Deep Water Reservoir Facies of the Late Jurassic Angel Fan, Dampier Sub-Basin, Australia

Dessy Dharmayanti, Alan Tait, and Richard Evans
Curtin University of Technology, Perth, WA, Australia

The Angel Formation is a sand-prone, deep water fan system that was deposited in the Dampier Sub-Basin on the NW Shelf of Australia. The Angel Formation hosts a number of oil and gas fields and is also a regional aquifer. Core (over 700 meters) from 15 wells were studied to define lithological facies and facies associations. The cores allowed the examination of the vertical facies variations and their correlation with wireline logs.

Three facies associations have been recognized in the Angel Formation:

1. Massive Sandstones are subarkosic to quartz arenites that are fine to coarse-grained, bioturbated, and glauconite-rich. Bedding is mostly massive, with occasional lamination and disruption by dewatering features.
2. Heterolithic facies consist of a mixture of sandstone and shale which is commonly bioturbated. Trace fossils include Chondrites, Zoophycus and Terebellina. Sandstone content in the heterolithic facies is variable, decreasing gradually basinward. Sandstone injection features are not uncommon.
3. (3) The claystone facies consists of up to 10 cm thick beds of claystone, which are burrowed by Chondrites. Claystones are less common and thinner towards the proximal part of the fan (NE).

These facies were deposited in a slope to basin floor fans system, supplied from a shallow marine shelf. Deposition was mostly by high-density turbidity flows, in water depths estimated to be 200 to 300 meters below sea level. Sandstone deposits are sharp based and gradually fine-upwards into laminated bedding. Occasionally beds have sharp indented tops with sandstone injection features. This suggests localised remobilization and modification of original sandstone geometries, which may impact upon reservoir performance.