Abstract

The prospective Sacramento Basin consists of approximately 4.6 million acres of Cretaceous-Tertiary area bounded on the west by the Sierra Nevada and on the east by the uplifted Coast Range terranes. The south margin extends to the Stockton Arch and the north productive limit lies near Red Bluff where Forbes objective section becomes diminished. Cretaceous objective section is dominated in the north by Forbes-Kione strata and in the south by the younger Starkey-Winters and equivalent strata. Overall the Sacramento basin is a south plunging trough containing over 40,000 feet of sediments in the Montezuma depocenter and interrupted by major structural features including the Rio Vista-Thornton Arch, transtensional Midland fault system, Sutter Tertiary Volcanics and transpressional Willows fault system. Productive zones range from about 2000 to 11,000 foot drill depths. With nearly 9500 wells drilled and about 3000 completed, the estimated ultimate recovery is nearly 10 trillion cubic feet (TCF) with an implied 14 TCF gas-in-place at 70% recovery. Ultimate recovery can be estimated by formation with nearly 40% from Tertiary rocks including 3.5 TCF from the Rio Vista field, 25% from the Cretaceous Forbes-Kione depositional system, and 35% from the Cretaceous Starkey-Winters depositional system. Severe overpressured marine Forbes strata in the North (Grimes-Sutter) area is largely due to tectonic uplift. Slight overpressure in marine Winters objectives at 11,000 feet is noted on the flanks of the Montezuma depocenter near Rio Vista. Both Forbes and Winters formation overpressure are capped in large part by thick slope shale sequences. Overlying Kione and Starkey deltaic facies and all Tertiary reservoirs are normally pressured. Present-day temperature gradients are extremely low, averaging 1.2 degrees F/100 ft. Vitrinite reflectance and source rock data suggest lean source rocks and low temperature gradients in most producing areas of the basin. With nearly all gas thermogenic in origin an implicit assumption obtains that an early oil phase was insufficient to saturate and allow significant expulsion. A later gas phase migrated from the deeper parts of the basin, particularly the Montezuma depocenter area and perhaps deeply buried Forbes source rock in the Grimes area, in general from below 12,000 feet. It is noted that fields on the flanks of the Montezuma area contain partial-retrograde condensate reservoirs and that heavy hydrocarbon components diminish in reservoirs at the basin margins. Nearly 90% of the recovered gas lies in fields located along major structural features including the largely stratigraphic Forbes and Winters plays. Structural trends appear to serve both as conduits for pervasive gas migration as well as the focus for the numerous structural and combination traps which account for most of the basin production to date.