Sedimentary and tectonic controls on Lower Carboniferous (Visean) mixed carbonate–siliciclastic deposition in NE England and the Southern North Sea: implications for reservoir architecture

Abstract

Discovery of the Breagh gas field in the Southern North Sea (SNS) has demonstrated the potential that the Lower Carboniferous (Visean, 346.7–330.9 Ma) Farne Group reservoirs have to contribute to the UK's future energy mix. New biostratigraphic correlations provide a basis to compare Asbian and Brigantian sedimentary cores from the Breagh Field and age-equivalent sediments exposed on the Northumberland Coast, which has proved critical in gaining an understanding of exploration and development opportunities. Thirteen facies associations characterize the mixed carbonate–siliciclastic system, grouped into: marine, delta front, delta shoreface, lower delta plain and upper delta plain gross depositional environments. The facies associations are interpreted as depositing in a mixed carbonate and siliciclastic fluvio-deltaic environment, and are arranged into coarsening- and cleaning-upward cycles (parasequences) bounded by flooding surfaces. Most cycles are characterized by mouth bars, distributary channels, interdistributary bays and common braided rivers, interpreted as river-dominated deltaic deposits. Some cycles include rare shoreface and tidally-influenced deposits, interpreted as river-dominated and wave- or tide-influenced deltaic deposits. The depositional processes that formed each cycle have important implications for the reservoir net/gross ratio (where this ratio indicates the proportion of sandstone beds in a cycle), thickness and lateral extent. The deltaic deposits were controlled by a combination of tectonic and eustatic (allocyclic) events and delta avulsion (autocyclic) processes, and are likely to reflect a changing tectonic regime, from extension within elongate fault-bounded basins (synrift) to passive regional thermal subsidence (post-rift). Deep incision by the Base Permian Unconformity across the Breagh Field has removed the Westphalian, Namurian and upper Visean, to leave the more prospective thicker clastic reservoirs within closure. Thematic collection: This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: https://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

Non‐palimpsested crowded Skolithos ichnofabrics in a Carboniferous tidal rhythmite: Disentangling ecological signatures from the spatio‐temporal bias of outcrop

“…The overall trace fossil assemblage of the Alston Formation (Boyd & McIlroy, 2017, 2018; Booth et al ., 2020; McLean et al ., 2020) is comparable to the recently described Phycosiphon and Rosselia ichnofacies (MacEachern & Bann, 2020). These ichnofacies have been defined to describe prodelta and delta front trace fossil assemblages and may be especially useful in subdividing Carboniferous cyclothem successions (Fielding, 2021).…”

“…The Alston Formation near Saltpan How has previously been investigated in detail from a facies model perspective by Reynolds (1992) and Booth et al . (2020), who both interpreted the presence of tidally‐influenced sedimentary facies [Reynolds’ (1992) Facies II tidal channel and Booth et al .’s (2020) Facies D1 interdistributary bay tidal flats]. However, neither noted the presence of rhythmite bundles, suggesting that this facies motif may be very spatially‐restricted and only apparent in the most recently exposed outcrop.…”

“…Diverse trace fossils have previously been documented from the Alston Formation in the Northumberland Basin, including reports of Arenicolites, Beaconites, Diplocraterion, Neoione, Macaronichnus, Palaeophycus, Planolites, Skolithos, Taenidium, Teichichnus, Thalassanoides and Zoophycos (e.g. Boyd & McIlroy, 2017, 2018; Booth et al ., 2020; McLean et al ., 2020). Despite this formation‐level ichnodiversity, the heterolithic package described here contains only a monospecific assemblage of multiple short, mud‐filled Skolithos (Fig.…”

Karstified and fractured Lower Carboniferous (Mississippian) limestones of the UK ‒ A cryptic geothermal reservoir

“…In South Wales, Southern England, North Wales and the Midlands lagoonal, marine carbonates accu mulated on the preexisting highs while the corresponding basinal areas comprise interbedded limestone turbidites and hemipelagic shales many of which are rich in organic matter. In northern England and Scotland, the depositional environ ment was typically marginal marine to paralic that produced the mixed carbonate, clastic and coals, known locally as Yor edale cycles (Tucker et al 2003;Booth et al 2020).…”

Early Carboniferous limestones of southern and central Britain: Characterisation and preliminary assessment of deep geothermal prospectivity

“…The limestones in these successions tend to be 1-5 m thick, interbedded in an upward-coarsening succession of marine mudstone and sandstone, and capped by a coal layer (Fig. 2; Kearsey et al 2019, Booth et al 2020). These limestones are formed by marine transgressions in response to glacio-eustatic sea-level changes (Davies & Fielding 2008).…”