AAPG Bulletin, Vol.104, No.8, 1627-1648, 2020
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 km(2) (39 mi(2)), 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.