Detailed x-ray diffraction analysis of 140 individual well cutting samples from Tenneco Oil Poundstone 30-1, located in the Grimes Gas Field, northern Sacramento Valley, California, was performed. The samples, taken every 30 feet, were from the Upper Cretaceous Forbes Formation, the primary natural gas producing unit in the Grimes. The minus 4 micron size fraction of each individual sample was analyzed. Clay size mineralogic changes seen below 6,700 feet appear to be in response to smectite dehydration and may provide additional formation pressure in the already overpressured Forbes. The approximate temperature for initiation of this reaction, here, is interpreted to be near 79 °C. Well log data indicates background gas increase and economic gas producing zones all occur below 6,700 feet. The relative integrated intensity of SiO2 (quartz) and relative permeability (from induction logs) may be related inversely in the reaction zones and likely play an important role in natural gas migration, accumulation, and formation pressure distribution present within the Forbes.