Amalgamated Sandsheet to Layered Sandsheet Architecture of Deepwater Deposits from the Upper Jurassic Angel Fan, Northwest Shelf, Australia

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

The Angel Fan is a deep water fan system that evolved as a series of stacked sand bodies. The fan is approximately 50 km wide and 100 km long and was deposited in the Dampier Sub-Basin on the NW Shelf of Australia. The fan sands form reservoirs in the Angel, Cossack, Dixon, Lambert, Legendre and Wanaea Fields, and acts as regional but limited aquifer for the oil and gas fields in the Dampier Sub-Basins. This study is based upon 15 wells in the Angel, Wanaea, Cossack and Lambert fields.

The Angel Formation in the Wanaea, Cossack and Lambert Fields has the attributes of slope and basin floor sandy fans. Fan architecture changes from a high net-to-gross, proximal fan system in the north-east to a moderate to low net-to-gross distal fan system in the south-west, suggesting a north-east source (e.g. Net-to-gross changes from 0.890 to 0.315 in 30km from Angel-1 to Madeleine-1). Individual sandbodies can be correlated relatively easily in the distal part of the fan (e.g. Wanaea-Lambert) where the sandbodies are layers separated by 4-10m thick bioturbated heterolithics. However, correlation becomes increasingly difficult in the proximal fan (e.g. Cossack-Angel) where the sandbodies are commonly amalgamated and the bioturbated heterolithics are less than 2m thick. The sandbodies are sometimes connected through the heterolithics by sandstone injection, while some pinch-out due to topographic or structural control. Despite the down-fan reduction in connectivity and net-to-gross, the sandstones maintain good reservoir quality. The best reservoir quality is found in the massive sandstone facies that has high porosity and permeability values.