Abstract: Diagenetic Modeling as an Exploration/Exploitation Tool: The Search for an Elusive Unconformity in the North Celtic Sea

S. A. Stonecipher, J. M. Spaw, U. Hammes

The "A" Sand interval of the Albian-age Greensand, North Celtic Sea, is the main reservoir at Kinsale Head and Ballycotton gas fields. The "A" Sand consists of stacked coarsening-upward sequences of poorly sorted, fine-to very fine-grained, bioturbated, glauconitic quartz sands topped by sandy shell lags. These sequences are in turn overlain by a glauconite-rich transgressive sand.

Petrographic evidence suggests that the "A" Sand was deposited as a broad sand ridge. Increased winnowing at the top of the ridge resulted in the observed concentration of bioclasts and reduction in detrital clay. In equivalent intervals, porosity and permeability vary across the study area by up to a factor of 1.5 for porosity and up to two orders of magnitude for permeability. Mosaic calcite cements and authigenic, coarsely vermiform kaolinite associated with altered micas are common in wells which exhibit good reservoir quality, but are absent in poor wells. It appears that the crest of the ridge was also a topographic high resulting in subaerial exposure and introduction of meteoric water during a drop in sea level. The meteoric water not only enhanced porosity by dissolving react ve micas and feldspars, but also caused dissolution of unstable fossil debris and precipitation of early formed, compaction-and clay precipitation-preventing carbonate cements. The unconformity does not show clearly in conventional sedimentologic, paleontologic, or seismic data, however, if petrographic traits such as the presence of kaolinite and the cathodoluminescent character of the mosaic calcite are mapped, the regional extent of the unconformity can be delineated.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994