Architecture and Sedimentology of the Miocene Moco T and Webster Turbidite Reservoirs, Midway-Sunset Field, California

Abstract

Abstract Thirty-five turbidite sand bodies from the Moco T and Webster reservoir zones were delineated for EOR projects in Mobil’s MOCO FEE property, south Midway-Sunset Field. The recognition of these sand bodies is based on: mappable geometries determined from wireline log correlations, log character, core facies, reservoir characteristics, and comparison to nearby age-equivalent outcrops. These turbidite sands are composed of unconsolidated arkoses of late Miocene age (“Stevens equivalent,” Monterey Formation). The sand bodies are dip oriented and parallel the northeast-dipping paleoslope. Reservoir quality in the sandstone is very good with average porosities of 33%, permeabilities of 800 to 4,000 md, and average oil saturation of 32 to 65%. The underlying Moco T Zone has 20 mappable sand bodies. Their mapped dimensions average 40 feet thick, 1,600 feet wide, and 5,000 feet long. These deposits are coarse grained, poorly sorted, amalgamated, thin and fine upwards, and contain shale intraclasts, dish structures, and slumped intervals. The sand bodies are lenticular, shale out to the east and west, and are interpreted to be channel-fill deposits. The overlying Webster Zone is divided into two units and 15 sand bodies. The Webster sands are similar in dimensions to those of the Moco T, but document a change in sedimentary character from medium-grained, sheet-like depositional lobes in the lower Webster Main to coarse-grained, lenticular channel-fill deposits in the upper Webster Intermediate. A channel/lobe transition zone occurs between the two facies. The depositional lobes average 28 feet thick, 6,000 feet wide, and 5,600 feet long; channel-fill sand bodies average 37 feet thick, 1,100 feet wide, and 4,000 feet long; and channel/lobe transition deposits average 33 feet thick, 3,200 feet wide, and 4,200 feet long. The depositional lobes contain sandstone beds with Bouma sequences and mudstone interbeds that are locally bioturbated, whereas the channel-fill units are poorly sorted, amalgamated, and contain boulders and shale intraclasts.