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Remote Sensing and Geophysical Footprints of Microseepages in Brazilian Onshore Basins: Possible Vectors towards Petroleum Plays

Abstract

Petroleum accumulations are not entirely sealed, thus allowing leaks of lighter hydrocarbons (HC) that through a migration process reach the surface and form microseepages. The indirect detection of HC microseepages through remote sensing (RS) is guided by mineralogical and geobotanical alterations that result from interactions between acid reduction solutions (derived from biodegradation of HC) and rocks, soils and vegetation. Acid solutions can also lixiviate K from clays – therefore, low-potassium anomalies in gamma-spectrometry data can be associated to seeps. In such environments, there can also occur an accumulation of uraninite, which is insoluble in reducing conditions. In this way, gamma-spectrometry data tend to present high concentrations of U, as indicative of HC plays. In this context, the objective of this work was to study the behavior of soils and vegetation in the Sao Francisco onshore Basin, central Brazil. The study used orbital multispectral (ASTER) and airborne hyperspectral (ProSpecTIR-VS) data, together with gamma ray spectrometric data, field reflectance spectroscopy data, and gasometry data. It is therefore a pioneering study in tropical terrains, directed to methods of petroleum exploration with a high level of detail, pursuing strategies to vector towards zones with higher possibility to host petroleum plays. Aerial geophysics data combined with multi-spectral ASTER images enabled the extraction of information of regional scope, and were efficient here as a preliminary stage to define potential targets for HC prospecting onshore. Terrestrial geophysical data coupled with hyperspectral imagery and field reflectance spectroscopy provided additional information to define areas of greater relevance from the prospective point of view, showing to have direct relation with gas geochemical anomalies recorded in the area. These anomalies are aligned in the direction NW-SE, thus coinciding with known gas wells and visible gas seeps along rivers. Efforts of this nature are able to reduce exploration costs during the stage of geochemical prospecting. Additionally, they help to understand the changes in geological material and in vegetation that are induced by the presence of gaseous HC in rocks and soils, based on detailed spectral features of interest.