THE INFLUENCE OF COMPOSITION, DIAGENESIS AND COMPACTION ON SEAL CAPACITY IN THE MUDERONG SHALE, CARNARVON BASIN

Abstract

The Muderong Shale blankets most of the northern Carnarvon Basin and is the top seal to over 90% of all commercial discoveries. This study examines the influence that vertical effective stress, mineralogy and diagenesis have on regional variations of seal capacity. Throughout the basin, threshold pressures (determined from Mercury Injection Capillary Pressure (MICP) analyses), range from less than 1,000 psi (equivalent to ~100 m gas column) up to 10,000 psi (~1,000 m gas column). Because the Muderong Shale varies in thickness (5 m to >900 m) and burial depth (~0.5–3.5 km), effective stresses and temperatures also vary. Effective stress and temperature significantly control pore geometry at different depths through compaction and diagenesis. The data from this study show that shale grain size has no direct influence over seal threshold pressure except that finer-grained Muderong Shale (36–45% particles <2 μm) is more compressible than the coarser grain fraction (26–35% particles <2 μm) with increasing vertical effective stress, resulting in higher seal capacities. When the shale reaches a point where pores are less susceptible to collapse under stress (generally at depths greater than 1.5 km in the Muderong Shale) temperature-controlled diagenetic alteration has a significant effect on threshold pressures. Compositionally, the Muderong Shale comprises inter-stratified illite/smectite, quartz, kaolinite and discrete illite, with actual abundances of each varying across the basin and with depth. Quartz-rich Muderong Shale mainly occurs in the inboard areas (Barrow and Exmouth Sub-basins and Southern Alpha Arch) while illite/smectite abundance increases to ~50% in the north of the basin (Rankin Platform, Dampier Sub-basin and northern Alpha Arch). One of the most common mineral reactions within smectitic shales is the transformation of smectite to illite with increasing temperature. This is an important reaction in the Muderong Shale, as variations in threshold pressures 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.