Six member sands are defined for the Winters Formation in the subsurface of the Sacramento Valley of California. Relations between these member sands and the S-5 member of the Starkey sands provide the basis for reconstruction of geologic events and basin geometry.

Two subbasins are recognized. Deposition of the Winters sands was concentrated in the northern subbasin where a more complete reconstruction of events is possible, though the largest gas field producing from the Winters sand is at Union Island in the southern subbasin.

The lower sands (Staten Island and Walnut Grove members) are inner bathyal deposits and part of an oblique tangential depositional pattern. These lower members are overlain unconformably by the S-5 member of the Starkey sands.

Due to partial filling of the basin, depositional conditions changed and subsequent sands (Putah Sink, Mound, and Unit members) are recognized as the deeper water facies of the S-5 member of the Starkey sands with an intervening area of shale representing deposition along the gentle prograding slope. For the uppermost part (McCune member), the basin filling had progressed to where there was no intervening shale between the Winters sand facies and the S-5 Starkey sand facies. The prograding slope can no longer be identified and the dividing line between the two sands is poorly defined.

Distributary channels served as conduits feeding sand into the central part of the basin where the Winters sands were deposited. For the lower members, stratigraphic trends associated with these distributary channels are favored areas for gas accumulation. For the higher members, gas accumulation is controlled primarily by faulting.

Discoveries to date total about 750 million Mcf recoverable reserves of which 400 million Mcf is estimated to be in the Union Island gas field. Future discoveries are anticipated in traps similar to those associated with existing discoveries.

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