Controls on reservoir quality in the Triassic Sherwood Sandstone of the Irish Sea

Abstract

The Lower Triassic Sherwood Sandstone Group constitutes the principal reservoir target in the East Irish Sea Basin. Interpretation of sedimentary facies allows recognition of major low sinuosity braided fluvial channel, minor ephemeral fluvial channel, aeolian dune and sandsheet, unconfined fluvial sheetflood and playa lake deposits that are discussed in terms of facies associations. Reservoir quality is highly variable, with porosity and permeability data revealing significant differences between fluvial, sheetflood and aeolian facies associations. The latter, in particular, preserve very high reservoir quality reflecting an open grain fabric and paucity of blocky cements. Variations in porosity between the facies associations relate primarily to the distribution of quartz cement which is significantly more abundant in fluvial channel sandstones than other facies. Authigenic illite, while severely limiting permeabilities in some reservoir zones, is not facies specific and does not contribute to the observed variations. It is suggested that the disparity in the abundance of quartz cement between facies associations relates to two distinct grain types that correlate broadly with fluvial and aeolian associations, respectively, and probably derive from different provenances. The evolution of the basin, in controlling the distribution of the major facies, influenced the distribution of the two grain populations and, thus, also the reservoir quality of the sandstones.