Abstract

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 composed of 1,200 to 1,800 feet of alternating brackish and marine clays and thin sands; the lower member, the Etchegoin, is 3,000 to 5,000 feet thick and is composed largely of marine shales and tight sands. 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 varies from 5 to over 50 feet, averaging 10 to 15 feet. The more important fields (Trico, Buttonwillow, Semitropic) are primarily structurally closed elongate domes. However, lensing and stratigraphic trapping are important contributory factors in accumulation in each field, and are the primary cause of entrapment at Harvester. The Atwell Island sands, 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 this area, and only one zone, the Randolph (at Semitropic) is productive. The Randolph is a series of fine grained silty sands in the lower portion of the Etchegoin. Seven wells are currently producing a total of about 125 B/D of 30° gravity oil from depths of 7,400 to 7,650 feet. At least 700 acres have so far been proven productive. A combination of faulting and permeability changes probably control the accumulation here. Noncommercial 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 of deep-seated faulting and, consequently, prospects for deeper production from Miocene and Eocene sands may be considerably greater than previously suspected.