Michael S. Clark¹, Karla E. Tucker², Oscar Yepes¹, William Kempner¹
(1) Chevron USA Production Company, Bakersfield, CA
(2) California State University, Long Beach, CA

Abstract: Controls on reservoir architecture and oil source in an active-margin estuary/shoreface complex, Miocene Temblor Megasequence, San Joaquin Basin, California

Temblor Formation heavy-oil reservoirs of West Coalinga field, San Joaquin basin, CA represent an active-margin, shallow-marine succession deposited during Miocene transgression. Despite complex stratigraphic geometries and updip breaching of the reservoirs, Coalinga has produced 850 MMBL of Eocene-sourced oil from fluvial, estuary and shoreface sandstones interbedded with claystones and diatomaceous shales. Although complex reservoir architecture complicates exploitation of 57 MMBO remaining reserves, 4D-seismic and flow simulation models facilitate well placement and steam management to produce heavy-gravity oils representing most of the current 26,000 BOPD production.

Onlap surfaces representing 5-20 my of missing section bound the Temblor and Big Blue Formations to delineate an unconformity bound megasequence up to 215 meters (700 feet) thick. Outcrop, core, 3D-seismic, and microfossil data indicate other unconformities subdividing this megasequence into four depositional sequences. At the base of the megasequence, braided fluvial strata onlap an alluvial valley incising middle Eocene shale source beds. Subsequent transgression deposited discontinuous estuary and tide-dominated shoreline facies. Truncation and onlap along a transgressive surface within these facies complicate reservoir correlations. Laterally continuous, low-permeability shoreface sandstones unconformably overlie the tidal succession, and the serpentine-bearing, non-marine Big Blue Formation unconformably caps the marine facies. Although most world oil reserves represent lowstand systems tracts, most Coalinga production is from transgressive reservoirs. Thus, Coalinga is an analog for other transgressive reservoirs, like the lower Cretaceous Peace River Group of Alberta and lower Eocene of Lake Maracaibo, Venezuela.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana