Gas and oil occur in the Pliocene in a number of fields located along three major northwesterly-trending anticlinal structures of low relief in the east-central portion of the San Joaquin Valley. The upper member of the Pliocene, the San Joaquin Formation, is 1,200-1,800 feet of alternating brackish and marine clays and thin sands; the lower member, the Etchegoin, is 3,000-5,000 feet of largely marine shales and tight sandstones. The majority of the production comes from the First Mya-B Zone of the upper portion of the San Joaquin, at depths ranging from 2,200 feet at Semitropic to 2,800 feet at Harvester. Thickness ranges from 5 to over 50 feet, averaging about 10 to 15 feet. The more important fields (Trico, Buttonwillow, Semitropic) are primarily structurally closed el ngate domes, but lensing and stratigraphic trapping are important contributory factors to the accumulation in each field, and are the primary causes at Harvester. The Atwell Island sandstones, one or more of which are productive at Trico, Harvester, and Garrison City (?), are next in productive importance. They occur in the lower portion of the San Joaquin and are of cyclical or repetitive depositional character in the Trico-Harvester area. Additional productive zones in the San Joaquin are present at Northwest Trico, Semitropic, Buttonwillow, and Bowerbank.

The Etchegoin is of considerably lesser productive importance than the San Joaquin, primarily because of lack of permeability. Productive gas zones include the Mulinia (Semitropic, Garrison City, and Bowerbank), Mitchel (Garrison City), and the "E-7" (Shafter).

Indications of oil in the Pliocene so far have been observed only in the Etchegoin in this area, and only one zone, the Randolph (at Semitropic), is productive. The Randolph is a series of fine-grained, silty sandstones in the lower portion of the Etchegoin. Five wells are currently producing a total of about 110 B/D of 30° oil from depths of 7,000-7,650 feet. At least 700 acres so far have been proved productive. A combination of faulting and permeability changes probably controls the accumulation here. Non-commercial oil showings in the Etchegoin have been encountered at Buttonwillow, Wasco, and in additional zones at Semitropic, but none have been reported in the several deep tests at Trico. Recent drilling at Semitropic suggests that the Pliocene structure may be the result o deep-seated faulting and, consequently, prospects for deeper production from Miocene and Eocene sandstones may be considerably greater than previously suspected.

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