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The Lathrop gas field lies near the city of Stockton at the northern extremity of the San Joaquin basin in an area known as the Manteca arch. The Stockton fault, a large cross-valley reverse fault with a complex structural history, nearly intersects the field's northern margin. Continental Plio-Miocene sediments overlie unconformably a thick series of Upper Cretaceous clastics of mid-valley facies at Lathrop.
Natural gas is trapped primarily in sandstones of Upper Cretaceous "E" zone age as a result of anticlinal folding. Shallow drilling on the Lathrop fold dates from 1937. Discovery of gas occurred 26 years later in October, 1961, with the completion of Occidental Petroleum's "Lathrop Unit A" 1 for an initial rate of 13,550 MCFD. Nine distinct "E" zone reservoirs occur at Lathrop, separated by thin shales. The 3,700-psi zone is the most extensive reservoir, with a productive closure of 2,400 acres, up to 600 feet of relief, and gas-phase pressure continuity through 550 ft. of section.
Thirteen dual-zone and seven single-zone wells have been completed for initial flow rates of up to 42,000 MCFD per well. Individual wells penetrate up to 600 feet of net "pay." Deliveries of gas commenced in January, 1963, and averaged approximately 50,000 MCFD for 1963. Independent volumetric reserve estimates have have ranged from 578 million to more than 700 million MCF.
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