The Forbes sandstones of the Sacramento Basin remain attractive targets for explorationists in California. This regional stratigraphic study s is located in the central Sacramento Basin (Figure 1). It encompasses an 1,139 square-mile area and includes most of Sutter and Colusa counties, as well as portions of Yolo county in California. The project, representing more than seven man-years of geologic research, used 587 wells to evaluate the stratigraphic framework of the Forbes formations in this portion of the Sacramento Basin.

     The Forbes Formation represents deposition in a southward prograding deep-sea fan system fed by a deltaic system. Migration of deltaic lobes and the associated deep-sea fan deposits result in a complex lateral and vertical distribution of sandstone bodies of the Forbes Formation in the Sacramento Basin.

     A great variety of traps are apparent based on this depositional model. The fan system is fed by numerous, smaller "gullies" which are derived from the Kione fluvial-dominated delta. The resulting fan system is not a well-defined, discrete fan, but a series of smaller fans, comprising a fan system. The delineation of channel systems associated with fan systems aids greatly in gas exploration and development.

     The majority of Forbes Formation gas production is from stratigraphic traps and a detailed analysis of stratigraphic relationships is essential to gas exploration. More than 1,000

well logs, 100 petrographic and paleontologic samples, 3,000 feet of core, and four measured sections (totaling over 6,350 feet) were analyzed to develop a depositional model and determine sandstone body orientations and geometries. The Forbes Formation can be subdivided regionally by two low-resistivity shale markers and can be further subdivided locally.

     Four outcrops and five cores of the Forbes Formation sandstones in the Sacramento Basin were described for this study. Lateral and vertical variations in lithologies as well as primary and biogenic sedimentary structures from this information were used to interpret the depositional sequences.

     The petrographic study of 76 thinsections provided data for interpreting textural and diagenetic characteristics of Forbes reservoir lithologies. Specific emphasis of the petrographic study was to determine the compositions of the sandstones, define the diagenetic sequence, and to relate cementation to variations in porosity and permeability.

     Three field areas in the study area were selected for the characterization of reservoirs from deep-marine depositional environments (Arbuckle, Grime, and Robbins fields). These field studies provide a detailed analysis of reservoir quality and reservoir development. Concepts derived in these field areas were applied to the regional evaluation of reservoir potential of the Forbes Formation in the Sacramento Basin.