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.