Air-Xe enrichments in Elk Hills oil field gases: role of water in migration and storage

Hydrocarbons from the Elk Hills Naval Petroleum Reserve (NPR#1), Bakersfiel d, CA, are enriched in heavy noble gases. The Xe-132/Ar-36 ratios are as hi gh as similar to 576 times the ratio in air and represent the largest relat ive Xe-enrichments ever observed in terrestrial fluids. The Xe isotopic com position is indistinguishable from air. We show that these samples cannot b e explained by equilibration of oil with air saturated water and secondary enrichment via a Rayleigh distillation gas stripping process. Based on labo ratory studies of others with potential petroleum source rocks, we believe the source of this enriched heavy noble gas component was adsorbed air init ially trapped in/on the source rocks that was expelled and mixed with the h ydrocarbons during expulsion and primary migration. Kr and Xe enrichments d ecrease with increasing Ar-36 concentration. We propose a model in which an initial Kr-Xe-enriched hydrocarbon becomes diluted with noble gases extrac ted from air saturated groundwater during expulsion, migration, and storage . The model generates an integrated water/hydrocarbon ratio for the product ion fluid which indicates a minimal role for water in hydrocarbon expulsion and migration. The results are interpreted to provide time/geometrical con straints on the mechanisms by which hydrocarbons can migrate as a separate phase. (C) 1999 Elsevier Science B.V. All rights reserved.