Project Final Report-07122-09-Tight-gas-sand reservoirs in the Rocky Mountains, western United States, represent a major natural gas resource. Major fields brought into production in the past 15 years, located in Colorado, Wyoming and Utah, represent more than 100 TCF of recoverable gas reserves. Despite the significance of these reserves, gas compositions in these fields are largely undocumented, and controls on gas composition are poorly understood. This study reports a large data set of gas compositions from three major tight-gas-sand reservoirs: Jonah Field, in the Greater Green River Basin, southwestern Wyoming; the Mamm Creek – Rulison – Parachute – Grand Valley complex of fields in the Piceance Basin, western Colorado; and Greater Natural Buttes Field, Uinta Basin, northeastern Utah. Data gathered included: fractions of hydrocarbon species and CO2 in production gas and mud samples; compound-specific carbon and hydrogen isotopes in production gas and mud gas samples; noble gas isotopes in production gas samples; fluid inclusion compositions from formations (Jonah Field and the Piceance Basin fields,14421 ); gas compositions generated from source rocks during hydrous pyrolysis experiments.