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The AAPG/Datapages Combined Publications Database

Pacific Section of AAPG


Geology of the Midway-Sunset Oil Field and Adjacent Temblor Range, San Joaquin Basin, California, 2001
Pages 1a-32
Regional Framework

Regional Geology of the Southwestern San Joaquin Basin, California

Tor H. Nilsen


The southern San Joaquin basin is a prolific oil-producing area that has had a complex tectonic and depositional history. It originated as the southern part of a large forearc basin in the late Mesozoic, was affected by both compressional and extensional tectonics in the Paleogene, and underwent a final episode of subsidence and infilling during the Neogene under the influence of strike-slip deformation associated with the North American-Pacific plate boundary. During the Neogene, thick diatomaceous oil-prone source rocks of upper Miocene Monterey Formation were deposited in the southern part of the basin, underwent subsidence and diagenesis, and generated large volumes of hydrocarbons. Numerous and thick sandstone bodies are interbedded with the diatomaceous strata and form the principal reservoirs. Shelfal clastic units surround the basin on the east, south, southwest, and north. Large submarine canyons were cut across the shelves, probably during lowstand periods, and funneled sands basinward, where thick sand-rich submarine fans accumulated.

During the Pliocene, shallow-marine and intertidal conditions were more prevalent throughout the basin, as it was gradually cut off from the Pacific Ocean. During the Pleistocene, thick nonmarine strata were deposited in the basin, dominated by fluvial, lacustrine, alluvial-fan, and fan-delta depositional systems of the Tulare Formation and related units. The basin continues to undergo subsidence in its axial portions, concurrently with uplift along the western, southern, and eastern margins.

The Midway-Sunset Oil Field and adjacent Temblor Range are present along the southwestern flank of the basin. A series of syndepositionally active folds in this area locally confined upper Miocene shallow- to deep-marine sandstones of the Monterey Formation, yielding numerous structural and stratigraphic traps. Uplift related to strike-slip motion along the San Andreas fault along the western flank of the basin generated numerous post-Miocene unconformities and additional traps for Miocene, Pliocene, and Pleistocene reservoirs.

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