Regional Distribution and Controls of Reservoir Quality of the Birkhead Formation
Oryang, Joel, P.
Geoscience Honours Degree, 2012
University of Adelaide
The Birkhead Formation presents a complex facies change from the clean fluvial Hutton Sandstone below into low energy fine-grained fluvial deltaic to shallow lacustrine facies. The Birkhead facies vary from upward fining to interbedded sandstone and siltstone zones overlain by thin lenticular coal seams. The abrupt lithological change from Hutton Sandstone to Birkhead Formation is accompanied by sharp changes in reservoir quality properties. The reservoir quality within the Birkhead Formation also varies, improving from east to west.
An investigation of the effects of provenance and diagenesis on porosity and permeability of the Birkhead Formation was carried out based on integrated core studies including detailed sedimentary logging (8 wells), thin section petrology (15 slides) and SEM analyses (7 samples) from cored holes across the basin.
Using data from routine core analyses, the porosity equivalent to 1mD permeability across the Birkhead Formation was mapped, showing that; above 18 % porosity is required for 1mD in the northeast, 12 – 15% porosity in the west and 11 – 12 % porosity in the south.
Determination and classification of mineralogy and plotting on ternary diagrams have revealed different provenance sources of the eastern, southern and western regions. The eastern sediments source is characterised by silt-size, lithic-rich provenance linked to a contemporaneous volcanic-arc located in the present day east of Queensland. The southern sediments were derived from a metamorphic terrane probably from a SW Cratonic origin or the SE Broken Hill region; while the western sediments are of mixed composition partly from metamorphic origin but largely of a granitic source from the NW basin areas.
Changes in provenance have exerted significant influence on the reservoir quality variation by conditioning the diagenetic pathway. The different mineralogy supplied responded differently to the factors of diagenesis. The unstable volcanic-arc sediments of the eastern region have been significantly altered to abundant clay minerals including kaolin, smectite, illite and chlorite. The clay matrix and their characteristic pore lining, pore bridging and pore filling properties have resulted into a system of isolated porosity, micro-porosity and micro-pore throats leading to severe loss in permeability in the eastern region. The southern to western study areas however, have experienced less diagenetic alteration and have preserved substantial open porosity with good permeability.