Petrovic and two co-authors reported in early 2008 that hydrocarbons leaking from subsurface petroleum reservoirs can create a reducing environment in the stratigraphic columns and soils above the reservoirs, and thus induce alteration of the rock and soil. This study combined different remote sensing and geochemical tools to detect, map, and analyze rock alteration caused by leaking hydrocarbons in Lisbon Valley, Utah. The authors concluded that the main pathway for the leaking hydrocarbons was the Lisbon Valley fault, which cut its way through the center of an anticline, and other smaller faults. This assumption is based on the proximity of altered zones to the fault zone.
The authors support the hypothesis that the alteration in the Wingate Sandstone is the result of hydrocarbon micro-seepages, based on the geochemical properties of the bleached rocks. All remote sensing processing techniques used for ASTER image classification, and its fusion with SAR radar data, were successful in mapping the alteration zones. The best results were obtained from the supervised classification and the fusion of ASTER and radar
data.
Differences within the Wingate Sandstone in Lisbon Valley, Utah, in the form of bleached beds were detected on the southwest and northwest part of the Lisbon Valley anticline, whereas the middle part appears to be unaltered. Point counting in thin sections showed a presence of significant amounts of clay minerals (9.6–14.9 %), and the lack of hematite cement in altered or bleached parts of the formation. The distribution of clay minerals suggested that they were formed by alteration of different feldspar minerals. Some feldspars showed a significant degree of alteration, with clay along the cleavage planes and edges. This formed a cloudy region, which in some places graded into well-developed secondary clay minerals. Also, spectroscopy showed that selected bleached samples from the Wingate Sandstone had significant abundances of clays, hydroxyl ions, and ferric ions, as well as calcite. The ICP analyses agreed with the thin section analysis with regard to the abundance of dominant minerals. It also revealed the presence of some minor and trace elements (nickel, vanadium, zinc, chromium) in concentrations that are in the range similar to their abundance in oil field fluids.
Results presented in this study affirm that bleaching and other large-scale alterations have been caused by leaking hydrocarbons. Remote sensing techniques proved to be successful when it came to distinguishing between altered and unaltered Wingate sandstones, as well as distinguishing the degree of alteration in some areas. If further analyses fully confirm this concept, than this type of research can be used as a new tool for petroleum investigations and explorations in areas where field work is difficult.
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