Exploration and development in the Carboniferous of the Southern North Sea: a 30-year retrospective

Abstract

A review is made 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. The 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 very 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 subbasin depocentres, and thus of basinal shale source rocks and potential hydrocarbon migration paths, remains poorly understood; 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. __________ Introduction.When exploration moved offshore into the southern North Sea in the early 1960s the impetus was provided by the discovery of Carboniferous-sourced gas in the lower Permian aeolian sandstone Gawthorpe 2003; Netherlands -Geluk et al. 2007; van Buggenum & den Hartog Jager 2007;Kombrink 2008). This major basin fill, deposited over a period of c. 60 ma between approximately 360 and 300 ma, comprises three stratigraphic elements,: a) the infill in early Carboniferous times of a series of major rift basins initiated during the late Devonian; b) a thick and broad regional drape, generally interpreted as a post-rift sag succession, deposited in mid Carboniferous times; and c) a dominantly red-bed succession associated with the early stages of Variscan basin inversion.In this paper the footwall highs of the rift basins are generally referred to as "blocks" and the rift depocentres as "basins". This is in keeping with a widely used informa...

Reservoir Quality Evolution of Upper Carboniferous (Westphalian) Tight Gas Sandstones, Lower Saxony Basin, Nw Germany

“…In addition to the Upper Carboniferous, potential plays may be present in the underlying Visean to Namurian succession which is known to consist of fluvio-deltaic deposits including channel sandstones in the North Sea area (Monaghan et al, 2017;Besly, 2018). With potential source rocks consisting of Visean coals and Visean-Namurian mudstones, stacked sandstone reservoir rocks, and both internal seals and the regional-scale Zechstein salt seal, the Lower Carboniferous petroleum system may be comparable to that in the Upper Carboniferous (Monaghan et al, 2017).…”

New insights on subsurface energy resources in the Southern North Sea Basin area

“…The Carboniferous was one of the main targets in the early days of hydrocarbon exploration in the UK and the Netherlands onshore. In the UK, this was based on the presence of prolific Carboniferous reservoirs in the Midlands and shale oil in Scotland (Besly 2018). Therefore, it is not surprising that the very first well drilled in the UK offshore (38/29-1, 1965) targeted Carboniferous rocks of the Mid North Sea High.…”

“…Therefore, it is not surprising that the very first well drilled in the UK offshore (38/29-1, 1965) targeted Carboniferous rocks of the Mid North Sea High. However, this well proved disappointing, together with a few more dry wells drilled at similar targets, and the Carboniferous was soon concluded to be unprospective for hydrocarbon exploration during this phase (Besly 2018). In the Netherlands, the discovery of gas in the Coevorden Field (1948) triggered exploration in the NE of the country, leading to a cluster of additional discoveries around that field.…”

Paleozoic plays of NW Europe: an introduction

“…c. 3400 bcm at 2005 from fields within the Anglo-Dutch and North German basins (Breunese et al 2010), or are estimated, e.g. estimated recoverable volume of 102 bcm (3.6 tcf 1 ) of gas in Carboniferous SNS gas fields in the UK sector at 2015 (Besly 2018). The discovery and development of fields such as Breagh with its lower Carboniferous reservoir and the Cygnus (Catto et al 2017) Rotliegend Group, Leman Sandstone reservoir have extended beyond the margins of the established plays ( Fig.…”

“…As a result, Paleozoic plays can be perceived as complex and risky, with expert knowledge built up through years of experience, and collaboration being particularly important. Historically, the loss of knowledge from company reorganizations related to fluctuations in the oil price has significantly hampered wider understanding of the plays (Besly 2018). Various leading explorationists from industry and academia have suggested that a paradigm shift is required in knowledge and play concepts to open up major new Paleozoic plays and reduce perceived risk.…”