In this paper, we analyze source, seal, trap, reservoir, timing, and migration to explain petroleum occurrence and exploration prospectiveness in the southernmost San Joaquin basin. The factors that control oil occurrence and field size vary greatly among the three productive geologic domains in the area. We term these three domains the Foothills, the Basin, and the Upturn. The Upturn lies in between the structurally high Foothills and the structurally low Basin, and is a near-vertical panel of highly faulted strata with 5,000’-15,000’ of structural relief.
Monterey Formation source rock in the southernmost San Joaquin basin has reached oil maturity mainly within the Basin. Oil generation there began about 4 Ma and continues today. However, the onset of anticlinal growth in the Basin at about 1 Ma dramatically changed oil migration pathways and delivery destinations. Prior to 1 Ma, oil migrating out of the Basin was delivered broadly across the entire east-west extent of the Upturn and Foothills. After 1 Ma, migration pathways became more complex and more east to west, resulting in very large oil pools in the southwestern San Joaquin basin.
In the Basin, abundant oil has been generated, seal is ubiquitous, and early-formed traps are present. However, reservoir presence, reservoir quality, and the availability of migration pathways limit the amount of commercial production. Commercial oil is found mainly within gently dipping upper Miocene Reef Ridge and Stevens submarine-fan sand stratigraphic traps. The upper Miocene is sand poor, so sand mapping is critical. Fortunately, modern 3-D seismic combined with well data allow reliable mapping of sand fairways and fan facies. Little oil production exists above the upper Miocene because the high-angle faults that facilitate migration from the lower Monterey hydrocarbon kitchen do not penetrate high enough to allow charging of shallower strata. Little commercial production exists below the upper Miocene because depths are too great for sands to retain reservoir quality.
The Upturn is well charged with oil, has abundant migration pathways, and contains numerous fault traps, many of which formed before oil migration began. However, it is sparsely drilled and difficult to image seismically, so exploration for the fault-footwall traps that are the typical targets is risky. Perhaps surprisingly given the active dense faulting, the presence of several oil fields in the Upturn indicates that seals can be effective. As in the Basin, sand is generally sparse, so sand presence is an important play control. The eastern Upturn toward the Tejon embayment is sandy, but the western and central parts generally are not. Where sand is present, reservoir quality is adequate even in lower Monterey and older strata that are tight in the Basin because burial of these strata was never deep.
Most Foothills fields are located in Quaternary surface anticlines. However, central and eastern Foothills anticlines are undercharged because their present-day fetch areas are small and their trap timing is late: most of the anticlines are extremely young, so were not available to capture early-generated oil migrating from the Basin. To date more oil has been produced in the eastern than in the western Foothills, partly because sand content increases eastward.
