Occurrence and origin of free carbon dioxide gas deposits in the Earth's continental crust

Deposits of CO2-rich gas (>50%) are present worldwide but in limited areas. Found during drilling for oil and gas, they can be an expensive nuisance or an economic resource.; A search was made for data about CO2-rich deposits worldwide to determine their tectonic setting and gas composition. The deposits are located in areas of tectonic extension including continental rifts, areas of tectonic escape, back arc basins, extensional cross trends in foreland basins, and plate bounding strike slip faults, often near basaltic volcanism.; Isotopic ratios of carbon in CO2 and of helium 3He/ 4He compared to air (Ra) aid in determining the possible source of CO2 gas. A detailed study was made of a typical CO2 deposit, Bravo Dome field, N.M., U.S.A., which contains 283 billion cu. meters (10 trillion cu. ft.) of 99% CO2. Gases from 14 wells spread across Bravo Dome field were specially sampled revealing a dynamic gas deposit in which noble gas contents vary systematically across the field. The western portion of the field far above the gas-water contact has noble gas concentrations similar to those in mid-oceanic ridge basalts (MORB). To the east, near the gas-water contact, the CO2 has higher noble gases concentrations because atmospheric and crustal noble gases entered the CO2 from the water.; The CO2 of Bravo Dome field is clearly of magmatic origin as shown by 3He/4He/Ra as high as 4.26 Ra, high CO2/3He ratio, delta13C values of -3.7 to -5.1 ‰ PDB in CO2 gas, by delta13O values of 8.08 ‰ SMOW in the west to 21.6 ‰ SMOW in the east, and by the relationships of noble gas concentrations.; It is concluded that in general CO2 deposits in sedimentary rocks are usually sourced from the mantle of the earth. Fractures in the crust in areas of crustal extension allow basic magma to rise. CO2 is expelled from the magma and enters porous reservoirs in sedimentary sections and, where adequate traps and seals are present, form CO2 deposits. CO2 is unrelated to hydrocarbons, migrating separately and at different times.