Multi-Disciplinary Approach to Assessing Top-Seal Quality in the Muderong Shale, Carnarvon Basin, Australia

Gillian Kovack
Australian School of Petroleum, The University of Adelaide, Australia
gkovack@asp.adelaide.edu.au

The Muderong Shale is the major regional top seal in the northern Carnarvon Basin. This study examines the influence that mineralogy and diagenesis have on regional and local variations of seal capacity in this shale. Across the basin, capillary threshold pressures range from less than 1000 psi up to 10,000 psi. The Muderong Shale varies in thickness (5m to >800 m) and burial depth (~0.5-3.5 km) and thus effective stresses and temperatures also vary. Effective stress and temperature significantly control pore geometry at different depths through compaction and diagenetic processes. The data from this study show that shale grain size has no direct influence over seal threshold pressure, although finer-grained Muderong Shale is more compressible than the coarser grain fraction resulting in higher seal capacities. When the shale reaches depths >1.2 km, temperature-controlled diagenetic alteration results in a significant increase in capillary threshold pressures in the Muderong Shale concomitant with the transformation of smectite to illite. This is an important reaction in the Muderong Shale as variations in threshold pressure correlate with total illite content (illite in mixed layer illite/smectite + discrete illite). Total illite is highest in the Barrow Sub-basin and Southern Alpha Arch, and lowest in the Exmouth Sub-basin. Although the Muderong Shale is deeply buried (>2.5 km) along the Northern Alpha Arch and Rankin Platform, total illite content is only moderate. Evidence from the Barrow Sub-basin shows that the transition from smectite to illite occurred in the Muderong Shale at temperatures greater than 60˚C and appears to be near completion by depths of 1.6 km (80˚C). Hence seal capacities in the Muderong Shale are controlled by both mechanical and diagenetic processes. While seal capacity is an important control when estimating potential hydrocarbon reserves, a study is proposed to highlight the importance of seal integrity as a potential risk with respect to the Muderong Shale.