Evidence for overpressure in the Belfast Formation, Shipwreck Trough, Otway Basin

Abstract

The Late Cretaceous Belfast Formation is the regional seal for many gas reservoirs in the Otway Basin. Analysis of this formation within the Shipwreck Trough has revealed several lines of evidence characteristic of over-pressure, which can be correlated over a significant area of the basin. A distinct decrease in sonic velocity and resistivity, relative to expected compaction trends, is observed in wells on the western (down thrown) side of the Sorell Transform. Increased drilling mud weights through this section in some wells supports this conclusion. The overpressure is interpreted to be a consequence of disequilibrium compaction due to rapid sedimentation during the Late Cretaceous rifting event and development of the Sorell Transform. Using the Eaton Method and sonic log data a maximum pore-pressure of approximately 30% higher than hydrostatic is calculated in the shale. This prediction, however, cannot be made without an implicit assumption regarding a 'normal' compaction trend; we utilise a Gardner density-derived velocity prediction in addition to a standard depth trend extrapolation method. Observations are consistent with recently published evidence for overpressure in Otway Basin west of the Shipwreck Trough. Implications for the interpreted overpressure will also be discussed, primarily focused on the impact of velocity analysis and depth conversion. The overpressure is not readily observable during velocity analysis of CMP gathers. This is partly due to the lack of reflectivity through the Belfast Formation and partly as the velocity of any reflector in this shale section will have a slower than expected velocity and can be potentially dismissed as multiple energy. This results in stacking velocities that are over predicted through the shale, which can limit their use in detecting overpressure and cause depth conversion errors.