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The West Thornton and Walnut Grove gas fields occupy a position astride the east-west-trending Thornton arch which extends from Lodi to Rio Vista and is one of the major structural features of the Sacramento Valley gas province. Production along this trend was first established in 1943 at the Thornton and Lodi fields which are areas of anticlinal closure. Down plunge to the west, additional production was subsequently developed in 1956 and 1958 with discoveries at West Thornton and Walnut Grove respectively.
The productive section at the West Thornton and Walnut Grove fields includes the Domengine and Midland sands of Eocene age, Paleocene Martinez sands, and Cretaceous Winters-Millar sands. Gas accumulation in these intervals is controlled by a combination of faulting and stratigraphic changes on the plunge of the Thornton arch. Cretaceous production of the Thornton arch was first established at Walnut Grove from the Wintersmillar sands in the Brazos Locke Unit No. 1 well in 1959. Three wells are presently productive from this interval. Erratic sand distribution in the lower Eocene Meganos sediments, attributable to either rapid facies changes or channel development similar to "Markley Channel,"
delayed definition of the West Thornton gas field for four years until further development drilling was undertaken in April of this year. Since this time five additional wells have been completed in the Midland pool.
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