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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 2.5 km) along the Northern Alpha Arch and Rankin Platform, total illite content is only moderate.

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In this paper we present a new approach to the estimation of Thomsen anisotropy parameters from laboratory data on cylindrical rock samples. Using tomography-style transducers array, ultrasonic P-wave ray velocities are measured on a transversely isotropic shale sample. This approach is applied to core samples cut along and normal to the bedding plane. Synthetic and actual laboratory data from an anisotropic shale specimen are used as examples. The fast simulated re-annealing method is used to search for the anisotropy parameters.

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