| The main objective of this work was to determine if there are characteristic mineral assemblages and chemical changes in areas affected by hydrocarbon microseepages. For this purpose remote sensing was utilized for mapping surficial rock alterations, and geochemical tools were used to understand the alteration processes. The key area chosen for this type of work were altered and unaltered Wingate Sandstone outcrops in Lisbon Valley, Utah. The Spectral Angle Mapper method was applied on HyMap hyperspectral images to classify the extent of altered and unaltered outcrops, as well as to map the changes in mineral content within the outcrops. The Spectral Feature Fitting and Band Ratio methods were used to identify lithological changes in the area. The commonly used band ratios for identification of calcite and kaolinite were modified for future use with multispectral data. Reflectance spectroscopy, thin section studies, major, minor, and trace element analyses, and stable carbon and oxygen studies on both bleached (altered) and unbleached (unaltered) samples were successfully used to delineate areas of similar rock composition and relate changes due to hydrocarbons leaking from underlying petroleum reservoirs. Unbleached Wingate Sandstone samples had higher hematite and feldspar content than bleached Wingate samples, which were characterized by larger amounts of clay, calcite, and pyrite. Some bleached samples also had higher concentrations of elements (U, Mo) characteristic of hydrocarbon-related reducing environments, and were depleted in 13C when compared to the unbleached samples. Based on these results, the following model of chemical reactions is suggested for diagnostic changes within Wingate Sandstone. Hydrocarbon-induced reducing environment caused the transformation of sulfate ion (obtained from groundwater or from oxidation of H2S) to sulfide ion, resulting in the reduction of hematite to pyrite, The released hydrogen ion from this reaction reacted with available feldspars in the rock, leading to precipitation of kaolinite. These conditions favor the reaction between bicarbonate ion and Ca2+ ions that can be obtained from the groundwater, leading to precipitation of calcite in pore spaces left open after the reduction and removal of hematite.;Results from geochemical analyses of Wingate Sandstone samples were compared to the results obtained from bleached and unbleached Dockum Group samples from Garza oil field, Texas. All results showed strong similarity, and could be successfully explained with the same model. This suggests that this model and the techniques that led to its development could be used in other parts of the world with similar surficial manifestations to explain the effects of leaking hydrocarbons on those two formations. |
