BREATHING NEW LIFE INTO THE EASTERN DAMPIER SUB-BASIN: AN INTEGRATED REVIEW BASED ON GEOPHYSICAL, STRATIGRAPHIC AND BASIN MODELLING EVALUATION

Abstract

The eastern Dampier Sub-basin on Australia’s northwestern margin has been subject to intensive exploration activity since the early 1960s. The commercial success rate for exploration drilling, however, has been a disappointing 8%, despite numerous indications of at least one active petroleum system. During 2002–2003, Woodside and its joint venture partners undertook an integrated review of the area, aimed at unlocking its remaining potential. Stratigraphy, hydrocarbon charge and 3D seismic data quality were addressed in parallel.

The eastern Dampier Sub-basin stratigraphy was upgraded from the existing, conventional, second-order tectono-stratigraphic framework to a third-order, exploration-scale, genetic stratigraphic framework. The new framework has regional predictive capability in terms of reservoir (and seal) presence and facies, and has led to recognition of new plays and an enhanced understanding of known plays. One new play involves shoreface sands within the Calypso Formation. New light has been shed on the known Lower Cretaceous M.australis sands play (K30), by the creation of gross depositional environment maps at third-order sequence scale. The Upper Jurassic deepwater clastics play of the Lewis Trough has also been developed, by recognition of four prospective, sand-rich gravity-flow intervals in the early Oxfordian (J42 play).

A 3D charge modelling study, underpinned by new geochemical analysis, has allowed delineation of areas of higher and lower risk in terms of hydrocarbon charge and phase (oil versus gas). Key source rocks for oil are identified in the early Oxfordian W.spectabilis biozone, although they are also a likely source for gas in the southwest of the area. The Bathonian-Callovian Upper Legendre Formation is a major source for gas, but could also have contributed minor oil in the northeast of the area. By a combination of geochemical fingerprinting and 3D forward modelling, most hydrocarbon occurrences in the area have been tied to these source intervals, complete with a consistent view of maturities and migration pathways.

Some 1,500 km2 of the Panaeus multi-client 3D survey were reprocessed, with close attention to multiple removal, velocities and imaging. A step-change improvement in seismic quality was obtained, together with improved velocities for depth conversion.

The prospect portfolio has been polarised and much enhanced through these studies, and the results of several existing wells have become better understood. Some new prospects were identified by apparent direct fluid indications, detected in one case by 3D volume AVO screening. Other new prospects are the result of a clearer seismic image, or of the revised velocity model for depth conversion. New plays are still being followed up, while the fresh light cast on existing plays (e.g. K30 and J42), in combination with improved seismic data, has led to development of several interesting opportunities.