Fluid Pressures in Overpressured Forbes Formation, Sacramento Valley, California

Carol A. Price

Initial shut-in pressures obtained from drill-stem tests, mud weights, and shale-compaction data were used to determine the formation fluid-pressure distribution in the southern Grimes gas field. Initial shut-in pressures in overpressured gas-bearing Forbes sandstones range from 3,252 to 6,660 psi. Pressure gradients range from 0.446 to 0.745 psi/ft.

Regional initial shut-in pressure data suggest that (1) pressures appear to increase dramatically at approximately 7,700 ft (2,339 m) below sea level, (2) pressures generally increase with depth above 8,200 ft (2,491 m), but are highly variable below 8,200 ft, and (3) the approximate depth to the top of geopressures is 5,500 ft (1,671 m) below sea level. Fluid pressures generally decrease from northeast to southwest through the region and from north to south within the westernmost fault block.

Mud-weight data give maximum pressures for any depth. The maximum possible pressure gradient in the southern Grimes gas field is 0.847 psi/ft.

The relationship between initial shut-in pressure and the amount of gas tested during drill-stem tests was also examined. More than 3,000 MCFGD was tested only where the fluid pressure gradient was between 0.510 and 0.700 psi/ft. Twice as many drill-stem tests showed little or no gas when the fluid-pressure gradient exceeded 0.700 psi/ft than when it was less than 0.700 psi/ft. Fluid-pressure gradients greater than 0.700 psi/ft were encountered only below 8,000 ft (2,430 m); therefore, the probability of testing, and possibly producing, 3,000 MCFGD decreases below 8,000 ft.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.