Thick-bedded, massive Winters sandstones are the principal reservoir facies of the Union Island gas field in the southern Sacramento Valley. The field is a structural-stratigraphic trap set up by structural closure against the Stockton Arch fault and by facies changes within the Winters Sandstone. Since the discovery of the field by Union Oil in early 1972, total gas production has been approximately 90,000,000 MCF from 16 wells.

Three major facies are recognized in the nearly 190 feet of conventional cores taken from four development wells in the field: (1) thick-bedded, porous (20 to 30%) massive sandstone, (2) inter-bedded thin sandstone and shale, and (3) laminated shale. Features in the cores as well as seismic, paleontologic, and regional subsurface data indicate a deep-water (bathyal) origin for the Winters Sandstone. X-radiographs of apparently “structureless”, massive sandstones at Union Island field exhibit horizontal or cross stratification in approximately 50% of the intervals examined. The stratification and other structures suggest that the massive sandstones as well as the thinner bedded sandstones were deposited by turbidity currents.

The Winters sandstones at Union Island field commonly show upward-fining and upward-thinning sequences, are elongate perpendicular to the depositional slope, and thicken toward the basin. They are interpreted to be coalesced channel deposits which formed at the base of the slope on the upper part of a sandy suprafan. Thinly interbedded sandstone and shale that separates the massive sandstones represents channel-abandonment and interchannel-overbank deposits.