SHAREGHI, E. ALLAN, JOAN R. BARMINSKI, and MICHAEL R. BRICKEY, Minerals Management Service, Camarillo, CA, JEFF D. MILTON, Shell Western E&P, Inc., Bakersfield, CA, and TERRY D. ADCOCK, Unocal Corporation, Santa Fe Springs, CA
Beta field (discovered in 1976 on Federal Leases OCS-P 0296, 0300, 0301, and 0306) is located in the offshore extension of the Los Angeles basin in water depths of up to 1000 ft. Stratigraphy at Beta consists of an assemblage of deep-water sands, shales, and silts presumably sourced from terrigenous rivers and deposited through a submarine fan and channel complex. The deeper middle Miocene section, deposited unconformably on the eroded surface of the Catalina Schist, averages 5400 ft thick and possesses a high percentage of turbidite sands. The shallower Pliocene section consists almost entirely of shale, which acts as a top seal for the Delmontian accumulations.
Forming approximately 2000 ft of fault closure, the right-lateral, northwest-trending Palos Verdes fault provides the trap for upturned Miocene turbidite sands. An average of 400 ft of net oil pay has accumulated at the top of Delmontian section in the east hanging wall. Productive depths range between 2700 and 4700 ft subsea.
Regional tectonics associated with convergence of the North American and Pacific crustal plates has caused normal and reverse faulting on both large and small scales. From reflection seismic data, the Palos Verdes fault is recognized as a reverse fault with over 2000 ft of vertical separation on basement. The shallow portion has been recurved past vertical by folding within the sedimentary section to form a typical "palm tree structure." On a smaller scale, over 45 other significant faults have been identified in the field. Three-dimensional seismic data suggests that a series of larger fractures generates successively smaller subsidiary splinter faults offset at 30-45 degrees . Splinter faults then originate their own smaller splinters at similar offset angles. Displacements along th se faults tend to diminish with distance from the originating flexure.
AAPG Search and
Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach,
California, May 5-7, 1993.