Mapping thermal maturity in the Chainman shale, near Eureka, Nevada, with Landsat Thematic Mapper images

Abstract

The purpose of this study was to determine if there is a correlation between measurements of organic matter (OM) maturity and laboratory measurements of visible and near-infrared spectral reflectance, and if Landsat Thematic Mapper (TM) images could be used to map maturity. The maturity of Mississippian Chainman Shale samples collected in east-central Nevada and west-central Utah was determined by using vitrinite reflectance and Rock-Eval pyrolysis. To establish the relations between maturity and spectral reflectance in the visible and near-infrared (VNIR) wavelength region, VNIR reflectance spectra of fresh and weathered whole-rock sample surfaces were measured in the laboratory. The spectra were convolved digitally with the Landsat TM filter band passes to facilitate th analysis, especially the relative sensitivities of individual band passes and ratios of band passes to spectral reflectance variations related to OM maturity. With increasing maturity, overall VNIR diffuse reflectance and mineral-absorption-feature intensities decrease, and the shape of the spectra changes from concave-downward to nearly flat. The spectral shape differences between mature and supermature samples remains distinctive in reflectance spectra of weathered surfaces. TM 4/TM 5 values correspond well to vitrinite reflectance and hydrogen index variations, and therefore this ratio was used to evaluate a TM image of the Eureka, Nevada, area for mapping thermal maturity differences in the Chainman Shale. First, the contribution of vegetation to the TM response was minimized using a linear regression technique, and then a TM 4/TM 5 density-sliced image was produced. Field evaluation of the TM 4/TM 5 density-sliced image shows that all the high values in the Chainman Shale, which correspond to high maturity, are located in the Diamond Mountains; in contrast, Chainman Shale in the northwestern Pancake Range exhibits low to moderate values. These results are consistent with published local maturity determinations. Locally, the presence of limonitic arenaceous exposures and colluvium causes anomalously low TM 4/TM 5 values, but these areas can be identified in TM images because of their diagnostic VNIR reflectance spectra.