A Mesoproterozoic (~1.25 Ga) ‘fossilised’ oil column in the Moroak Sandstone of the Beetaloo Sub-basin, NT

Abstract

The Beetaloo Sub-basin hosts a large unconventional ‘shale gas’ resource within organic-rich Mesoproterozoic shales of the Velkerri and Kyalla formations; however, little has been done to understand conventional oil charge to the associated sandstone reservoirs. Using Grains containing Oil Inclusions, we show that an ancient ‘fossilised’ oil column once existed at the top of the Moroak Sandstone in the Elliott-1 well. The column had a minimum height of 18.02 m, with a possible paleo–oil–water contact at the base of a resistive zone on logs at ~1348 m and a change in core colour. Fluid inclusion oil is entrapped in quartz cement and is inferred to be a light oil of >42°API gravity from its near-blue fluorescence colour. Post-oil solid bitumen is still present in the reservoir sandstone, and we assert that this helped to retard the crystallisation of quartz cement that elsewhere occluded porosity. From burial history models that utilise a refined source rock maturity evaluation method, we claim this to be the remnant of Australia’s oldest yet known conventional oil accumulation at ~1.25 Ga. Analogous bituminous paleo-reservoirs from the North China Craton, dated at 1327 ± 2 Ma, were likely sourced from similar oil-prone cyanobacterial shales that formed during their shared connection with the North Australian Craton on the ancient supercontinent Nuna during the Mesoproterozoic.