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Although the Etchegoin Formation is considered to be Pliocene in age, K-Ar and 87 Sr/86 dating indicates that the lower part of the formation may be of late Miocene age. Molluscan fauna and foraminiferal assemblages indicate it was deposited in shallow-marine to intertidal and estuarine environments. Trapping of oil in the Etchegoin is caused by the pinchout or truncation of individual sandstone members often against the flanks of anticlines. The Etchegoin Formation produced most of the first one billion barrels of Midway-Sunset’s 2.4 billion barrels of cumulative oil production indicating much future potential remains.

The San Joaquin Formation is the youngest unit of the upper Miocene and Pliocene Etchegoin Group and provides a record of the late Neogene history of the San Joaquin basin. During Pliocene time, the San Joaquin basin had become an inland sea that was connected to the Pacific Ocean through a narrow strait along the western side of the basin. The Pliocene San Joaquin Formation unconformably overlies the Etchegoin Formation and underlies conformably to unconformably the Tulare Formation. At Midway-Sunset, the San Joaquin Formation ranges in thickness from 150 ft in the north to 1100 ft in the south. It is divided into two main zones, the Mya (Mya Tar) and Top Oil (Scalez). The reservoirs deposits are clay-rich, very fine to fine grained sandstones alternating with siltstone and claystone. Fossil assemblages indicate shallow water deposition with alternating brackish and freshwater conditions.

Production from the San Joaquin Formation, at the Midway-Sunset field, is minor with production figures not available because it has been commingled with production from the Etchegoin and Tulare Formations. The oil is 12° to 28° API gravity and is found in both stratigraphic and structural traps. San Joaquin Formation oil production at Midway-Sunset field has been maintained in part by use of waterflooding and steamflooding since 1965.

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