Sand-rich Submarine Fans, Mio-Pliocene of Santa Monica Basin Offshore California: Untapped Exploration Targets

Jeffrey A. May and Kenneth J. McMillen

Santa Monica Basin lies directly west of Los Angeles Basin, one of the world's most prolific oil provinces. Published literature suggests that Santa Monica Basin was starved of coarse clastics during the late Miocene through Pliocene. However, seismic sequence stratigraphy indicates that deposition of sand-rich fans alternated with mixed-load systems throughout The Delmontian and Repettian stages. Seismic sequences and facies are calibrated to seismic and well data from Beta Oil Field, in San Pedro Basin to the south.

Eustasy evidently played a dominant role in controlling sedimentation. Variations in fan lithology, thickness, and basinward extent correspond to worldwide changes in sea level. Regional erosion surfaces apparently signify drops in sea level. Overlying thick seismic packages display hummocky to chaotic seismic facies separated by high- to low-amplitude continuous reflections. These configurations are interpreted as inner- to mid-fan channels separated by overbank deposits in sand-rich lowstand fans. High-amplitude basinwide reflections bound the tops of the sand-rich intervals, and likely represent condensed sections formed during sea-level rises. Thin seismic intervals above the condensed sections display downlap, and are interpeted as interbedded sandstones and shales of prograding ighstand fans.

Sediment input to Santa Monica Basin, based on seismic-facies and isochron patterns, was predominantly from the (present-day) north, with subordinate input from the east. High-amplitude eustatic variations dominated deposition and sequence development even in this tectonically active basin Previously unidentified sand-rich fans are present, and have not been drilled.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California