Reservoir Characteristion of the Birdrong Formation, Griffin #1 Well, Northwest Shelf, Western Australia.
Honours Degree, 1993
University of Adelaide
The Birdrong Formation at Griffin #2 has been divided into 8 facies. The sediments deposited in this formation were deposited on a shallow shelf, low energy environment with periods of high energy during storms. These storms churned up sediments and thus reworking occurred. These sediments are the main reservoir facies and make up over 60 percent of the formation. Probe permeability showed that Facies 3 is the best reservoir facies with an average permeability of 52.6 mD and characteristic increasing or decreasing permeability trends depending on grain size trends of each bed in the facies. Facies 2 is a reworked sediment with an average permeability of 4.8 mD. This facies has poorer sorting quality than Facies 3 with higher silt content. Facies 2 and Facies 3 were produced from sandy and silty bioturbated sediments respectively.
Probe permeability showed that most of the facies are relatively tight. Histograms show exponential trends towards the lower permeabilities with the exception of Facies 3 which has a normal distribution. Porosity - Permeability plots show that Facies 2 and Facies 3 were deposited under the same conditions. Facies 4 and Facies 5 also show deposition under the same conditions. Facies 7a and Facies 7b were deposited under different conditions. The lateral extent of the reservoir facies extend over the whole Griffin area with relatively constant thickness due to their sheet-like structure.
From XRD analysis the main components seen in the facies are quartz, siderite, glauconite, kaolin, feldspar and various other minerals and clays. Pore throats vary throughout the facies. Facies 3 has the largest pore throats with a maximum diameter of 15 microns. The maximum pore throat size of Facies 2 is 10 microns. The tightest facies are Facies 7a and Facies 7b. The seal for the oil column is the Mardie Greensand. It displays tight properties and very small pore throat diameters. Thin section analysis shows that pore throat size is affected by siderite and glauconite cements, quartz overgrowths, kaolin and various other clays. Secondary porosity has been produced by the dissolution of siderite cement and shrinkage of glauconite throughout the formation.
Directional permeability conducted on Facies 2 showed preferential high permeability directions at 30 and 150 degrees. Facies 3 shows a weak trend at 30 degrees. The directions could correspond to wave action at 30 degrees and long-shore drift at 150 degrees.