The Ordovician Acacia Sands, Canning Basin, Western Australia.
Honours Degree, 1992
University of Adelaide
Regionally, the Willara Formation is an Ordovician argillaceous limestone deposited in a shallow epieric seaway. However, in four wells Acacia #1, Acacia #2, Solanum #1 and Dodonea #1, an anomalous dolomite / sandstone sequence was intersected. In some zones of the sandstone, there are porosity values up to 20 percent. This part of the Acacia sandstone is characterised by well connected pore throats.
Cores from these wells are interpreted as sabkha, lagoon and shoal type sediments, determined from their lithologies and sedimentary structures in core and in thin section. The porous sands were aeolian, blown into the sabkha and reworked into the calcitic lagoonal sediment by wind and channel processes. Porosity in the sands was first occluded by dolomite cements generated by brine reflux, a gravity flow mechanism. Porosity was then enhanced to the present level by the dissolution of dolomite and any evaporite cements. Fluids that dissolved the cements possibly came from the dewatering of either the underlying Nambeet Formation shales or the overlying Goldwyer Formation shales. Fluid flow was most likely along fault planes.
Well to well correlation shows that the area around Acacia #1, Acacia #2, Solanum #1 and Dodonea #1 was topographically higher at the time of deposition, supporting the gravity flow interpretation for the dolomitic fluid.