Abstract: From Tar Seeps to Anticlines and Back: The Rise and Fall of the Anticlinal Theory in the California Transverse Ranges
CLARK, MICHAEL, Chevron USA Production Co, Bakersfield, CA
Exploitation of the Sespe oil fields, Ventura basin, California has moved full circle since the 1870s when prospectors drilled tar seeps seeking a few barrels of heavy crude. These five pools, discovered on the uplifted basin margin between 1877 and 1905, demonstrate a historical progression from drilling oil seeps to anticlines to fracture traps to oil migration paths. Early entrepreneurs drilled shallow wells on tar seeps near Sespe Creek to produce low-gravity crude at low rates. Later wells, higher in the mountains, away from seeps, and presumably closer to oil sources drilled deeper to find lighter-gravity crude at higher rates. Location of these "uphill" wells on anticlines supported the "anticlinal theory of oil migration" and led to drilling structural highs. Yet, subsequent drilling extended production to structural lows as well.
In reality, most of the 43 MMBO produced derives from fractured, non-marine, Sespe Formation sandstones in the upper plate of the San Cayetano thrust--a dual-permeability system producing from high-permeability fractures and low-permeability matrix. Thus, many wells deviate to maximize fracture production, and fracture stimulations enhance matrix production. These fields probably represent "kinetic accumulations" where subthrust sources supply oil to overthrust reservoirs faster than oil leaks updip to surface seeps. Because oil accumulates behind "bottlenecks" to form pools, undiscovered accumulations may exist between the field and surface seeps. Thus, the story goes full circle. Although early explorationists moved away from the seeps to anticlines in search of oil, future development may move back towards the seeps seeking kinetic accumulations.
AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California