SNODGRASS SLOUGH GAS FIELD: A WINTERS STRATIGRAPHIC TRAP

Snodgrass Slough Field produces gas from the up-dip pinch-out of an Upper Cretaceous Winters Formation turbidite fan at the mouth of an abandoned feeder channel. The channel carried sand from the coeval Starkey Formation deltaic facies, across the Sawtooth Shale pro-delta facies, and deposited it in a basinal submarine fan facies. The field is located on the eastern side of the Sacramento Valley in Sacramento County, California. There are numerous Winters feeder channels along the east side of the Valley, several of which form stratigraphic traps in settings similar to Snodgrass Slough. Isopachous mapping of the net Winters sand shows large east-trending re-entrants of the zero sand line of the Winters into the pro-delta shale facies, representing the location of the mouths of the feeder channels. Fan deposits coalesce to the west, where Winters sands are hundreds of feet thick and blanket the region.

A 3D seismic survey delineates the trap and shows several isolated, productive sand bodies. The Winters fan facies is characterized by a series of gently dipping, off-lapping reflections, with younger sands located at the top of the Winters interval to the west, and older sands located to the east. The Sawtooth pro-delta shale facies is expressed as a series of prograding clinoforms that down-lap onto the Winters. Seismic modeling shows a linear relationship between amplitude and pay thickness, as does observed data collected from the wells in the field. Gross pay maps, built from the amplitude maps, yield reserve estimates similar to the actual produced volume from this water-drive reservoir.