Chris J. Ballentine¹, Martin Schoell², Dennis D. Coleman³, Bruce A. Cain⁴

(1) IGMR, Zuerich, Switzerland
(2) Chevron Research and Technology Co, Richmond, CA
(3) Isotech Laboratories, Inc, Champaign, IL
(4) Altura Energy LLP, Houston, TX

ABSTRACT: 300 Ma Old Magmatic CO2 Precedes Natural Gas Migration in West Texas

Resolving magmatic CO2 from crustal sources of CO2 is not possible with stable isotopes alone because magmatic and crustal carbon stable isotope ranges of the CO2 are non unique. In contrast 3He is an unambiguous tracer of magmatic volatiles. Combined with stable isotopes we provide clear evidence for a dominant magmatic origin of the CO2 in the JM Brown Bassett Field in West Texas. For example, CO2/3He ratios are indistinguishable from the pristine magmatic range (1-6x109). This range is small compared to the range typically found in natural gases (105-1015), and clearly shows the presence of magmatic CO2 in this foreland basin. Furthermore, Rayleigh fractionation from a degassing magmatic source is consistent with a systematic regional spatial trend in CO2/3He and C13(CO2). This finding has the important geologic implication that filling of the reservoirs with magmatic CO2 has to pre-date hydrocarbon generation in the basin, which started at ~280Ma. Replacement of old CO2 by later hydrocarbon gases is also consistent with a small magmatic 3He component resolved in the CH4 endmember. Isotopic signatures of all gas components suggests that the gases remained in absolutely tight reservoir compartments since the time of entrapment. Such closed system behavior of natural gas in a trapping structure of some 300Ma contradicts the much lower residence times predicted by diffusion models in other systems. Our data, therefore, provides support for the viability of natural gas exploration in deeper, older and therefore more unconventional locations.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado