Uplift and Erosion, Distribution of Mature Source Rock, and Burial Histories for Two Gas Systems in the Sacramento Basin, California

Hosford Scheirer, Allegra¹, Leslie B. Magoon¹, Marilyn E. Tennyson², Kenneth E. Peters¹ (1) U.S. Geological Survey, Menlo Park, CA (2) U.S. Geological Survey, Denver, CO

For a 2006 U.S. Geological Survey assessment of undiscovered gas resources in the Sacramento Basin of California, we quantified key aspects of its two major gas systems, which are presumably derived from the Upper Cretaceous Dobbins shale and Winters shale. We mapped the present-day depth to the threshold of thermal maturity for gas generation (vitrinite reflectance, %Ro = 0.6). We used this map to quantify uplift and erosion of sedimentary rocks, assuming a standard geothermal gradient (25°C/km), surface temperature (20°C), and threshold for hydrocarbon generation temperature (120°C). Results indicate little to no uplift south of the Stockton Arch but significant uplift (14,000') on the basin's west side in the vicinity of the Coast Ranges. We also mapped the areal extent of pods of active source rock by determining where the tops of source rocks lie beneath the Ro=0.6 depth. Results show that thermally mature Dobbins shale extends for 180 miles along the basin axis and deepens from -2,300' below sea level in the north (implying significant uplift) to -14,000' below sea level east of Rio Vista Field. In contrast, the pod of active source rock for the shallower Winters shale is restricted to a 60-mile long swath from the Stockton Arch to northwest of Rio Vista Field. The pod is deepest (14,000' below sea level) in the Rio Vista Field west of the Midland fault. One-dimensional burial history models suggest that the Dobbins shale began to generate gas ~75 Ma (0.6% Ro), reached peak maturity (0.9% Ro) at 45 Ma, and became overmature (1.2% Ro) about 10 Ma. Generation began in the Winters shale about 35 Ma.