DEPOSITIONAL SEQUENCE OF THE WITHNELL FORMATION, DAMPIER SUB-BASIN

Abstract

The Late Cretaceous Withnell Formation has attracted very little exploration attention because of the perception that it has poor hydrocarbon potential. This unfavourable perception has arisen from the fact that very little is known about its depositional environment and lithofacies and therefore, its petroleum prospectivity.

A sudden fall of relative sea level occurred at the end of the Santonian, and was followed by the deposition of the siliciclastic Withnell Formation. The occurrence of a number of channels and canyons at the base of the formation, over the old shelf and slope on the southern margin of the sub-basin, supports the hypothesis that the Withnell Formation began as a lowstand systems tract. The thickness distribution of the formation and the progradation direction of seismic packages suggest a southeasterly provenance. Correlation of seismic data and well logs, and rock descriptions demonstrate the presence of units deposited during increasing water depths and subsequent highstand systems tract.

Much more information, both seismic and well data, is required to establish the facies distribution within the Withnell Formation which may contain sand-prone lowstand facies such as basinfloor or slope fans. The presence of such reservoir facies would enhance the petroleum prospectivity of the Withnell Formation.