The Beetaloo Sub-Basin in the Northern Territory is a Proterozoic basin filled predominantly with siliciclastics rocks, the deeper sections only found in subcrop. The Beetaloo Sub-Basin hosts significant unconventional hydrocarbon resources within three identified shale reservoir intervals of the Mesoproterozoic Velkerri Formation. The origin and mode of preservation of organic carbon in the Velkerri Formation is currently debated. A greater understanding of the sedimentology and structural geology of the host formations is required to constrain plausible causal relationships within these units.

Borehole image logs were used to investigate the structural geology and sedimentology of the Beetaloo Sub-Basin ranging from the Velkerri Formation through to the Kyalla Formation in the following wells: Kyalla 117 N2 1, Beetaloo W 1, Kalala S 1, Amungee NW 1, and Tanumbirini 1. These wells provide control points within the present day basinal axes and the southern margin of the Beetaloo Sub-Basin at an average correlation length of 55 km. In addition to standard sedimentary bed dip picking, image facies assignment, and interpretation of image facies associations, sedimentological features were identified and mapped including small-scale erosional features (gutter casts) and v–shaped resistive features.

Gutter casts are first abundant at the base of the Wyworrie Member of the Velkerri Formation and continue up into the base of the Moroak Sandstone, appearing again in the lower portion of the Kyalla Formation. V–shaped resistive features first appear in the upper part of the Wyworrie Member and continue up into the Moroak Sandstone. Both v–shaped resistive features and gutter casts disappear where the Moroak Formation becomes dominantly arenaceous.

Image facies analysis indicates that the study interval consists of offshore, turbidite fan, offshore/prodelta, lower shoreface/distal delta front, and upper shoreface/proximal delta front palaeodepositional environments. The appearance upsection of gutter casts followed by v–shaped resistive features, interpreted as synaeresis crack fills, is interpreted to indicate the basinward progradation of the deltaic system; in the more distal position/deeper water depths, erosional scours are formed by either storm-rip currents or hyperpycnal bottom-hugging currents that are then filled by sediment transported in these flows. In the more proximal location/shallower water depths, synaeresis crack fills form by the introduction of brackish water into a predominantly saline environment.

Acknowledgments

ImageStrat would like to thank Origin Energy and Tamboran Resources for permission to present this work.