STRATIGRAPHY OF THE SOUTHERN SAHUL PLATFORM

Abstract

This study of the southern Sahul Platform area in the Zone of Cooperation is based on the identification of depositional sequences, their distribution and relationship to structuring events in order to predict the locations of favourable combinations of source, seal and reservoir facies with increased confidence. A sequence stratigraphic approach integrating well logs, palynology and seismic data was used to identify and map significant seismic horizons such as the Aptian and Tithonian unconformities.

Early to Middle Jurassic sediments were deposited in a broad, northeast-southwest oriented sag basin with a northeastward sediment transport direction. Depositional environments range from non-marine to marginal marine in the Plover Formation to the shallow marine sediments of the Elang Formation. The Elang Formation, comprising two depositional sequences, represents the last of the sediments deposited before the Breakup Unconformity. These formations comprise the dominant reservoir facies, containing a number of oil and gas discoveries. Porosity degradation occurs in Jurassic reservoirs below 3,360 m.

The Callovian Breakup Unconformity resulted in the initiation of the narrow, confined depocentres of the Sahul Syncline, Malita Graben and a series of east-west troughs. The Sahul Platform and Londonderry High comprise the flanks of these depocentres but were originally located within the depocentre of the Early to Middle Jurassic sag basin. The Flamingo Syncline is a younger feature developed in the Albian.

Late Jurassic and Early Cretaceous sediments are confined mainly to the Sahul Syncline and Malita Graben and are absent or represented by thin, condensed sections on the flanking highs. The condensed sections on horst blocks are a result of sediment bypass rather than considerable erosion. Reservoir facies of Tithonian-Berriasian age are interpreted to occur within east-west troughs constituting another reservoir section apart from the Bathonian-Callovian sediments. Wells distant from the Sahul Syncline and Malita Graben, have encountered hydrocarbons, indicating that the area contains mature source rocks, capable of charging traps away from the immediate vicinity of the depocentres.