While exploring for hydrocarbons in rift-related basins, volcanic and volcaniclastic rocks and their erosional products are commonly encountered lithologies. At the reservoir level, the presence of rock types derived from volcanism or affected by post-volcanic re-deposition leads to highly varied complex lithologies with wide ranges of inherent rock properties.
In the Santos Basin, a number of exploration wells have encountered basalts emplaced along the basin hinge line that have controlled the sedimentation and basin geometry. The main reservoir units of the basin consist of complex Upper Cretaceous sandstones deposited during periods of active volcanism. For one of the main exploration targets, the Senonian Ilhabela Formation, integrated geological and geophysical interpretation of recently acquired seismic and well data has led to a refined sedimentological model, an enhanced AVO model as well as a deeper understanding of the petroleum system. The encountered sandstones are of shallow-marine and deltaic origin, with local occurrence of mafic volcanic and subvolcanic rocks. Dependent on the presence of volcanic rocks in the vicinity of the particular reservoir, two different types of sandstones with different AVO characteristics have been identified. The presence of volcanic rocks may be interpreted from seismic data by their distinct geometry, usually high amplitudes and unique AVO behaviour, expressed as a strengthening of positive amplitudes with increasing offset.