Estimating reliable earth properties using PGS Ultima
De-risking potential prospectivity in frontier areas of Canada’s petroleum basins
Seismic attributes are widely used in hydrocarbon exploration and play a key role in prospect identification. Pre-stack seismic inversion has typically been the solution to derive earth properties, particularly velocity ratio and reflectivity derivatives, which are then used to calculate various attributes. Traditionally, a sequential workflow using Full Waveform Inversion (FWI) followed by Least- Squares Reverse Time Migration (LS-RTM) has been employed to invert for subsurface velocity and reflectivity models. Recently, PGS introduced a new seismic inversion scheme that combines both inversions into a single process, PGS Ultima.
A key aspect of the novel approach is the separation of the low- and high-wavenumber components of the earth model, enabling the simultaneous update of the velocity and reflectivity with minimum crosstalk. The approach is equivalent to performing FWI and LS-RTM simultaneously, where both velocity and reflectivity are continuously updated at each iteration. The iterative inversion compensates for incomplete acquisitions and varying illumination in the subsurface to provide true-amplitude earth reflectivity.
Extracting reliable earth properties
Using a wave equation parameterised in terms of velocity and reflectivity removes the need for a density assumption in the PGS Ultima multi-parameter inversion process. Velocity and reflectivity outputs from the inversion can in fact be used to extract additional properties, such as relative impedance and relative density, for prospectivity assessment in a reliable and data-driven approach.
Inversion for prestack reflectivity
The simultaneous inversion workflow has recently been extended to the pre-stack angle gather domain, which is crucial for improving our understanding of subsurface elastic properties. A key aspect of this approach is the extraction of angle information using elements obtained from the solution of the reflectivity-based wave equation.
The inverted velocity and reflectivity models along with the derived relative impedance and density, and inverted pre-stack angle gathers, provide reliable information for subsequent amplitude versus angle (AVA) analysis and quantitative interpretation (QI).
Application in a frontier exploration area
PGS’ South Bank 3D seismic survey is located in the Salar Basin, which is an Early Cretaceous, isolated rift basin with passive margin fill from Late Cretaceous period and onward. Many fan systems have been identified along the margin using existing seismic data. They are interpreted as Oligocene in age, and the main prospectivity is believed to lie in these fans originating from the shelf and shelf-edge deltas. Class II and Ip anomalies are observed in the reservoir interval, along with class IV responses in the deeper section analogous to a modeled source rock in the region.
Figure 1 shows the PGS Ultima velocity model and stacked reflectivity from angle gather output from the simultaneous inversion. The resolution in the velocity model allows accurate spatial positioning of seismic events, while the reflectivity output aids improved stratigraphic and quantitative interpretation.
The following two figures (Figure 2) represent estimation of relative Vp/Vs ratio over a key prospect, which has three vertically stacked levels. The difference between the images is that the section on the top was produced in a conventional flow using the final and fully processed Kirchhoff migration data, whereas the bottom section is using PGS Ultima angle-dependent reflectivity. Note that the input to PGS Ultima was limited spatially. Yet, the PGS Ultima Vp/Vs response has an improved signal-to-noise ratio and good top and base definition of the layers compared to the Kirchhoff migration output.
A reliable solution
The simultaneous inversion products, i.e., velocity, angle-dependent reflectivity, and the derived relative impedance and density, improve individual lead evaluation and provide better property constraints for QI analysis and anomaly interpretation. The high-resolution velocity model constrains lithologic relationships in the subsurface, resulting in higher confidence quantitative analysis. As such, PGS Ultima assists in de-risking potential prospectivity in frontier areas of Canada’s petroleum basins.
