Porosity of Coastal Deltaic Sandstones, Cerro Prieto Geothermal Field, Baja California, Mexico

Abstract

Core porosity values for sandstone and density log derived porosities for sandstone siltstone shale sequences indicate a range from less that 1% to 40% at the Cerro Prieto geothermal field, Baja California, Mexico. Mean porosity values indicate that a general trend of decreasing porosities with increasing depth from 35% at 600 m to 10% at 2300 m is complicated by the 15 to 30% porosities in the 350°C hot water zone at about 2700 m depth. Scanning electron microscopy documents secondary dissolution porosity, mineral overgrowths, and abundant clay minerals. Core permeability ranges from 0.1 to 1000 millidarcies for the more that 50 cores studied. The porosity variability indicated that geothermal systems provide an ideal setting for testing concepts of dissolution porosity and increased secondary dissolution permeability that could be useful for nuclear waste storage as well as petroleum reservoir engineering.