Unravelling the Triassic Mungaroo Formation within North Carnarvon Basin using Regional Stratal Slice Volumes

Keywords: maximum flooding surface (mfs), sequence boundary (sb), transgressive surface (ts).

The Northern Carnarvon Basin (NCB) along the NW margin of Australia covers a vast area of approximately 535 000 km² and represents Australia’s leading hydrocarbon province. The majority of Chevron interests fall within the Exmouth Plateau, Barrow sub-basin and along the Alpha Arch and Rankin Platform. This area is blanketed by a patchwork of 3D seismic surveys of various vintages, processing types and data sources presenting a significant challenge for regional exploration studies. Chevron’s main exploration targets occur in the 4+ km-thick succession of alluvial to shallow-marine deposits of the Late Triassic Mungaroo and Brigadier Formations and the overlying Early Jurassic North Rankin Formation. This interval is characterised by relatively isopachous strata that was later deformed during Jurassic and early Cretaceous rifting events (Fig. 1).

Fig. 1.  Seismic section illustrating the isopachous nature of the Late Triassic Mungaroo Formation and variable thickness of the Jurassic and Early Cretaceous rift related stratigraphy. Inset: TWT structure map (red = shallow, blue = deep) illustrating the complex nature of the Jurassic to Early Cretaceous rifting events.

The vast geographical scale and the diversity of available seismic data led the Chevron Australian Business Unit (ABU), to investigate ways to merge imaging results from adjoining seismic surveys in the Mungaroo Formation with the goal of providing an un-interrupted view across the basin. A regional amplitude calibration methodology was developed to account for amplitude imbalances between surveys allowing relative amplitude differences to be preserved. This work evolved into the development of the Regional Stratal Slice Volumes (RSSVs) technology currently deployed in the Chevron ABU. The primary goals of the RSSV project were to identify new exploration opportunities and provide a way to de-risk reservoir, trap and seal presence.

Improved regional understanding of the distribution of sequence boundaries, regional marine flooding surfaces and transgressive events through detailed biostratigraphic analysis (e.g. Payenberg et al. 2013; Marshall and Lang 2013) improved regional seismic correlations and provide a robust framework for regional stratigraphic imaging in the Mungaroo. Five regionally extensive seismic surfaces (maximum flooding and transgressive surfaces) within the Mungaroo were systematically mapped within this framework across the basin and were subsequently used in stratal flattening.

Due to the relatively isopachous nature of the Triassic Mungaroo/Brigadier Formations, supported by multiple regional flooding events, proportional flattening was the preferred method for constructing the Mungaroo RSSV. The main Jurassic break-up unconformity eroded many horst blocks where exploration potential exists. To flatten data where the regional horizons have been eroded, a systematic re-construction process was employed.

A set of attributes from near and far angle-offset stacks were computed for individual surveys and combined into a merged regional mosaic forming an RSSV. As part of this process a proprietary survey alignment methodology was used to align stratal slices between surveys to ensure synchronisation of stratigraphic imaging between surveys. RSSV attributes were selected based on several criteria. Optical stacking of near and far amplitude data were selected to mitigate the ‘porpoising’ effect encountered as the stratal slice departs from datum surface control. These displays also aided in evaluation of polarity reversal and AVO class identification. Spectral decomposition which incorporates a thicker vertical window was selected as it improves channel belt continuity, enhances shoreline identification and aids in environment of deposition (EOD) interpretation (Fig. 2).

Fig. 2.  Group of RSSV attributes used for EOD interpretation and subsequent prospecting work.

Paleo-environments, interpreted through the integration of biofacies (such as marine fauna and plankton associations, terrestrial versus marine palynomorph assemblages, freshwater algae, composition and preservation of organic matter and the grain size of organic particles) and RSSV-based seismic stratigraphy enabled confident interpretation of shoreline and marginal marine to non-marine boundaries over a 30-million-year time period throughout the Triassic Mungaroo. These boundaries were used to produce regional scale EOD maps showing marine, marginal marine and non-marine depofacies belts (Fig. 3). Stacking of the EOD maps highlighted how these facies belts have shifted laterally through time. The temporally-systematic fluctuations of the Mungaroo shoreline observed in the RSSVs, combined with similar trends observed in the biofacies, have clarified ABU’s understanding of the sequence stratigraphic significance of well-based regional stratigraphic framework picks. This new framework provided a predictive stratigraphic context for the basin, which can be applied to not only the shallow, well-imaged portion of the basin, but also to the deeper, less well-imaged portion of the basin. Within this framework, maximum regressions (sequence boundaries) tended to be associated with wider, thicker, more proximal deposits within Chevron’s acreage, while maximum transgressions (maximum flooding events) were associated with less-wide, thinner and more distal deposits.

Fig. 3.  (a) Seismic geomorphology combined with biostratigraphic data allow for a robust interpretation of paleo-shorelines (red line) and marginal marine to non-marine transition lines (white line). The result of this work separates the RSSV into three distinct environmental zones: marine, marginal marine and non-marine. (b1) Spores/pollen/freshwater algae: indicative of non-marine depositional environments. (b2) Dinocysts/acritarchs: indicative of marginal marine depositional environments. (b3) Nannofossils: indicative of marine depositional environments. (c) Temporal variations in the location of environmental zones results in a systematically changing vertical succession of depositional environments throughout the study area – as demonstrated in example well data.

Progradational and retrogradational trends in the shoreline position, when combined with distinct basinward and landward shifts in biofacies, have greatly increased Chevron’s understanding of the stratigraphic evolution of the area. Shoreline regressions were characterised by periods of active delta-building with broad, well-developed marginal marine zones. During periods of active transgression, the marginal marine zone is often cannibalised by the quickly advancing shoreline, which juxtaposes non-marine and marine zones, possibly allowing for the deposition of fluvial sands directly into a marine environment.

RSSV attribute volumes were combined with automatically generated closures integrating stratigraphy with structure/trap presence and provided a platform to screen for prospectivity. Knowledge gained from improved stratigraphic and depositional environment interpretations provided valuable information for play fairway identification, lead and prospect evaluation and reservoir modelling inputs.

The use of RSSVs has provided a step change in understanding the Mungaroo Formation of the NCB, an area comparable in size to the Gulf of Mexico. RSSVs have enabled the visualisation of detailed geomorphological information across a large region of the NCB, which in turn provides insight into reservoir and seal distribution, understanding of regional changes in AVO behaviour and understanding of chronostratigraphic/lithostratigraphic relationships throughout the Mungaroo section without having to map a significant number of horizons. RSSVs have enabled the rapid screening of the 4+ km thick Mungaroo Formation for prospectivity and the population of robust channel dimension data bases. It has also provided stratigraphic information to populate static reservoir models, and assisted in analog selection for volumetric inputs in prospect reviews. The RSSVs, in conjunction with biostratigraphy, have led to play and sand fairway prediction, seal and reservoir risk assessment, and a better understanding of how the quality of these units are related to EODs.

The authors declare no conflicts of interest.