Abstract

The Sacramento basin is filled with more than 15,000 m of Mesozoic clastic sediments deposited in a marine fore-arc basin. The Delta Depression, located in the southern part of Sacramento basin, has approximately 3,000 m of Tertiary sediments in addition to the Mesozoic section. Hydraulic pressure distribution was determined using sonic and conductivity logs of more than 200 wells along with drill-stem test and published pressure data. Temperature gradients were obtained from uncorrected log temperatures for more than 3,000 wells.

Geopressured zones having hydraulic pressure gradients greater than 9.8 kPa/m (>0.433 psi/ft) occur at relatively shallow depths (<1,200 m) in the eastern, western, and northern parts of the basin. These zones occur at progressively greater depths toward the center of the basin where they may occur at depths greater than 3,000 m. In the Delta Depression, geopressured zones occur at depths as shallow as 1,500 m in Tertiary rocks.

Regional high pressures in the Sacramento basin probably result from the late Cenozoic tectonic compression of the Great Valley rocks between the Sierran-Klamath and Salinian blocks. The Tertiary rocks of the Delta Depression are geopressured due to loading by the rapidly deposited sediments and thermal expansion of the enclosed waters. Locally, overpressures are controlled by faulting and transmissivity of the rocks.

Temperature gradients in the Sacramento basin are generally low ranging from 18° to 25°C/km, they are generally higher in the Delta Depression with values between 25° and 35°C/km. Higher gradients in the Delta area are a result of the low thermal conductivity of the Upper Cretaceous rocks buried deep beneath the Tertiary rocks of the Delta Depression.