american geologists gradually became aware that there were huge volumes of gas-saturated sandstones with permeability too low (generally less than 0.1 millidarcy) to recover the gas economically. starting in 1973, massive hydraulic fracturing was used in thousands of gas wells in the san juan basin, denver basin, the piceance basin, and the green river basin, and in other hard rock formations of the western us. other tight sandstone wells in the us made economically viable by massive hydraulic fracturing were in the clinton-medina sandstone (ohio, pennsylvania, and new york), and cotton valley sandstone (texas and louisiana).