Reservoir Facies Architecture in a Microtidal Barrier System Frio Formation, Texas Gulf Coast

Sandstone reservoirs deposited in microtidal barrier systems contain large oil and gas reserves in several Gulf Coast Basin plays. Three representative Frio Sandstone reservoirs in West Ranch field show that barrier-island sand bodies are complex mosaics of barrier-core, inlet-fill, flood-tidal-delta, washover-fan, barrier-flat, and shoreface facies. The proportions of these facies differ within progradational, aggradational, and transgressive barrier sand bodies. Detailed isolith and log-facies maps, based on closely spaced production wells, combined with analysis of resistivity distribution and sparse core control, provide the basis for interpretation of the 41-A, Glasscock, and Greta reservoirs.The 41-A reservoir is a progradational barrier sand body. The most important producing facies include the barrier core and crosscutting inlet fill. Permeability and distribution of irreducible water saturation reveal depositional patterns and subdivisions of the sand body into numerous facies-controlled compartments. Both original hydrocarbon saturation and irregularities in water encroachment show that the facies compartments locally affect fluid movement within the reservoir.The Greta reservoir is an aggradational barrier complex. This massive sand body consists of intermixed barrier-core and inlet-fill units. Prominent resistivity compartments are dip oriented, indicating the importance of inlet development during barrier aggradation. Despite the uniform appearance of the Greta reservoir, water encroachment has been irregular.The Glasscock reservoir is characterized by comparatively low permeability and is an atypically thin and discontinuous Frio reservoir. It is interpreted to be a transgressive barrier deposit, and it consists mainly of large washover-fan and associated barrier-flat sands. Hydrocarbon saturation, drainage, and injection response all reflect the facies geometry typical of a transgressive barrier complex.Recovery efficiency of Frio barrier-island reservoirs is high. However, projected primary and secondary recovery from the three reservoirs studied range from 56 percent of oil in place in the 41-A reservoir to 39 percent in the Glasscock sandstone. The Greta sandstone is intermediate, having a projected recovery of 42 percent of oil in place.