300-Myr-old magmatic CO2 in natural gas reservoirs of the west Texas Permian basin

Abstract

Except in regions of recent crustal extension¹, the dominant origin of carbon dioxide in fluids in sedimentary basins has been assumed to be from crustal organic matter² or mineral reactions^3,4. Here we show, by contrast, that Rayleigh fractionation caused by partial degassing of a magma body can explain the CO₂/ ³He ratios and δ¹³C(CO₂) values observed in CO₂-rich natural gases in the west Texas Val Verde basin and also the mantle ³He/²²Ne ratios observed in other basin systems⁵. Regional changes in CO₂/ ³He and CO₂/CH₄ ratios can be explained if the CO₂ input pre-dates methane generation in the basin, which occurred about 280 Myr ago⁶. Uplift to the north of the Val Verde basin between 310 and 280 Myr ago⁶ appears to be the only tectonic event with appropriate timing and location to be the source of the magmatic CO₂. Our identification of magmatic CO₂ in a foreland basin indicates that the origin of CO₂ in other mid-continent basin systems should be re-evaluated. Also, the inferred closed-system preservation of natural gas in a trapping structure for ∼ 300 Myr is far longer than the residence time predicted by diffusion models^7,8.