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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.

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