Print this pageAbstract: Sequence Stratigraphy and Trapping Mechanisms, Miocene Temblor Formation, Coalinga Field, California
CLARK, MICHAEL, Chevron USA Production Co, Bakersfield, CA; KARLA TUCKER, Californa State U Long Beach, Long Beach, CA; WILLIAM KEMPNER, Chevron USA Production Co, Bakersfield, CA
Temblor Formation reservoirs at Coalinga field, San Joaquin basin, CA represent an active-margin, nearshore to non-marine succession deposited during Miocene transgression. 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. Other unconformities subdivide this megasequence into four smaller sequences. At the base of the oldest sequence, braided fluvial strata onlap an alluvial valley incising middle Eocene shale source beds. Subsequent transgression deposited discontinuous estuary and tide-dominated shoreline facies in this and the overlying sequence. Laterally continuous, low-permeability shoreface sandstones compose a third sequence, and impermeable, serpentine-bearing, non-marine strata of Big Blue Formation make up the upper sequence.
Despite 850 MMBO production, the megasequence is breached along an anticlinal axis with active oil seeps that ceased flowing with field development. Despite the absence of a trap, enough oil accumulates beneath tar mats and tight outcrop to degrade to low-gravity crude produced primarily by steam injection. Biomarkers indicate Eocene derivation of the oil (Kreyenhagen Formation), even though Eocene source rocks at the surface are immature. Because recent folding of the anticline indicates field formation is also recent, and source rocks in the basin center only recently reached sufficient burial depths for oil generation, oil migration into the field is ongoing. Thus, the field represents a "kinetic accumulation", in which oil is continually replenished by new supplies migrating in from below, and oil not produced slowly migrates to surface seeps where it is lost.
AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California