PGS Access™ Westralia: Quantitative Interpretation through Well Log Integration

PGS Access™ Westralia is a multi-disciplinary 2D geoscience study with the aim of offering new insights into the hydrocarbon potential and prospectivity of the Westralian Superbasin of the Australian North West Shelf geological province.

A key step to extracting maximum value across an area close to 850 000 sq. km is the harmonization of well information with the 11 776 km of high-fidelity prestack seismic data (Figure 1). The project facilitates preliminary basin screening for future exploration in a prospective region, and high-resolution well logs provide the essential link to calibrate the migrated prestack seismic image into geologically plausible models.

Figure 1: The stream-lined Access™ Westralia interpretation workflow highlighting the multi-disciplinary combining Regional Geology with Petrophysics/Rock Physics and Geophysics/Reservoir ImagingFigure 1: The streamlined Access™ Westralia interpretation workflow highlighting the multi-disciplinary combining Regional Geology with Petrophysics/Rock Physics and Geophysics/Reservoir Imaging

The first step, and the most important for a regional-scale study, was the careful selection of well control across the entire survey footprint. The confidence increases when well data points are near enough to the regional seismic grid to perform a seismic-to-well tie (Figure 2), although the final well selection is not always governed by their geographical position, but is strongly dependent on log data availability and quality. 

Figure 2: Example of updated partial-stack seismic-to-well ties based on the application of targeted reservoir-oriented processing to ensure AVO-complianceFigure 2: Example of updated partial-stack seismic-to-well ties based on the application of targeted reservoir-oriented processing to ensure AVO-compliance

 

The objective of this well selection process was to identify and select those with shear-wave information to derive robust relationships between reservoir properties and elastic parameters (Figure 3). These wells were then passed through a consistent petrophysical workflow over the entire log interval (and not just at the reservoir level), with the subsequent interpretation being used to evaluate lithology, reservoir properties, and fluid content. The depth-dependent, statistical approach was used for rock physics analysis. The resultant multi-dimensional rock physics probabilistic space was subsequently sampled within a depth dependent Monte-Carlo scheme (stochastic forward modeling) to generate a wide range of possible outcomes and to derive statistical conclusions not limited to scenarios (lithology and fluid combination) encountered in the wells. This stochastic forward modeling provides invaluable, direct information which ultimately drives our Quantitative Interpretation (QI) workflow decisions.

 Figure 3: Example of depth-dependent rock physics analysis (lithology and fluid) at the approximate Vulcan-Plover Formation level, Browse BasinFigure 3: Example of depth-dependent rock physics analysis (lithology and fluid) at the approximate Vulcan-Plover Formation level, Browse Basin

Our hand-picked selection of 20+ conditioned wells provides a good spread across the variable velocity regimes of the Westralia Superbasin, but there are obvious limitations to creating a reliable low-frequency model for absolute seismic inversion over such a vast area. Few lines have access to good quality, nearby well information, making it challenging to calibrate the prestack seismic inversion with the same confidence everywhere. For these more data-constrained areas, we were able to leverage the prestack fidelity and broad bandwidth of the GeoStreamer® gathers and employ an additional AVA screening workflow. This workflow is entirely data-driven, transforming the seismic input into relative Ip, Is and Vp/Vs based on Shuey’s approximation of AVO reflectivity. Reasonable conclusions could, therefore, be drawn in areas with limited to no well control, as we have also successfully demonstrated with intersecting 3D surveys in the region.

This screening workflow (relative elastic attributes estimation) was performed for the full 11 776 km of reprocessed 2D along with the generation of Intercept, Gradient and IxG attribute SEGY’s to complement the AVO compliant N/M/F/UF and relative amplitude stacks. The aim was to leverage the improved data quality to further investigate the oil potential of the Inner Browse fairway by calibrating to key wells across the Yampi Shelf based on our petrophysical study.

The interpretation integrates all relevant information including wireline and non-wireline logs, lithological descriptions, core analysis, petrographical studies and well test information, supplemented by the in-region experience of the PGS Reservoir team.

By using a multi-disciplinary reservoir approach, geoscientists at PGS are gaining new insights into both the regional- and reservoir-scale geology in Australia’s North West Shelf offshore margins. The PGS Access™ Westralia project demonstrates the value of broad data integration for extracting maximum information. The additional insight gained from this integrated product will lead to better decision-making as to where to best invest time and resources along the margin, and increase the likelihood of exploration success.