Understanding the petrophysical properties, seismic responses and impacts of the basalt-sediment transition in prospective sedimentary basins

Volcanic rocks form major components in many prospective sedimentary basins around the world and are increasingly being found in association with significant hydrocarbon reserves. Application of seismic forward modelling to volcano-sedimentary sequences has been crucial to cover the gaps between outcrop observations and seismic data. Within this study we combine 3D virtual outcrop modelling from the North Atlantic Igneous Province with laboratory petrophysical measurements from Paraná-Etendeka Igneous Province field outcrop analogues to develop synthetic seismic forward models for four different basalt-sediment transition scenarios. A base-model comprising a lava flow package overlying homogenous sandstone is presented with subsequent models developed to incorporate known field based variations including layered sedimentary rocks, intrusions and invasive lava flows with associated peperite. The models were run using source wavelets with different frequency spectrums in order to assess variations in seismic reflection character and detectability of the volcanic features along with the basalt-sediment transition. The results of this study clearly demonstrate how geological variability associated with the basalt-sediment transition can influence seismic imaging and complicate interpretation. In particular, features such as invasive lava flows may be challenging to differentiate from sill intrusions in seismic data, and therefore their potentially very different impacts on reservoir properties and connectivity must be appraised carefully. This study demonstrates the power of integrating outcrop analogue studies with seismic forward modelling to improve seismic interpretation in prospective mixed volcano-sedimentary systems.