Description
Initial shut-in pressures (ISIP) 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. ISIP's in overpressured gas-bearing Forbes sandstones range from 3252 psi to 6660 psi. Pressure gradients range from .487 to .745 psi/ft. Regional ISIP data suggest that (1) pressures appear to increase dramatically at approximately 7700 feet (2339 m) below sea level; (2) pressures generally increase with depth above 8200 feet (2491 m) but are highly variable below 8200 feet; and (3) the approximate depth to the top of geopressures is 5500 feet (1671 m) below sea level. Fluid pressures generally decrease from NE to SW 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 .847 psi/ft. Shale compaction data and the equivalent depth method applied to semi-logarithmic plots of shale interval transit time versus depth were used in an attempt to determine formation fluid pressures. Pressures calculated from shale compaction data were consistently less than ISIP's measured at the same depth. Accuracy of calculated pressures decreased with depth, particularly below 7700 feet. The discrepancy between measured and calculated pressures may be due to many factors, including: (1) undercompaction is not the primary cause of overpressuring in the Sacramento Valley; therefore, shale compaction data do not reflect porosity changes with depth; and (2) sandstone pressure may not be in equilibrium with shale pressure. The relationship between ISIP's and the amount of gas tested during drill-stem tests was also examined. More than 3000 Mcf/day of gas was tested only where the fluid pressure gradient was between .510 and .700 psi/ft. Twice as many drill-stem tests showed little or no gas when the fluid pressure gradient exceeded .700 psi/ft than when it was less than .700 psi/ft. Fluid pressure gradients greater than .700 psi/ft were encountered only below 8000 feet (2430 m); therefore, the probability of testing, and possibly producing, 3000 Mcf/d of gas decreases below 8000 feet in the southern Grimes Gas Field.
