The Forbes formation of Upper Cretaceous age consists of marine shale, siltstone, and interbedded sandstone, and lies stratigraphically between the younger Kione deltaic sandstone facies and the older Dobbins shale. On the west side of the Sacramento Valley, the Kione formation is truncated and the Forbes formation is overlain by the Capay (Eocene) and/or Tehama (post-Eocene) formations. In the Sacramento to Red Bluff area, the Forbes formation attains a thickness of up to 5,000 ft (1,524 m).

The importance of the Forbes formation as a source of gas production in the Sacramento Valley is well established. Gas was first produced from the Forbes formation near the south edge of the Marysville Buttes in 1953. The formation is now productive in over 20 fields in the Sacramento Valley with cumulative production to January 1, 1980, of 1.23 billion Mcfg.

The discovery and development of gas from the Forbes formation declined considerably in the late 1960s. As a result of new CDP seismic reflection profiling, however, drilling for gas from the Forbes formation has increased dramatically since 1978. Moreover, careful integration of modern seismic data with detailed subsurface stratigraphic mapping has resulted in new discoveries.

The Grimes gas field provides an example where these investigative techniques can be supplied.

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