Assessment of Undiscovered Natural Gas Resources of the Sacramento Basin Province of California, 2006

National Assessment of Oil and Gas Fact Sheet

The U.S. Geological Survey (USGS) recently completed a new assessment of undiscovered natural gas resources of the Sacramento Basin Province of California. Using a geology-based assessment methodology, the USGS mean estimates of undiscovered, technically recoverable resources are 534 billion cubic feet of natural gas and 323 thousand barrels of natural gas liquids in the Sacramento Basin Province. Additional undiscovered oil accumulations larger than 0.5 million barrels are considered unlikely.

Introduction

The Sacramento Basin forms the northern half of California’s Central Valley. It is an elongate, northwest-trending structural trough filled with more than 20,000 feet of Early Cretaceous (~145 to 100 million years old) and younger marine and nonmarine sedimentary rocks. The Sacramento Basin oil and gas province has been extensively explored for petroleum resources, and more than 9 trillion cubic feet of natural gas have so far been produced.

Recently, U.S. Geological Survey (USGS) scientists finished an assessment of the undiscovered natural gas potential of the Sacramento Basin Province. (The southern half of the Central Valley, the San Joaquin Basin, a prolific oil province, was assessed in 2003.) Their assessment of the Sacramento Basin indicates the probable existence of some still undiscovered, technically recoverable natural gas resources—in other words, those resources that can be discovered, developed, and produced by using current technology. The assessment is based on the geologic elements used to define a Total Petroleum System (TPS), including hydrocarbon source rocks (source-rock type and maturation and hydrocarbon generation and migration), reservoir rocks (sequence stratigraphy and petrophysical properties), and hydrocarbon traps (trap formation and timing). In the Sacramento Basin Province, the USGS defined two TPSs—the Dobbins-Forbes TPS and the Winters-Domengine TPS—and four assessment units, two in each petroleum system.

Resource Summary

The USGS assessment strategy provides estimates of the volumes of undiscovered petroleum (mainly oil, natural gas, and natural gas liquids) that are technically recoverable. For the Sacramento Basin Province, the USGS estimates that total undiscovered natural gas resources range between 139 and 1,067 billion cubic feet (BCF; 95% and 5% probabilities of greater than these amounts, respectively), with a mean estimate of 534 BCF. Of this mean natural gas estimate, 40%, or 211 BCF, is expected in the Dobbins-Forbes TPS. Nearly all of that amount (176 BCF) is anticipated to come from Upper Cretaceous (~100 to 65 million years old) submarine-fan and deltaic-sand reservoirs of the Forbes-Kione and Older Assessment Unit (AU); a minor amount (35 BCF) is estimated to exist in Cenozoic (~65 million years old to present) reservoirs of the Lower Princeton Canyon Fill and Northern Nonmarine Rocks AU. Nearly two-thirds of the estimated undiscovered natural gas resources in the province (323 BCF) is expected in the Winters-Domengine TPS. This TPS is subdivided into the Late Cretaceous Deltaic and Submarine Fan AU, with an estimated undiscovered 261 BCF of natural gas, and the Shallow Marine Sands and Canyon Fill AU, with an estimated undiscovered 62 BCF of gas in Cenozoic reservoirs.

Sacramento Basin Province assessment results. [MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MBNGL, thousand barrels of natural gas liquids. Results shown are fully risked estimates.

In addition to the natural gas resources already discussed, the discovery of further petroleum resources of significant size in the Sacramento Basin Province is improbable. Natural gas liquids account for just 323 thousand barrels (mean estimate). Few minor oil accumulations have been found in the Sacramento Basin, and additional undiscovered accumulations larger than 0.5 million barrels are considered unlikely. The possible presence of a continuous, basin-centered gas accumulation in the Dobbins-Forbes TPS was considered, because of its numerous overpressured reservoirs. These reservoirs, however, exhibit few of the characteristics of a basin-centered accumulation, and therefore such an accumulation is very unlikely, at least at currently drilled depths.

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