The Grimes gas field in the northern Sacramento Valley, California, produces from sandstones in the Upper Cretaceous Forbes Formation. These sandstones form a complex of lenticular stratigraphic traps. The Grimes field, with present annual production of 29 billion cu ft, is the third largest dry gas producer in the state.

The original Grimes prospect had as primary objectives the shallow Upper Cretaceous "Winters" sandstones; Forbes sandstones were secondary objectives. Subsurface data, supplemented by minor seismic information, were used to delineate the prospect. After the discovery in 1960, an extensive seismic survey resulted in modification of previous conceptions of the structure and helped greatly in development. Seismic information did not help delineate the lenticular sandstone traps. The Forbes sandstones were deposited in moderately deep water by deep-water currents, probably turbidity currents. Individual lenses are semielliptical, oriented generally in a northwesterly direction. They range in length from less than 1 to 8 or more miles (1.6-13 km).

A series of tectonic episodes, ending with the emplacement of igneous plugs at the Colusa high and Marysville Buttes in Pliocene time, has given the area its present structural configuration. The northern part of the area is dominated by the highs of the Marysville Buttes and Colusa areas, whereas the Grimes area proper is simply a southwest-dipping homocline. Faults of pre-Eocene origin are present in the Grimes area and are partly responsible for the Forbes accumulations there.

Fluid-pressure relations in the Grimes area are somewhat unusual. Formation pressures are well above hydrostatic and increase with depth at a gradient greater than the hydrostatic gradient; however, formation pressures decrease eastward from the west edge of the Grimes field. The pressure relations are probably important to the nature of presently existing gas accumulations.