A FLUID INCLUSION STUDY OF CEMENT AND VEIN MINERALS FROM THE CELTIC SEA BASINS, OFFSHORE IRELAND

Fluid inclusion petrography and microthermometry have been carried out on Jurassic sandstone cements (quartz, calcite and dolomite) and late r fracture-filling calcite(+/- quartz) veins from the North Celtic Sea and the Fastnet Basins. Fluid inclusions in the sandstone cements are small (1-8 mu m) monophase (liquid only) or liquid rich, two-phase (l iquid+vapour) aqueous inclusions; all display irregular to elliptical morphologies. The two-phase aqueous inclusions homogenise to the liqui d phase between similar to 60-122 degrees C and have salinities rangin g from similar to 7.0-16.0 wt% NaCl equivalent, indicating precipitati on of the cements by moderate salinity waters. In contrast the veins h ost both aqueous and petroleum inclusions. The aqueous inclusions are liquid rich, two-phase (liquid+vapour) inclusions (similar to 2-30 mu m long) that are typically primary and show irregular and negative cry stal shape morphologies. Their homogenisation temperatures (to liquid) range from 50-145 degrees C and salinities from 6-20 wt% NaCl equival ent, indicating broadly similar salinities coupled with a wider range of minimum trapping temperatures to those recorded from the cements. Y ellow-brown, two-phase (liquid+vapour) petroleum inclusions are hosted by vein calcite from several wells in the central North Celtic Sea Ba sin. Some of these inclusions appear to be primary and all are spatial ly associated with the aqueous inclusions. The two-phase (liquid+vapou r) petroleum inclusions homogenise, mostly to the liquid, between simi lar to 50-130 degrees C. Petroleum inclusions in the vein calcites ind icate that petroleum was transported via fractures, at a late stage, t hrough the Jurassic and Cretaceous strata of the Celtic Sea Basins, an d apparently not through the sandstone matrix. (C) 1998 Elsevier Scien ce Ltd. All rights reserved.