Seismic Modelling of Sandstone Intrusions With Unresolvable Geometry

Abstract

Seismic modelling of outcrop data from a sand injection complex enables detection of subseismic sandstone intrusions, but the geometry and orientation of individual intrusions remain unresolved. Sand injection complexes are increasingly recognised as a common shallow-crustal process, comprising millimetre- to decametre-scale, close to bedding-concordant sills and strongly bedding-discordant dykes, as well as intrusions with less regular geometry. These features can act as basin-scale fluid migration conduits, hydrocarbon reservoirs and possible sites for CO₂ sequestration. This paper presents 2D point-spread function (PSF) seismic modelling of a digital outcrop model containing a wide range of intrusion geometries, including thin, complex and interconnected features. The results provide insights into the seismic response of subseismic (unresolved) geological features and enable evaluation of the effects of illumination, lateral resolution, dominant frequency and noise on seismic imaging. Multiple densely spaced thin intrusions generate interference as a function of wavelength, producing complex seismic patterns caused by the dense spacing and cross-cutting geometry of intrusions. The seismic patterns show little resemblance to the geometry of the intrusions. Increases in dominant frequency improve the resolution and interpretation of large intrusions from seismic data and preferentially intensify some seismic characteristics, sometimes creating bedding-like, sub-horizontal features that do not exist in the outcrop data. This ambiguity caused by enhancement of sub-horizontal intrusions relative to sub-vertical intrusions can lead to misinterpretation of sandstone presence and distribution. Individual intrusions with a thickness of 1 m may be detected under favourable conditions but are not directly resolvable in seismic data and increased dominant frequency does not necessarily result in improved geological interpretation. High-angle dykes (> 45^o) display linear zones with amplitude dimming, which are attributed to their cross-cutting character, thus facilitating their interpretation. Seismic amplitudes from host strata interact with those of intrusions, diminishing the clarity of the seismic response of intrusions. Limited illumination reduces the accuracy of interpretation. The addition of noise increases the complexity of intrusion-related seismic responses, both enhancing and reducing amplitudes associated with intrusions, specifically in intervals with complex intrusion networks.