Sedimentology and diagenesis of the Jurassic and Cretaceous of the North Celtic Sea and Fastnet Basins

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Abstract
The North Celtic Sea and Fastnet Basins both contain a relatively complete Jurassic and Cretaceous section. These sediments were examined in order to gain an insight into the nature and extent of sedimentological and diagenetic processes which affected the basins. Significantly variable depositional environments existed in the basins through time. The Early and Middle Jurassic were dominated by low-energy marine conditions, below storm wave-base, with higher-energy storm events locally depositing coarse-grained sediments. The Upper Jurassic succession contains sediments deposited in marine, marginal marine and continental environments, with significant quantities of coarse-grained material. Lower Cretaceous sediments were initially characteristic of marginal marine to continental conditions but increasing marine influence resulted in deposition of fully marine sediments. Three sequences were examined petrographically, concentrating on those considered to have potential for reservoir development. In general, porosity and permeability appear to be principally related to primary depositional factors, or to the control of these primary factors on the subsequent diagenetic evolution. Quartz cementation appears to have been the most destructive in terms of porosity occlusion, but locally, calcite cements are significant. Significant reservoir potential is considered to remain in the three sequences examined in the area, with good sandstone development postulated in undrilled areas.