TRIASSIC-JURASSIC DEPOSITIONAL HISTORY OF THE DAMPIER AND BEAGLE SUB-BASINS, NORTHWEST SHELF OF AUSTRALIA

Abstract

Very large gas accumulations were discovered in the Dampier Sub-basin on the Northwest Shelf of Australia during the early 1970's. The majority of the reservoirs are fluvial and marginal marine sandstones within a thick intra-cratonic clastic sequence of Early Triassic to Middle Jurassic age. Parts of this succession have been penetrated by more than 60 wells within the Dampier Sub-basin and the adjacent Beagle and Barrow Sub-basins.

Broad regional palaeoenvironmental episodes have been established using oriented core supplemented by seismic, palaeontologic and wireline log data. The earliest episode was an Early to Middle Triassic transgressive-regressive cycle which led to marine and marginal marine deposition, followed during the Middle to Late Triassic by dominantly fluvial deposition. A transgression began in the early Rhaetian, characterised by deltaic, estuarine, lagoonal and coastal sedimentation which gave way to fully marine conditions during the Early Jurassic (Hettangian); this was followed by a regressive cycle.

A regional tectonic episode followed which resulted in development of troughs, and brought about associated extensional faulting. Marine conditions which typified the Early Jurassic (Sinemurian) persisted until the beginning of the Middle Jurassic (Bajocian) in the Dampier Sub-basin, but passed rapidly to a fluvio-deltaic cycle in the Beagle Sub-basin. Regressive conditions extended gradually westward until sedimentation was interrupted by the onset of major continental break-up toward the end of the Middle Jurassic.

The deposition of an alternating sequence of thick shale and sand during mainly restricted marine and continental conditions has provided the area with potential source and reservoir rocks.