Undiscovered Gas Resource Potential of the Sacramento Basin, California: Results of the 2006 USGS Assessment

Hosford Scheirer, Allegra¹, Marilyn E. Tennyson², Leslie B. Magoon¹ (1) U.S. Geological Survey, Menlo Park, CA (2) U.S. Geological Survey, Denver, CO

In 2006, a team of U.S. Geological Survey scientists completed a new assessment of undiscovered gas resources in the Sacramento Basin of California. The team created a series of rock unit distribution maps from more than 7,000 well picks. These maps were used to make stratigraphic columns for the north and central basin as well as the area south of the Stockton arch. We created two- and three-dimensional maps of key source and reservoir rocks, constructed a thermal maturity map using vitrinite reflectance, evaluated overpressures in the basin, delineated the pods of active source rock, and calculated burial and maturation histories for the two gas source rocks, the Upper Cretaceous Dobbins and Winters shales. In addition to the geological analysis, the field discovery history was analyzed as a function of time and drilling.

The Dobbins-Forbes Total Petroleum System was assessed as two assessment units (AU), the Forbes-Kione and Older AU and the Lower Princeton Canyon Fill and Northern Nonmarine Rocks AU. The Winters-Domengine Total Petroleum System was also assessed as two assessment units, termed the Late Cretaceous Deltaic and Submarine Fan AU and the Shallow Marine Sands and Canyon Fill AU. The greatest potential for undiscovered gas is in Cretaceous-aged reservoir rocks located above the Sacramento Shale, a regional seal. Our analyses allow for the remote possibility of conventional gas accumulations in the Marsh Creek Formation, a southern equivalent of the prolific Forbes formation. Our analyses further suggest that the Forbes formation probably lacks a continuous gas accumulation. Although a few oil accumulations occur in the Sacramento Basin, additional undiscovered accumulations larger than 0.5 million barrels are unlikely.