Deposits of CO₂-rich gas (>50%) are present worldwide but in limited areas. One hundred twenty-one (121) have been identified and classified worldwide, but many others remain to be identified and studied. If encountered while drilling for oil and gas, CO₂ can be either an expensive nuisance or an economic resource. Traditionally, explorationists have only wanted to know how to avoid CO₂ deposits or at least learn how to calculate the risk of finding them. In certain areas there is now a desire to find CO₂. Evidence of the source of the CO₂ deposits is in their geologic setting and in the gas itself.

Geologic Setting

The 121 known deposits of CO₂ are typically located in areas of tectonic extension. They are distributed among: cratonic arches, 14; cratonic basins with basic igneous intrusions, 47; continental rifts, 13; areas of transtensional basins related to tectonic escape, 12; back-arc basins, 13; cross-trends in foreland basins just in front of thrust sheets, 12; platebounding strike-slip faults, often near basaltic volcanism, 8; and in thrust belts, 2. Although the geologic settings of these deposits suggest mantle CO₂ rising with mantle-derived basalts into the crust, other evidence is needed to support a reliable model for a source of CO₂ deposits.

Gas Composition and Stable Isotopes

Evidence of source is available from the gas itself in the stable isotopic ratios of carbon in CO₂ and in the content and isotopes of noble gases, especially of helium ³He/⁴He. The isotopic ratio of carbon 13 to carbon 12 reported as difference in parts per thousand from the PeeDee belemnite standard (d ¹³C ⁰⁄₀₀ PDB) can aid in distinguishing different sources of CO₂. The ratio can vary from below -10 ⁰⁄₀₀ for CO₂ derived from organic matter to 0 ^0⁄₀₀ for CO₂ from calcined limestone. Mantle CO₂ is around -5 but overlaps with CO₂ from metamorphism of limestone. The ratio ³He/⁴He as compared to a standard that is the ratio in air (Ra) is another useful measure. Helium 3 is a marker that can document access to the mantle as proved by values on the midoceanic spreading centers. The use of both these isotopes will be shown in the talk.

Example of a Typical CO₂ Deposit

A detailed study was made of a typical CO₂ deposit, Bravo Dome Field, New Mexico, U.S.A., which contains 283 billion cu. meters (10 trillion cu. ft.) of 99% CO₂. It is a combination structural-stratigraphic trap, with Permian Tubb Formation sandstone pinching out on a basement nose and sealed above by anhydrite. Gases were specially sampled and analyzed, revealing a dynamic gas deposit in which the noble gas content varies systematically across the field from near mantle values in the west, far above the gas-water contact, to higher concentrations in the east at the gas-water contact. We interpret that CO₂ entered the lowermost sandstone on basement at the west side of the field from a basalt dike below, sweeping the connate water of the sandstone down-dip as the trap filled. The field is a window to the mantle because mantle gases are preserved to the west, while in the east, atmospheric and crustal noble gases enter the CO₂ from the water below. The CO₂ of the deposit is dissolving down-dip into the water. That the CO₂ of Bravo Dome Field is clearly of magmatic origin is shown by d ¹³C values of - 3.7 to -5.1 ⁰⁄₀₀ PDB in the CO₂ gas, by the relationships of noble gas concentrations, by the isotopic ratio ³He/⁴He being as high as 4.26 Ra, and by the high CO₂/³He ratio. This will be illustrated by maps, charts and graphs.

End_Page 17---------------

Exploration Use and Conclusions

Examination of other CO₂ deposits worldwide will be shown to illustrate that a general model of generation of CO₂ deposits is possible; the use of the model in exploration to either avoid or find CO₂ will be explained.

We conclude that, in general, CO₂ trapped in sedimentary rocks came from the mantle. Fractures in the crust in areas of 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, forms CO₂ deposits. CO₂ is unrelated to hydrocarbons, migrating separately and at different times.

End_of_Record - Last_Page 19---------------