Volcanic rock characterisation using the concept of Extended Elastic Impedance: A case study from a Middle Jurassic gas reservoir in offshore Western Australia

Successful identification of volcanic rocks is critical in reservoirs where they have been previously intersected. This is because they impact on reserve estimates and influence fluid flow behaviour. Various studies using seismic inversion data were performed to try to characterise volcanic rocks in a sandstone reservoir in the Plover Formation. We noted that traditional techniques such as cross-plots between P-Impedance (Ip) and Vp/Vs was not very effective in this reservoir due to significant facies overlap at seismic resolution and inversion data quality. Therefore volcanic rock identification was attempted using advanced seismic attribute analysis. This involved testing and evaluating other elastic attributes, either individually or in combinations, to try and segregate volcanic rocks from other lithofacies. Two approaches were adopted to find out a suitable single attribute to identify the volcanic rock: (i) scaling of elastic logs with a non-elastic trend; (ii) generating a single attribute using Ip, Is and LMR cross-plots. Log and seismic scale analysis proved the suitability of both methods in volcanic rock identification. Subsequently, Extended Elastic Impedance (EEI) was applied to generate the EEI equivalent of those single attribute yielding positive results.