Bravo Dome CO2 Gas Field, New Mexico, USA and Associated Noble Gases: Type Example of Accumulation of Carbon Dioxide and Window to the Mantle

Cassidy, Martin M.¹, Chris J. Ballentine², Barbara Sherwood Lollar³, James Lawrence⁴ (1) University of Houston, Houston, TX (2) University of Manchester, United Kingdom (3) University of Toronto (4) University of Houston

Bravo Dome CO₂ gas field was studied as a type example of a pure CO₂ gas deposit associated with young basaltic lavas in an extensional regime. The field is a structural -stratigraphic trap containing about 10 TCF CO₂ in Permian Tubb sandstones on basement sealed above by anhydrite. Well logs and data from 450 field wells were available for study. Compositional and isotopic analysis of CO₂ and coproduced noble gases were made of gas from 14 wells across Bravo Dome field. Samples were taken with great care to avoid air contamination. The noble gases ³He, ⁴He, ²⁰Ne, ³⁶Ar, ⁴⁰Ar, and ⁸⁴Kr contents vary systematically across the field as shown in maps. In the west, far from the gas-water contact, noble gas concentrations and other data indicate a mantle source. Helium in the CO₂ has a ratio of ³He/⁴He compared to air (Ra) as high as 4.26. Original gas of mantle composition is preserved in the west. However, in the east of the field noble gases are more concentrated in the reservoir, being left behind by dissolution of the CO₂ in the water at the gas-water contact and by atmospheric and crustal noble gases entering from the water. We conclude that Bravo Dome CO₂ gas is young and of volcanic mantle origin. Noble gas concentrations are changing with time, and the CO₂ is dissolving into the underlying water zone. Generalizing: CO₂ deposits depend upon access to the mantle and evolve with time.