Reservoir characteristics of a braid-plain depositional system: The Upper Carboniferous Pennant Sandstone of South Wales

Braid-plain depositional systems form important hydrocarbon reservoirs in Palaeozoic, Mesozoic and Cenozoic basins throughout the world. Production from these reservoirs is frequently complex due to the nature and scales of permeability barriers inherent to braid-plain deposits. An analysis of the nature and distribution of permeability barriers from the well exposed braid-plain deposits of the Upper Carboniferous Pennant Measures of South Wales has revealed a variety of scales of heterogeneity. Potential permeability barriers include: (1) conglomerate channel lag deposits, (2) thick accumulations of plant detritus, (3) distribution of fines on foreset, set and coset boundaries, (4) laterally discontinuous mudstone bodies parallel or transverse to channel azimuth and (5) periodic basin-wide coal and seatearth development. Comparison of the Pennant outcrop with producing braid-plain reservoirs indicates that these permeability barriers are commonly encountered during production and are likely to be present (to some extent) in all potential braid-plain reservoirs. Account of heterogeneity distribution during production/development of braid-plain reservoirs will aid cost-effective production strategy.