An overlooked play? Structure, stratigraphy and hydrocarbon prospectivity of the Carboniferous in the East Irish Sea–North Channel basin complex

Abstract

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...

Paleozoic plays of NW Europe: an introduction

“…Paleozoic-sourced potential trapped within inversion structures is highlighted by a number of papers. In the Irish Sea, Pharaoh et al (2018) document Variscan inversion structures superimposed on Carboniferous fault blocks and highlight potential prospectivity within the deeply buried Carboniferous-Permian succession and around the margins of the oil-and gas-producing East Irish Sea Basin. Butler (2018) documents a regional seismostratigraphic framework of central-southern England that also includes a significant history of inversion and unconformity development within deeply buried sequences, including Variscan inversion over the Midlands Microcraton.…”

“…Carboniferous source rocks and Carboniferous, Permian and Triassic reservoirs and seals have formed a highly productive gas province in the East Irish Sea, the Southern North Sea (SNS) and onshore into the Netherlands and Germany since the 1960s (e.g. Meadows et al 1997;Glennie & Underhill 1998;Fraser & Gawthorpe 2003;Underhill 2003;Cameron et al 2005;Breunese et al 2010;Gast et al 2010;Kombrink et al 2010;Pletsch et al 2010;Pharaoh et al 2018;Figs 1 & 2). Substantial quantities of gas have been produced, e.g.…”

Impact of in situ stress and fault reactivation on seal integrity in the East Irish Sea Basin, UK

“…Hydrocarbons were generated principally by organic-rich marine mudstones of Carboniferous age (Cowan et al 1999;Jackson et al 1995;Pharaoh et al 2016). The Carboniferous strata were deposited in basins associated with NNW-SSE extension, later folded along reactivated WSW-ENE Caledonian structures during the Variscan Orogeny.…”

“…Production to date has been exclusively from the Triassic, with a Sherwood Sandstone Group reservoir capped by the Mercia Mudstone Group (Figure 2). Aside from untested structures associated with this conventional petroleum system, future exploration might focus on prospective Palaeozoic targets such as the potential Carboniferous plays recently identified by Pharaoh et al (2016;in press). The EISB is also of interest in terms of its potential for carbon capture and storage (Armitage et al 2013;Lewis et al 2009), whereby emissions are captured from anthropogenic sources such as power plants, and injected to subsurface reservoirs for long-term storage.…”

The Môn–Deemster–Ribblesdale fold–thrust belt, central UK: a concealed Variscan inversion belt located on weak Caledonian crust

“…Continuity of the fold belt (and ancestral basin) into the offshore of the Irish Sea has been postulated on a speculative basis by numerous authors (e.g. Jackson et al 1995, Corfield et al 1996, Woodcock & Strachan 2012 but was first convincingly demonstrated by Pharaoh et al (2016bPharaoh et al ( , 2018 during the course of a review of Palaeozoic prospectivity in the Irish Sea for the 21 st C Exploration Roadmap Project (for a DECC/OGA/industry consortium). The location of exploration wells suggests that the prospectivity of the concealed fold belt has nevertheless long been recognised by the hydrocarbon industry.…”