Despite having been affected by several stages of exhumation during the Cretaceous and Cenozoic, the contemporary stress state of the East Irish Sea (EISB) is poorly characterised. As the basin is mature in terms of exploitation of hydrocarbons, future exploration beyond the conventional Sherwood Sandstone Group reservoir (Triassic) necessitates a greater understanding of the in situ stress field, while proposed natural gas storage and carbon sequestration schemes also require detailed stress field information. Using petroleum well data, the in situ stress field of the EISB has been characterised to assess the mechanical seal integrity. A strike-slip stress regime most-likely prevails in the basin, meaning the Maximum Horizontal Stress (SHmax) is the greatest of the principal stresses. Interpretation of stress orientation data suggests that SHmax is oriented 152˚ ± 12˚, consistent with mean stress orientations across the wider region associated with plate boundary forces. Some degree of structural control appears to influence the orientation of SHmax, with orientations locally aligned sub-parallel to major Permo-Triassic basin-bounding faults. Fault reactivation risk is evaluated through modelling the pore pressure increase required to induce failure on pre-existing faults. Vertical faults striking 30˚ from SHmax are optimally-oriented to become reactivated under elevated pore pressure conditions. For any project relying on an element of fault seal for the containment of buoyant fluids at the average reservoir depth of 800 m, pore pressure increase should be less than 3.3 MPa to avoid reactivating pre-existing optimally-oriented faults. Higher pressure increases would be required to initiate reactivation of faults with other orientations. Vertical faults striking perpendicular to SHmax are least likely to become reactivated, and in the absence of halite, seal integrity would instead be limited by caprock strength and capillary-entry pressure. Major faults affecting the basin have been analysed for their slip tendency (ratio of shear to normal stress), which provides an indication of their susceptibility to become reactivated. Although the analysis is limited due to lack of an accurate 3D representation of the fault network, the results suggest that many of the fault orientations observed in the EISB exhibit high slip tendencies, including N-S striking faults to the north and west of the East Deemster Fault, where the SHmax orientation is NW-SE. Faults striking perpendicular to SHmax, such as the Lagman Fault, are least likely to become reactivated due to higher normal stresses that inhibit frictional sliding, while faults striking parallel or very close to SHmax also exhibit low slip tendency as they are not subjected to significant shear stresses.
Seismic mapping of key Palaeozoic 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 major Carboniferous tilt-block is identified beneath the central part of the (Permian to Mesozoic) East Irish Sea Basin (EISB), bounded by carbonate platforms to south and north. The importance of the Bowland Shale Formation as the key source rock is reaffirmed, the Pennine Coal Measures having been eroded over wide areas as a result of Variscan inversion and erosion prior to Permian deposition. Peak generation from the Bowland source rock coincided with maximum burial of the system in late Jurassic/early Cretaceous time. A multiphase history of Variscan inversion has generated numerous structural traps whose potential remains essentially unexplored. 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. The productive oil and gas fields of the EISB evidence a working, Carboniferous-sourced petroleum system. Whilst a great deal may be known of the Triassic reservoir and seal (Meadows et al. 1997), little is known about Carboniferous and Permian petroleum systems at depth and in adjacent basins, that may offer significant additional potential. Following the Wood Review (2014), Palaeozoic plays, including that of the greater Irish Sea area were identified as priority for building regional digital datasets and stimulating exploration. In response, the 21 st Century Exploration Roadmap: Palaeozoic Project running from 2014-2016 and openly released in 2017, undertook regional scale seismic and well interpretation, source and reservoir screening studies and basin modelling. This paper provides a re-interpretation of the structural history of the greater Irish Sea, and its influence on potential Carboniferous and Permian prospectivity including the marginal basins. The Carboniferous structure and stratigraphy of the UK sector of the East Irish Sea-North Channel region has been reviewed using all available well and seismic reflection data. The project interpreted about 40,0...
This review of the article by Kenomore et al. (2017) on the total organic carbon (TOC) evaluation of the Bowland Shale Formation in the Widmerpool Gulf sub-basin (southern Pennine Basin, UK) reveals a number of flaws, rooted mostly in an inadequate appreciation of the local mid-Carboniferous stratigraphy. Kenomore et al. use the ΔLog R, the 'Passey' method after Passey et al. (1990), to evaluate the TOC content in two boreholes in the Widmerpool Gulf: Rempstone 1 and Old Dalby 1. We show here that Kenomore and co-authors used maturity data, published by Andrews (2013), from different formations to calibrate their TOC models of the Bowland Shale Formation (Late Mississippian-Early Pennsylvanian); the Morridge Formation in Rempstone 1 and the Widmerpool Formation in Old Dalby 1. We contest that this gives viable TOC estimates for the Bowland Shale Formation and that because of the location of the boreholes these TOC models are not representative over the whole of the Widmerpool Gulf. The pyrite content of the mudstones in the Widmerpool Gulf also surpasses the threshold where it becomes an influence on geophysical well logs. Aside from these stratigraphic and lithologic issues, some methodological flaws were not adequately resolved by Kenomore and co-authors. No lithological information is available for the Rock-Eval samples used for the maturity calibration, which because of the interbedded nature of the source formations, has implications for the modelling exercise. We recommend that more geochemical data from a larger array of boreholes covering a wider area, proximal and distal, of the basin are collected before any inferences on TOC are made. This is necessary in the complex Bowland Shale system where lithological changes occur on a centimetre scale and correlations between the different sub basins are not well understood.
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