The complex petroleum system of the Rankin Trend and Brigadier Trend, Northern Carnarvon Basin, Australia. New insights from the Triassic Mungaroo Formation in Ironbark-1.

Ironbark-1 was drilled towards the end of 2020 to test the hydrocarbon potential of the fluvial TR20.0 sub-play in the Lower Mungaroo Fm on the Brigadier Trend. While proven in the Northern Carnarvon Basin this sub-play remained untested in the area. Areally extensive incised valleys containing thick amalgamated sand bodies had been mapped across the Brigadier Trend similar to the Upper Triassic fluvial reservoir systems on the Rankin Trend and Alpha Arch. The pre-drill geological model showed that such sands would provide sufficient reservoir thickness for an economic development, be a precursor condition for porosity preservation at depths over 5 kmBML and form an effective migration path from the modelled gas kitchen. The primary target interval below TR21.1 TS (legacy TRC2, bp T75) was water bearing and sands were significantly less well developed than expected. The 313 m gross interval contained 151 m of relatively thinly bedded very fine to medium grained and extensively cemented sandstone. Gas compositional and carbon isotope analysis indicated the presence of low concentrations of low maturity thermogenic gas. Post-well evaluation and remapping of the channel systems showed that a poorly connected low porosity reservoir system had a significant adverse effect on the hydrocarbon charge. Our evaluation also identified significant differences between the channel systems on the Brigadier Trend and Rankin Trend. Remapping of the Rankin Trend identified a large braidplain 10 km north of the main gas fields and we demonstrated that this braidplain forms a very effective multi-Tcf migration path from the Victoria Syncline to the main producing reservoir in the Goodwyn field. This is a significant new insight as hydrocarbon charge from either the north or south has been debated by industry for several decades. This evaluation shows the importance of detailed mapping of carrier beds using high quality 3D seismic data and integration with the basin model to understand the potential hydrocarbon charge. This work can be especially valuable for prospect evaluation in the absence of direct hydrocarbon indicators.