Spectral characterization of hydrocarbon seepages in brazilian continental basins

Petroleum reservoirs are not entirely sealed, thus allowing leaks of lighter hydrocarbons (HC) to reach the surface and form microseepages. Gas microseeps are usually undetectable by the naked eye. However, the HC biodegradation changes the pH and Eh of the environment, induces alterations in geological materials (resulting in mineralogical and geophysical anomalies) and causes stress in vegetation. Remote sensing and radiometric data are potentially useful to identify these alterations and to discover new petroleum plays. The goal of this research was to investigate the mineralogical changes induced by oil and gas seeps in Brazilian sedimentary basins under different climatic conditions and geological settings. The Diápiro do Cinzento (Bahia (BA) State) and Remanso do Fogo (Minas Gerais (MG) State) are considered archetype areas in Brazil for oil and gas seeps research, respectively. These areas are located in the Recôncavo (BA) and São Francisco (MG) sedimentary basins. The Recôncavo basin hosts the second largest oil producing fields among onshore basins in Brazil. Despite the large number of productive brownfields, the basin is still the focus of exploration activities. In contrast, the São Francisco Basin is yet in the exploration stage. Several wells recently drilled in greenfields in the basin showed evidences of gas. Gasometric data, airborne and terrestrial radiometric data, reflectance spectroscopy data of soils, rocks and vegetation, X-ray diffraction data, ASTER and RapidEye multispectral imagery and ProspecTIR-VS hyperspectral imagery were employed in the research. Continuum-removed VNIR and SWIR spectra of soils in the Remanso do Fogo area were used to vectorize the location of microseeps based on the absorption bands of kaolinite and iron oxides/hydroxides. Vegetation stress was mapped using both ASTER multispectral and ProSpecTIR-VS hyperspectral data. Ground gammaspectrometric data collected around proven microseeps show loss of potassium and enrichment of uranium, as verified in several sites. In the Diápiro do Cinzento, the mineralogical assemblage of jarosite-pyrite-goethite is intrinsically related to hydrocarbon degradation. Such minerals, which indicate acidic and sulfate-rich conditions, were promptly detected by reflectance spectroscopy and X-ray diffraction. Far from the macroseepage zone, iron minerals such as hematite and goethite predominate. RapidEye multispectral images were effective for mapping these minerals remotely in the Diápiro do Cinzento, reflecting environments with different pH conditions. The main contribution of this research was to improve the knowledge of mineralogical changes induced by oil and gas hydrocarbon seeps in Brazilian sedimentary basins formed under different tectonic settings. Additionally, this research demonstrated the usefulness of remote sensing data yielded at various spectral and spatial resolutions to map the seepage phenomena. Evidences at multiple scales show that individual seeps can be more spatially restricted that previously envisaged and display specific mineralogical and geobotanic features in a small area. The overall results of the research offer new perspectives for petroleum exploration in Brazilian offshore basins (AU)