Sedimentological control on the reservoir and caprock properties of a bleached palaeoreservoir in the Entrada Formation at Salt Wash Graben, Green River, Utah

The stratigraphy and sedimentology of the Entrada Formation are central to understanding the origin of the bleached palaeoreservoir exposed at Salt Wash Graben. Aeolian sandstones of the Slick Rock Member formed high-permeability carrier beds which distributed reducing fluids laterally within the formation. The overlying Earthy Member is a massively-bedded succession of low permeability mudflat/sabkha deposits which acted as a caprock: albeit one that formed an imperfect seal and allowed the upward diffusion of reducing fluids for a distance of up to 4 m above the top of the reservoir. The Salt Wash Graben is located on the crest of the Green River Anticline and was filled by buoyant fluids that may have migrated updip along this north plunging fold; alternatively these fluids may have entered via the northern fault of the Salt Wash Graben. The role of this structure in forming an updip seal during the filling of the reservoir is uncertain. The reservoir was probably filled in the early Cretaceous at a depth of around 1 km or less, during the subsidence phase of the Paradox Basin. The fluids are likely to have been an admixture of H2S, CO2 and CH4. The reservoir was later breached by NNW-trending fractures related to extension across the crest of the Green River Anticline.