Sedimentology of the Cretaceous Greensand, Quadrants 48 and 49, North Celtic Sea Basin: a progradational shoreface deposit

The Aptian-Albian Greensand interval is the main reservoir unit in the North Celtic Sea Basin. The Greensand comprises three units: 1) the Lower Claystone, 2) the "A' Sand, and 3) the Gault Claystone. Sedimentological and wireline log studies of the Lower Claystone and "A' Sand units within Irish Quadrants 48 and 49 indicate an overall 35 to 50 m thick, coarsening-upward profile from inner shelf mudstones to middle and possibly upper shoreface fine-grained sandstones. The "A' Sand can be sub-divided into three coarsening-upward packages which show a progressive upward increase in higher-energy facies and thickness. Previous interpretations have suggested that the Greensand interval represents a series of tidal sand bars formed during transgression. However, the wide lateral extent and the pulsed, progradational nature of the coarsening-upward units indicates deposition in a (wave-dominated?) shoreface environment during a punctuated regression. The overlying Gault Claystone represents an abrupt return to open marine shelf conditions. -from Author