Shelf Sheet-Sand Reservoir of the Lower Cretaceous Greensand, North Celtic Sea Basin, Offshore Ireland

Abstract

Core and log data show that the marine, early to middle Albian A Sand of the Aptian to lower Cenomanian Greensand-Gault interval, North Celtic Sea Basin, offshore Ireland, was deposited as an approximately tabular sand body in shelf water depths. The A Sand is the major reservoir interval at Kinsale Head and Ballycotton gas fields. The reservoir sandstone is bioturbated, variably glauconitic, shell rich, and least muddy toward its provenance in a local area of the Irish massif and finer grained southeastward into the basin. Thickness and coarseness of the A Sand are related, in part, to distance from a narrow area of the paleoshore. Bathymetric control of sand thickness was superimposed on proximal-distal trends. Specifically, thick intervals are inferred to have be n deposited in shelf lows, and thin zones were deposited over bathymetric highs. The A Sand was not deposited as a ridge sand, and positive relief on the depositional sand body appears to have been minor. Deposition of the Greensand occurred during thermal subsidence of the North Celtic Sea Basin following Early Cretaceous rifting. Overall late Early Cretaceous to Late Cretaceous transgression was interrupted by progradation of the A Sand, probably caused by a relative drop in sea level (forced regression). The A Sand sheet consists of several units within the central depositional basin. Three slightly coarsening-upward units up to 15 m thick probably are the consequence of high-frequency fluctuations in sea level during progradation. The top of the A Sand consists of a several-m ter-thick, very glauconitic, muddy sandstone to sandy mudstone. The upper unit accumulated in deepening water following the maximum lowstand.