Continued gas and condensate discoveries in the Late Cretaceous Winters Sandstone of the southern Sacramento Valley provided impetus for detailed paleoenvironmental analyses of that formation. The Winters Sandstone interval, cored in the Cities Service Nixon Community No. 1 well, Solano County, California, was analyzed to ascertain its sedimentary and petrographic characteristics and to determine the depositional environment in which it accumulated.
The Winters Sandstone core extends from 8855 to 8882.4 feet in the Nixon Community No. 1 well and includes strata divisible into three lithologic units. The lower unit, Unit 1 (8862.5 to 8882.4 feet), is a massive-appearing porous (25%) and permeable (150-950 md) sandstone with nongradational upper and lower contacts with shale. X-ray radiographs reveal numerous subhorizon-tal laminations throughout much of what otherwise appears to be a structureless sandstone. Two vertical sequences based on differences in grain size measurements can be distinguished, indicating vertical stacking of genetic subunits within the sandbody. These sedimentary features, developed within a well-defined sandstone unit, correspond to typical B-facies sandstones of the Mutti-Ricci Lucchi submarine fan model. As such, Unit 1 is interpreted to be turbidites deposited in a deep-water setting.
Overlying the sandstone are shale and silty shales divisible into two units. Small diameter burrows of indeterminate origin are abundant, though discernable on polished core surfaces only where highly concentrated. A modest foraminiferal fauna suggestive of slope or deeper depths was also recovered from the shale interval. The middle unit, Unit 2 (8859-5-8862.5 feet), is a horizontally laminated shale with subhorizontally bedded to contorted silty and sandy zones and with small obliquely to vertically oriented sandstone dikes. These sedimentary features suggest deposition from the fine-grained distal end of one or more turbi-dite flows and are typical of the D-facies of the Mutti-Ricci Lucchi submarine fan model.
Unit 3, silty “shale” (8855-8859-5 feet) is generally lacking distinct stratification features, being almost massive in appearance. It’s mode of occurrence presumably was from suspension which, together with its included moderately deep water microfauna, suggests an environment devoid of bottom turbidites, probably at slope depths. Unit 3 is classified as a G-facies deposit of the Mutti-Ricci Lucchi model.
