The Petrology Laboratory of Bendix Field Engineering Corporation (BFEC) at Grand Junction, Colorado, is performing mineralogie and petrologic analyses on core samples from the Multi-Well Experi-ment (MWX) project for Sandia National Laboratories. The samples studied to date include some fluvial sands, transitional sands, and shoreline blanket sands. The sands are generally fine-grained feldspar-and lithic-rich quartz sandstones, with minimal textural and mineralogie variation. Porosity is predominantly microporosity in clays which fill secondary pore space. The primary factor limiting the permeability of these sands appears to be the clogging of the pore space by clay minerals.