Quantification of the Giant Triassic Mungaroo Formation Fluvial-Distributive System, NWS, Australia

When producing hydrocarbon or aquifer reservoirs in fluvial deposits, it is notoriously difficult to define reservoir volumes connected to completed well intervals from sparse subsurface data and to forecast production behaviours like initial delivery rates and long-term drawdown curves. A fluvial reservoir architecture classification is used to improve subsurface characterization and is applied to the Triassic Mungaroo Formation, Northwest Shelf, Australia. High-quality 3D seismic data covering large parts of the depositional basin allowed the mapping and interpretation of individual channel-belts through an approximately 2 km thick, Triassic section. Integrated interpretations using seismic and borehole data, suggest the Mungaroo Formation was deposited by very large rivers with low channel sinuosity and low channel belt sinuosity and rugosity traversing very low-gradient flood plains. Mean channel-belt widths of 1100 m (ranging from 150–3000 m) and mean thicknesses of 22 m (ranging from 5–68 m) are used to support the interpretation that these deposits formed in very larger rivers. Along individual stratigraphic intervals, thicker, wider channel-belts with longer edge rugosity wavelength in the proximal part of the system pass down depositional dip into narrower, thinner and lower wavelength rugosity belts. These spatial changes in channel-belt geometry are used to define changes in the fluvial styles across a broad fluvial system spanning hundreds of kilometres. The comprehensive integration of high-quality seismic data with borehole core data, provenance data and scaling relationships has led to a robust paleogeographic interpretation as an overall large, low-gradient distributive river system.