Diagenesis and reservoir quality of the Sherwood Sandstone (Triassic), Corrib Field, Slyne Basin, west of Ireland

The Triassic Sherwood Sandstone in the Corrib Field, Slyne Basin west of Ireland on the European continental margin, is a dry gas reservoir with a Mercia mudstone top-seal. Petrographic analysis combined with X-ray diffraction, stable isotope, fluid inclusion and core analysis have been used to assess: timing of growth and origin of mineral cements, whether sandstone diagenesis involved mass flux, the controls on reservoir quality and how reservoir quality is likely to vary in more deeply buried sections. Depositional and early diagenetic characteristics of the Sherwood in Corrib are typical of a semi-arid fluvial environment, containing groundwater of meteoric origin. Early diagenesis included the development of copious dolomite cement, in the form of dolocrete, as well as the formation of abundant clay while less volumetrically important, grain-coating haematite and K-feldspar cement also grew. Burial diagenesis witnessed the initial growth of minor chlorite and albite. Quartz overgrowths and ankerite followed and are the most important burial cements growing over a temperature interval between 100 and 1165 degreesC. Albite commenced growth at about 90 degreesC, quartz cement at 100 degreesC and ankerite at 110 degreesC. These cements reached the zenith of their development at 105 - 110 degreesC for albite, 125 - 130 degreesC for quartz and 135 - 145 degreesC for ankerite. Siderite and arrhydrite are relatively minor, late-stage cements. The formation water has been consistently highly saline during burial and, in terms of mineral cement-derived oxygen stable isotope values, is likely to be a diagenetically evolved version of the initial depositional water suggesting longterm stagnation of the pore-fluids. The diagenetic reactions that formed burial cements all seem to be essentially isochemical including the ankerite that has carbon isotope characteristics of the cannibalisation of dolocrete. Reservoir quality is mainly controlled by early diagenetic dolomite and clay although grain size is also important because only sandstones with > 200 mum grain size have > 50 md permeability. Both shallower and deeper sections than Corrib may have similar porosity and permeability since temperature-dependent diagenetic controls on reservoir quality, e.g. quartz and illite cement, are peripheral. (C) 2004 Elsevier Ltd. All rights reserved.