Data integration for a geologic model of hydrocarbon microseepage areas in the Tonã Plateau region, North Tucano basin, Brazil

Abstract

A diversified database was used for investigating the major factors controlling the distribution of hydrocarbon microseepage in the region of the Tonã Plateau, North Tucano sedimentary basin, Brazil. Visual analysis of this database suggested the existence of spatial relationships between anomalous geochemical soil gas data with rift faults, lithologic contact, and groundwater discharge areas. As a result, a geologic model was proposed. According to this model, rift faults constitute the primary pathways for the upward migration of gaseous hydrocarbons from a deep-seated source rock in the depocenter of the basin. Cherty limestone layers in the Tonã Plateau act as a seal to prevent the escape of the hydrocarbons to the surface. As a result, hydrocarbon microseepage occurs preferentially in the contact zone of the cherty limestones of the Tonã Sequence with porous sandstones of the Marizal Formation, mainly in areas of groundwater discharge. The importance of these three patterns for controlling the near-surface distribution of the anomalous soil gas samples was estimated through Bayesian probability analysis. According to this interpretation, lithologic contact is the spatial pattern more strongly associated with the anomalous soil gas samples, followed by groundwater discharge areas. Rift faults show the weaker spatial correlation with the anomalous near-surface areas of hydrocarbon microseepage.