Sedimentary facies and the context of dolocrete in the Lower Triassic Sherwood Sandstone group: Corrib Field west of Ireland

The Lower Triassic Sherwood Sandstone Group (SSG) reservoir in the Corrib Field is predominantly fine- to medium-grained fluvial sandstone with minor aeolian and playa sediments that formed under semi-arid to arid conditions. There is no evidence of roots, burrows or organic-rich soils in the SSG suggesting an environment with sparse vegetation in the catchment and sedimentation from a predominantly braided sand-dominated river. Dolomite is the main porosity-occluding cement (< 25 vol.%) in the Sherwood Sandstone Group reservoir in the Corrib Gas Field, offshore west of Ireland. Dolomite is demonstrably early diagenetic since it is non-ferroan, brightly luminescent, occurs in rocks with high (uncompacted) intergranular volumes and grew before all burial diagenetic cements. Conversely, calcrete is the dominant early diagenetic cement in well 27/5-1 in the SSG, ∼50 km south of Corrib. Dolocrete and/or calcrete are represented throughout the entire SSG interval and typically form under conditions of very low sediment accumulation rates. Dolocrete occurs equally in all facies showing that it is not a result of depositional processes. Core goniometry data in the Corrib Field revealed that the general palaeoflow direction was from south to north. During progressive evaporation of river- and ground-water calcite formed first, followed by dolomite further downstream with gypsum formed last if evaporation continues. This well-established pattern indicates that well 27/5-1 has calcrete and Corrib has dolocrete since they both result from evaporative concentration of ground- and river-water during flow down the very shallow Triassic palaeogradient.