Petrovic and two co-authors reported in 2008 that long-term hydrocarbon micro seepages develop diverse arrays of chemical and mineralogical changes in rocks and soils. Alterations that are reported to have affected the Wingate Sandstone, Lisbon Valley, Utah, include bleaching of red beds, precipitation of high limonite and carbonate concentrations, anomalous weathering patterns, and high clay content.
To investigate sandstone color alteration patterns, spectroscopy, ASTER multispectral remote sensing and Synthetic Aperture Radar (SAR) data were used as well as geochemical analyses of selected samples. Using ASTER satellite images and SAR data, the altered areas of the Wingate Sandstone were identified and mapped at the northwest and southwest part of the Lisbon Valley anticline. Based on sandstone color alteration patterns three different zones were identified in the Wingate Sandstone; the southwest zone is the most altered, the middle part is unaltered, and the northwest part shows minor alteration.
Spectroscopy and thin section studies revealed higher content of clay minerals in altered than in unaltered zones. Major elemental concentrations suggest removal of iron cement and addition of calcite during diagenesis. Trace elemental concentrations in selected altered sandstone samples are characteristic of geochemical halos forming over oil fields. The methodology being developed in this work can provide a completely new technique for petroleum exploration in remote regions of the world where field work is difficult. [It may also have application to uranium exploration.]
The authors concluded 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 geo- chemical tools to detect, map, and analyze rock alteration caused by leaking hydrocarbons in Lisbon Valley, Utah. It is believed 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. This research supports the hypothesis that the alteration in the Wingate Sandstone is the result of hydrocar-
bon microseepages, based on the geochemical properties of the bleached rocks.
All remote sensing processing techniques used for ASTER image classiï¬cation, and its fusion with SAR radar data, were successful in mapping the alteration zones. The best results were obtained from the supervised classiï¬cation 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 signiï¬cant 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 signiï¬cant 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 signiï¬cant 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 ï¬elds.
Results presented in this study afï¬rm 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 conï¬rm this concept, than this type of research can be used as a new tool for petroleum investigations and explorations in areas where ï¬eld work is difï¬cult.
[Also see additional papers below this URL from ResearchGate on similar subjects]
[Also see the first page of the article by Segal, et al., (1986) "Remote Detection of Anomalous Mineralogy Associated With Hydrocarbon Production, Lisbon Valley, Utah, in The Mountain Geologist, Vol. 23, No. 2 (April) p. 51-62]
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