Magmatic CO2 in natural gases in the Permian Basin, West Texas: identifying the regional source and filling history

The origin of regional CO2 occurrences within basinal fluids cannot be reso lved with stable isotopes alone because magmatic and crustal delta C-13(CO2 ) ranges are non-unique. He-3/CO2 from magmatic systems such as mid-ocean r idges fall within a tight range (10(9)-10(10)) compared with values found i n crustal fluids (10(5)-10(13)). Although crustal fluids with He-3/CO2 with in the magmatic range clearly have a significant magmatic component, the po ssibility of crustal CO2 addition or reactive loss of CO2 makes a quantitat ive assessment of the magmatic contribution non-trivial. In this paper we p resent a combination of high-precision delta C-13(CO2) and He-3/CO2 from th e CO2-rich (20-55%) JM-Brown Basset gas field, West Texas. In this field He -3/CO2 is in the magmatic range and varies between 4.1 and 6.2 x 10(9). Thi s variation is directly correlated with %CO2. We use the very small range i n delta C-13(CO2) (-2.70 to -3.06 parts per thousand) to rule out either pr ecipitation or crustal addition of CO2 being the cause of the He-3/CO2 vari ation. He-3 has no significant crustal sources or sinks, A model of variabl e magmatic outgassing (up to 20%) is consistent with the range in observed He-3/CO2 and delta C-13(CO2). Within the context of this model, all of the CO2 in this gas field is magmatic. Furthermore samples preserving the highe st. He-3/CO2 represent the earliest phase of magmatic degassing and provide a temporal and spatial filling history for the gas field. In the regional context we show that our results are consistent with a regional magmatic CO 2 source to the North of the Marathon Thrust Fault system. This CO2 must ha ve been in place before significant generation of CH4 and other hydrocarbon s filled these reservoirs from the same direction, overprinting the magmati c CO2. (C) 2000 Elsevier Science B.V. All rights reserved.