Multiple atmospheric noble gas components in hydrocarbon reservoirs: a study of the Northwest Shelf, Delaware Basin, SE New Mexico |
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| Authors: | BM Kennedy T TorgersenMC van Soest |
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| Institution: | 1 Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
2 Department of Marine Science, University of Connecticut, Groton, CT 06340-6097, USA |
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| Abstract: | The Northwest Shelf of the Delaware Basin, SE New Mexico is the site of several large and productive oil and gas fields. The most productive reservoirs are located in the late Pennsylvanian Morrow and early Permian Abo formations. Production from the latter more important play is predominately from fluvial Abo red beds of the Pecos Slope Field. The oxidizing conditions implied by the reddish color of the formation require an external hydrocarbon source. To test the existing migration model for the region and constrain the location of potential hydrocarbon sources, we measured the elemental and isotopic composition of noble gases produced along with the hydrocarbons. We found the hydrocarbons to be highly enriched in radiogenic 4He, 40*Ar and nucleogenic 21*Ne F(4He) = 44,000-250,000; 40Ar/36Ar = 400-3145; 21Ne/22Ne = 0.044-0.071]. The greatest enrichments occur in the Pecos Slope gas fields. The hydrocarbons also contain three independent nonradiogenic noble gas components each with an atmospheric isotopic composition. One component is most likely air-saturated water (ASW). The second component is enriched in the heavy noble gases F(130Xe) > 8.5] and is derived from the hydrocarbon sources. The third component is enriched in Ne F(20Ne) > 0.8] that we believe is degassed from sources within the reservoirs. This component is correlated with but decoupled from the dominant source of radiogenic 4He and 40*Ar. Very high concentrations of 4He (up to ∼1% by volume) in the Pecos slope reservoirs require a source external to the reservoirs, such as the underlying Precambrian basement granites and sedimentary equivalents. Structural buckles cutting through the Pecos field may act as high flux vertical pathways for the radiogenic 4He. If the hydrocarbons in the Pecos slope fields have migrated northward from the deeper Delaware Basin, as suggested by compositional trends, then perhaps the buckles also play an important role in the distribution and filling of the Pecos slope reservoirs. |
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