2012 – Salinas Basin Field Trip

The Salinas Valley and Santa Lucia Range of central California provide a unique opportunity to observe all elements of the Salinas Basin petroleum system in outcrop. Neogene uplift of the sedimentary fill along the basin margins has exposed the critical components of a petroleum system that are responsible for the formation of the half-billion-barrel San Ardo oil field. That the petroleum system components are identifiable here in outcrop is significant, because this affords the opportunity to understanding the evolution of the Salinas Basin oil fields, to potentially predict undiscovered petroleum accumulations, and to identify distribution limits of known fields.

A petroleum system comprises four elements and two sets of processes, which are requirements for any oil or natural gas accumulation to form. The four petroleum system elements are: (1) hydrocarbon source rock, (2) reservoir rock, (3) seal rock, and (4) overburden. These elements are influenced by two sets of processes: (1) hydrocarbon generation, migration, and accumulation, and (2) trapping mechanisms to ultimately form the accumulation. Critical to the success of a petroleum system is the sequence in which these elements form and in which the processes occur. For example, reservoir rock must form in time for oil to migrate into it, and the trap rock must develop prior to migration of oil to ensure accumulation. This field trip differs from other recent Pacific Section SEPM Fall Field Trips, in that it emphasizes a holistic view of all elements of a petroleum system, rather than focusing on a single element, such as the sedimentary character of a particular reservoir unit.

The petroleum system elements of the Salinas Basin are enclosed nearly entirely within the Miocene Monterey Formation. The western side of the basin reveals exposures of the organic-rich source rock of the Sandholdt Member of the Monterey, as well as the thick siliceous Hames Member of the Monterey Formation. It is both the appreciable thickness of the siliceous part of the Monterey Formation and the tectonically-induced structural burial that constitutes the overburden element of the petroleum system. About 3 km (~2 mi) of overburden in some places has led to sufficient maturation of organic matter within the Sandholdt Member as source rock to have generated oil. Reservoir rocks and seal rocks are also contained within the upper Monterey Formation, although these occur in the subsurface on the eastern side of the basin. Cores through these reservoir and seal rocks from the San Ardo oil field provide insight into these petroleum system components. There are few sedimentary units with matrix permeability levels that are conducive to hydrocarbon migration in this basin. However, highly fractured chert facies within the Hames Member of the Monterey and transpressional faults provide alternative pathways through which petroleum migrates from source rock to reservoir. Although the Salinas Basin system is contained primarily within petroleum-rich Miocene and younger strata, the petroleum-poor Cretaceous through Oligocene strata exposed along the far western margin still serve as outcrop analogs to reservoir facies in other California basins.

This 2-day Pacific Section SEPM Fall Field Trip will sequentially investigate key elements of the Salinas Basin petroleum system. The Saturday segment, on October 20th, will highlight source-rock exposures in the northern part of the basin. The trip progresses southward on Saturday afternoon and Sunday, October 21st, to explore regional and localized structural features, exposures of sedimentary rock as basin fill, and “dead oil” that had migrated and decayed to tar through biodegradation. The final portion of the trip will tie in petroleum-system characteristics viewed on Saturday as they apply to the San Ardo oil field.