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

Martin H. Link, Blaine R. Hall

Thirty-five turbidite sandstone bodies from the Moco T and Webster reservoir zones were delineated for enhanced oil recovery 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 arkosic late Miocene sandstones (Stevens equivalent, Monterey Formation). They were deposited normal to paleoslope and trend southwest-northeast in an intraslope basin. Reservoir quality in the sandstone is very good, with average porosities of 33% and permeabilities of 1 darcy.

The underlying Moco T zone has 20 mappable sand bodies. Their mapped dimensions average 40 ft thick, 1,600 ft wide, and 5,000 ft long. These deposits are coarse grained, poorly sorted, amalgamated, thin, and fining upward and contain shale intraclasts, dish structures, and slumped intervals. The sand bodies are lenticular, shale to the east and west, and are interpreted to be channel-fill deposits.

The overlying Webster zone is divided into 15 sand bodies. The Webster sands are similar in dimension to those of the Moco T but document a change in sedimentary character from medium-grained, sheetlike 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 channel-fill sand bodies average 37 ft thick, 1,100 ft wide, and 4,000 ft long; channel/lobe transition deposits average 33 ft thick, 3,200 ft wide, and 4,200 ft long; and depositional lobes average 28 ft thick, 6,000 ft wide, and 5,600 ft long. The channel-fill units are poorly sorted and amalgamated, and they contain boulders and shale intraclasts; the depositional lobes contain sandstone eds with Bouma sequences and mudstone interbeds that are locally bioturbated.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.