Carlos Cevallos

Basin architecture from high-resolution gravity gradient, magnetic, and seismic data, King Sound, Canning Basin, Western Australia

An interpretation of geologic structure at King Sound in the Canning Basin was completed using airborne gravity gradient, magnetic, and seismic data. During the Late Devonian and Mississippian periods, the elevated part of the basement in the north was rimmed by carbonate reefs and redeposited carbonate debris, whereas in the south, siliciclastic submarine fans and turbidites were deposited along the margin of the basement in a deep-marine environment. Three principal lithologic units were identified from the vertical gravity gradient (GDD) in the basin: (1) the Fairfield Group carbonates of high density are interpreted to be the source of prominent positive gravity anomalies; (2) forereef debris and carbonate clastics reworked from carbonates higher up the slope or from the carbonate platform are interpreted to be the source of medium-density responses; and (3) turbidites, debris flows, and associated clastic basinal sequences of low density are interpreted to be the source of prominent negative gravity anomalies. Depth slices of GDD indicate the channelized nature of turbidite flows. In the lower section of the basin, intrasedimentary intrusives were identified from magnetic, GDD, seismic, and well data. Depth to magnetic basement calculation indicates that the surface of the Archean to Paleoproterozoic basement ranges from 3200 to 130 m (10,499–427 ft) below sea level. The northwest- and northeast-oriented south-dipping faults cut the basement and propagate upward into the sediments. A three-dimensional geologic model constructed for King Sound satisfies all known geologic constraints and is consistent with the gravity, magnetic, seismic, and well data.

Basin Architecture From Gravity Gradiometer and Seismic Data, South-Western Margin of the Fitzroy Trough and Gregory Sub-Basin, Canning Basin, Western Australia

The Canning Basin is an under-explored frontier basin. Most of the available information about the subsurface structure is from ‘vintage’ 2D seismic lines and approximately 250 exploration wells. Buru Energy has acquired a FALCON ® Airborne Gravity Gradiometer (AGG) survey (38,800 km 2 ) over the SW margin of the Fitzroy Trough and Gregory Sub-basin, also covering parts of the Jurgarra and Barbwire terraces, and the Broome and Crossland platforms. A new workflow was used to reinterpret ‘vintage’ seismic data with the aid of the AGG data to produce a geological model. An initial seismic interpretation was performed by Buru Energy. The following integrated interpretation of the AGG, seismic, magnetic, well, and other available data allowed for an improved understanding of sub-surface structures and stratigraphy. A basement structure map, two intra-sedimentary structure maps and a distribution map of interpreted gravity sources, many of which are carbonate reservoirs, were produced. The interpretation of 16 seismic traverses with the assistance of AGG data and validation through 2.5D gravity modelling is a key component to this interpretation workflow. The results, integrated in a 3D geological model, produced with SKUA-GOCAD TM and validated by forward modelling and heterogeneous property inversion in VPmg, show that overlying the basement is a sequence of Ordovician carbonate and shale bearing formations of relatively constant thickness and clear definition in the AGG data. The internal structure of the platforms and terraces is well defined due to low vertical gravity gradient (GDD) values in the fault heave area of these formations. In the northern part of the survey, thickness variations in the Ordovician-Silurian Carribuddy Group are linked to large listric growth faults, which form the WNW Fitzroy Trough trend. These faults, less important in the south, predate the Devonian to Carboniferous faults of the NW Gregory Sub-basin trend. Devonian carbonates have a pronounced appearance in the AGG data. Formed during the Devonian Pillara Extension they define the Gregory Sub-basin. The Pillara Extension was near parallel to the Ordovician-Silurian listric faults, which reactivated as transfer faults. Deposition of the Devono-Carboniferous Fairfield Group was followed by the Meda Transpression. After deposition of Permian sequences in the Gregory Sub-basin and Fitzroy Trough, the Triassic Fitzroy Transpression inverted particularly the WNW trending growth faults in the north as well as the major faults between platforms terraces and troughs.

3D geophysical model of the Glyde Basin, Northern Territory, based on curvatures derived from airborne gravity gradient data

This paper presents automatic 3D geophysical model generation based on equivalent pseudodepth slicing of the shape index of the equipotential surfaces derived from airborne gravity gradient data. The method is carried out in three steps. First, the pseudodepth slices of the vertical gravity gradient and the magnitude of the differential curvature components are generated. Second, the equivalent pseudodepth slices of the shape index are generated. Finally, 3D interpolation is carried out to obtain the final model. The method is applied to FALCON airborne gravity gradiometer data from the Glyde Basin, Northern Territory and compared to an independently interpreted, integrated 3D geological Earth model.

Interpreting 2D seismic with the assistance of FALCON® Airborne Gravity Gradiometer data in the Canning Basin

The interpretation of ‘vintage’ seismic data acquired in underexplored frontier basins is often challenged by their sparse coverage. This example from the Canning Basin illustrates how FALCON® Airborne Gravity Gradiometer (AGG) data greatly enhances the 2D seismic interpretation, facilitating exploration in such frontier basins. The initial seismic interpretation was performed by Buru Energy, and given the ‘vintage’ data, was limited at best. The integration of the AGG, magnetic, well, and other available data allowed the improvement of seismic interpretation. A basement structure map, and two intra-sedimentary structure maps were produced, resulting in an overall geological model. In particular, the initial seismic interpretation of seismic traverses perpendicular to strike across the AGG survey could be significantly improved by using images of the AGG data and AGG profile data (GDD and gD). The AGG data and the structure maps were used to constrain fault locations and depths as well as thickness distributions of geological units. The interpreted seismic traverses were validated by 2.5D gravity modelling, ultimately resulting in a conceptual geological model. This is a key-method to constrain the interpreted geology, providing a more confident interpretation of ‘vintage’ reflection seismic data with sparse coverage.