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

Abstract

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.