ABSTRACT

Researchers cite the Patrick Draw, Wyoming, section of the Joint NASA/Geosat Test Cast Project as an example of a vegetation community influenced by hydrocarbon microseepage. The community presumed to be so influenced is visible as a tonal anomaly on satellite imagery. This study and the NASA/Geosat study indicate that evidence supporting this assumption is not yet substantiated. Examination of the dominant sagebrush subspecies at Patrick Draw shows its general growth form is stunted both inside and outside the study area. Therefore, stuntedness is not a specific indicator of hydrocarbon microseepage. The sparse distribution of vegetation results in more soil exposure, and this, rather than the stuntedness of the sagebrush, best accounts for the appearance of the area as a tonal anomaly on imagery. Air photos and images taken on various dates indicate that vegetation began to deteriorate to the point that it became visible on unenhanced imagery sometime between 1978 and 1983, corresponding to a period of increased annual average precipitation. Soils within the anomaly had a high sodium absorption ratio compared to soils outside the anomaly, indicating soil osmotic factors may be stressing the vegetation. Soil gas surveys and a biogeochemical survey provide the strongest evidence that the area is influenced by hydrocarbons. However, the die-off of sagebrush and interspace grass, and hence, the appearance of the tonal anomaly on imagery, may be specifically related to anaerobic conditions in the root zone and/or dissolved salts in the soil-water solution resulting from the increased precipitation. The coincidence of a fault and contact of the Wasatch and Fort Union Formations within the anomaly may explain why the anomaly occurs where it does, but we do not see distinctive evidence linking when the anomaly appeared on imagery to when gas and water reinjection occurred in wells.

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