Stratigraphic and structural architecture across the central North West Shelf – implications for Triassic petroleum systems

Abstract

The Triassic succession in the Roebuck Basin and parts of the adjacent sub-basins is the subject of a regional mapping program at Geoscience Australia that addresses stratigraphic and structural aspects of exploration risk. Seismic horizons of regional significance are integrated into a new tectonostratigraphic framework that also includes lithostratigrapy, gross depositional facies and tectonic extension phases. The main Triassic depocentre extends over the Beagle Sub-basin and north-east Exmouth Plateau, with a re-entrant into the Bedout Sub-basin. A smaller north-east oriented depocentre is located over the outer Rowley Sub-basin and extends into the Barcoo Sub-basin. The Triassic succession thins markedly between these depocentres over a regional palaeogeographic high. Above a basal marine mudstone, the Triassic succession is mainly fluvio-deltaic, but outboard the uppermost Triassic succession it is carbonate-rich. The Rowley depocentre is unusual in that it comprises Lower–Middle Triassic volcanics and Middle–Upper Triassic carbonate-rich facies. Syn-depositional half graben in the Barcoo Sub-basin extend part-way into the adjacent Roebuck Basin. The Roebuck Basin structural architecture is simpler than the neighbouring sub-basins. The contiguous development of fluvio-deltaic facies across the main Triassic depocentre indicates that the Middle Triassic petroleum system may extend beyond the Bedout Sub-basin. Syn-depositional extension may have formed isolated sub-basins favourable for source rock deposition in the Barcoo Sub-basin. The Triassic carbonate play may also be of interest, as carbonate build-ups are locally well imaged in seismic profiles.

Triassic petroleum systems on the central North West Shelf – Learnings from the greater Phoenix area seismic mapping and geochemical studies

“…The stratigraphic framework has been updated using new biostratigraphic analyses (Hannaford et al, 2016;. Recent seismic and well interpretations ( Figure 3; Abbott et al, 2019;Nguyen et al, 2019;Orlov et al, this volume) were used to produce paleogeographic maps showing the distribution of gross depositional facies (Abbott et al, this volume) and preserved extent of gross depositional facies at key intervals (TR10.0_SB-TR15.0_SB, TR15.0_SB-TR17.0_SB and TR17.0_SB-Top Triassic; Figures 4b, d and f, respectively) to predict source rock facies distribution and their depositional environments. The correlation of organic geochemical data with biostratigraphy and lithology identified regional intervals with source rock potential.…”

The influence of mantle flow on intracontinental basins: Three examples from Australia

“…Regionally, evidence for Triassic igneous activity is reported at the north‐west edge of the Canning Basin (Gleadow & Duddy, 1984; Reeckmann & Mebberson, 1984) and in the Rowley Sub‐Basin (Abbott et al., 2019a; MacNeill et al., 2018). On the north‐west shelf of Australia, late Permian uplift, faulting and tilting was accompanied by extensive intrusion and extrusion of mafic igneous rocks during the Bedout Movement (Forman & Wales, 1981), which is proposed to be either compressional based on observations of faulting, folding and erosion over the Bedout High (Shaw et al., 1994), or extensional (Colwell & Stagg, 1994; Forman & Wales, 1981; Reeckmann & Mebberson, 1984) and possibly caused by active mantle upwelling (Colwell et al., 1994) based on the extent of volcanism.…”

“…Recent (2014) 3D seismic acquisition in the Rowley Subbasin revealed clinoforms dipping 20°-30° landward, interpreted to represent a 10 km thick lava complex that prograded southeast from the Argo/West Burma landmass (Abbott et al, 2019b;MacNeill et al, 2018). Regional stratigraphy suggests the lava flows could be between 250-240 Myr old (Abbott et al, 2019a;MacNeill et al, 2018), and comparable seismic facies from the northwest Rowley Sub-basin may represent younger volcanism (240-210 Ma) (Abbott et al, 2019b). MacNeill et al (2018) suggest an Early-Middle Triassic mantle plume could have produced the lava complexes; which is consistent with our model, in which a plume occurs within 10 Myr and 650 km of the Rowley Sub-basin lava flows (Figure 12).…”

Palaeogeographic evolution of the Triassic succession, central North West Shelf

“…Palaeogeographic mapping is based on revised structural and stratigraphic frameworks for the CNWS (Abbott et al, 2019;Orlov et al this volume). Constraints for palaeogeographic mapping are limited over most of the study area, so a gross depositional facies (GDF) approach was adopted based on extrapolation of well lithology data, seismic facies, and information from analogous intervals in adjacent basins.…”