Extensive testing, measurement, and data collection at the Multiwell Experiment (MWX) field test site in northwestern Colorado characterize low-permeability gas-bearing sandstone reservoirs in the 1220-m (4000-ft)-thick Mesaverde Formation in the Piceance Basin and can be used to understand such reservoirs elsewhere in the Rocky Mountain region. These data show that although there is little change in porosity with depth, permeability is degraded by the increasing water saturations near the top of the gas-bearing section. Confining stress and pore pressure also combine to reduce permeability, with the result that conventional laboratory measurements on cores overestimate the actual matrix permeability. However, restored-state laboratory permeability measurements seriously underestimate the system permeability, as measured by well tests, because the system permeability is dominated by natural fractures. Both matrix permeability and fracture permeability are also sensitive to changes in stress: increased confining stress or decreased pore pressure (or both) during production decreases the system permeability by several orders of magnitude because of closure of fracture apertures and intergranular pores. Natural fractures cause horizontal permeability anisotropies of up to 100:1, measured by carefully controlled well testing, with the long axis oriented parallel to the dominant fracture trend and to the maximum horizontal compressive stress orientation. This fracture permeability system is susceptible to damage by stimulation fluids. Calculations of pore pressure based on drilling mud weights underestimate measured formation pressures. The system is overpressured at 18.1 kPa/m (0.8 psi/ft), but by only 12 kPa/m (0.53 psi/ft) if local topography is considered. Coals in the section are barriers to heat flow, causing a stepped geothermal gradient. Although collected in the early 1980s, these data comprise what is perhaps the most comprehensive and mutually supporting suite of geological and engineering data from a single site and strongly support the basin-centered gas model at this location.