The late Eocene to Oligocene Sacate to Sespe formations of the Central Santa Ynez Mountains have been measured and correlated using aerial photographs. The preliminary fieldwork has established a sequence stratigraphic framework and facies partitioning that can be correlated to offshore subsurface data, where these strata form reservoirs.

The upper Sacate, Gaviota, Alegria, and Sespe formations (Refugian-Zemorrian California stages; Priabonian-Chattian stages) form a package ~4,000 feet thick and an overall retrogradational to progradational sequence. The succession starts with an intra-Sacate angular unconformity and ends with the incision of the Vaqueros Formation. Within this large-scale facies evolution, the succession can be further subdivided into higher-frequency sequences separated by a distinct flooding event at the top of the Gaviota and two significant erosional events within the Sespe.

The Gaviota Formation shows a transgressive to regressive sequence with granular sandstone at the top followed by a rapid flooding event. In places large articulated oysters mark this contact with the Alegria. Shallow marine Alegria sandstones with channels and mud rip-ups grade into lower Sespe tidal deposits showing numerous fining upward parasequences topped by supratidal red siltstones. A channelized incision follows and bounds a transgressive-regressive sequence marked at the base with several hundred feet of marine medium sandstone containing oyster fragments. Fluvial sandstones and overbank siltstones at the top of this sequence are cut by a second Sespe unconformity and filled with marine sediment followed by nonmarine fluvial sandstones and siltstones.

The nested hierarchy of sequences discussed may affect reservoir compartmentalization.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah