The Relationship Between Permeability and Grain Size in the Birkhead Formation Reservoir, Eromanga Basin, Australia
Heya, Martin M.
Master of Science Degree 2008
University of Adelaide
The Eromanga Basin hosts the Middle to Late Jurassic Birkhead Formation, a succession of interbedded dark grey and brown siltstone, mudstone and buff fine to coarse grained sands with thin lenticular coal seams. The formation sands are characterised by labile volcanic lithics. Calcite and siderite cements occur and quartz overgrowths, illite and kaolinite are present as cement with the clays largely filling the primary pores.
The Birkhead Formation sandstone reservoir which is the focus of this study exhibits grain size, porosity and permeability distributions that show considerable spread with bimodal pattern.
A series of samples were selected from the end of plugs used for routine core analysis. The petrology of the reservoir was studied using techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM), and thin section analysis. Three grain size analysis methods for lithified rocks were used; dry sieving, optically measurement of grain size from thin sections and strewn mounts. Mean grain sizes and sorting were calculated for each plug using each method. The results for each method were plotted against permeability derived from routine core analysis.
Results indicate correlation between grain size and permeability. Where the permeability is higher or lower than expected for a particular grain size, the differenced can be explained by variation in sorting and patches of calcite cement. The strewn mount method for grain size analysis was found to be the most reliable grain estimate for Birkhead Formation sandstone as it allows for anomalies resulting from incomplete separation of grains from the whole rock or overcrushing during grain liberation. These results can be used for drill cuttings to identify zones of high permeability, the production zones where oil has accumulated. However because of the way in which drill cuttings are collected it can be difficult to establish zone boundaries accurately. Wire line logs can be used to further define these zones more precisely.