Multi-disciplinary approach to fault and top seal appraisal; Pyrenees–Macedon oil and gas fields, Exmouth Sub-basin, Australian Northwest Shelf

Abstract

The Pyrenees–Macedon (P–M) fields in the Exmouth Sub-basin of the Northern Carnarvon Basin, Australian Northwest Shelf are currently under-filled relative to available closure despite being a regional focal point for Cretaceous to recent charge. Late structural development of the P–M trap with respect to charge was thought to be the reason for under-filling. However, seismic amplitude anomalies and gas shows above the reservoir suggest vertical leakage may have controlled column heights. Hydrodynamic analysis of pressure data also suggests that faults separating the fields act as barriers to the migration of hydrocarbons and water, whilst faults within the Macedon Field do not. The reasons for hydrocarbon leakage and the difference in fault seal capacities are investigated by integrating field observations, analysis of pressure and stress data, the appraisal of top (mercury porosimetry measurements) and fault (Shale Gouge Ratios; SGR) membrane seal capacities, constraining geomechanical properties (top and fault seals) and well-based fracture analysis. The top seals are at a low risk of capillary failure, but vertical leakage is possible via dynamic failure along pre-existing faults and conductive fractures, and lateral leakage across reservoir against thief zone fault juxtapositions. The difference in observed fault seal capacities between different faults is explained by a combination of the spatial distributions of SGR and buoyancy pressure. The procedure presented delivers a robust description of the key risks concerning reservoir connectivity and the integrity and capacity of seals where static (geological timescale migration) and dynamic (tectonically related flow) conditions must be considered.