Seal failure assessment of a major gas field via integration of seal properties and leakage phenomena

We present a seismic- and well-based interpretation of a large leakage zone above the Scarborough gas field, Exmouth plateau, North West shelf of Australia. This leakage zone, well imaged on three-dimensional seismic data, extends over a region of 100 km2 (39 mi2), encompassing both the crest and flanks of the anticlinal trap, and is termed here as distributed crestal leakage. The present-day gas–water contact is 85 m (278 ft), and the spillpoint is 110 m (328 ft) below the crest, implying that the trap is underfilled at present. The leakage zone comprises more than 500 pockmarks at the present-day seabed with no crosscutting or cannibalization, suggesting that they formed in a short interval of time. These are underlain by sediment remobilization features and amplitude anomalies, consistent with a relatively high flux leakage of gas from the underlying Cretaceous deep-water sand-rich reservoir. By analyzing the geometrical relationship between the leakage zone, the top-seal properties, and the gas–water contact, we conclude that the mode of leakage in this specific setting is not the result of gradual addition of gas charge but is instead consistent with a sudden increase of aquifer overpressure. We suggest two alternative models for seal failure in this case study: a conservative model consistent with a modest but rapid increase in aquifer overpressure leading to membrane seal failure and a model dominated by high aquifer overpressure leading to leakage through hydraulically dilated faults and fractures.