Integrated Seismic and Rock Physics Modelling of Oil-Producing Volcanic Sills in the Neuquén Basin, Argentina
Igneous sills are commonly observed in sedimentary basins around the world and may have significant influence on the related petroleum systems. Seismic reflection data play a key role for the characterisation of sill complexes. It is often difficult to validate seismic interpretations, because few wells are drilled into intrusions. Furthermore, the comparison of outcrops to kilometre scale seismic images is challenging due to resolution issues. Most of the seismic interpretation work is focussed on mapping intrusion geometries, while quantitative seismic analysis is challenging and rarely performed. In this study, observations from geological fieldwork, 3D virtual outcrop models, core samples and well logs are integrated and linked to qualitative and quantitative seismic analysis of an oilproducing volcanic sill complex in the Neuquén Basin, Argentina. The link is established by performing integrated seismic forward modelling and rock physics modelling. The seismic modelling is based on a realistic, metre-scale geological model obtained from a virtual 3D model of a large-scale analogue outcrop. The rock physics modelling uses fracture properties obtained from fieldwork and core samples to investigate the effect of fractures on elastic properties. Results from seismic modelling show characteristic waveforms with remarkable similarity to observations from seismic field data. These waveforms arise from interference and include branching and steps of reflections. This can be used to interpret corresponding intrusion geometries. Additionally, the seismic amplitudes related to sills can be much more variable than usually assumed. Rock physics modelling suggests that this variability may be partly explained by the effect of fractures on the seismic velocities of the sills. Fracture analysis from outcrop data indicates the presence of several fracture sets of preferred orientations. The corresponding rock physics modelling predicts a marked drop in P-wave velocity as well as P-wave anisotropy of up to 14%. However, with rising complexity of the fracture pattern, the incident angles that are required to recover this anisotropy from azimuthal seismic field data increase from > 30◦ to > 60◦. In fact, such azimuthal anomalies in sills have been observed in real seismic data from the oil fields. The approach presented in this study illustrates how various geoscientific datasets can be linked and thereby used to increase the reliability of qualitative and quantitative seismic interpretations of volcanic sill complexes.