evaporites of the Cumbrian Coast Group in the EISB.