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The Effect Of Diagenesis And Facies Distribution On Reservoir Quality In The Permian Sandstones Of The Toolachee Gas Field, Southern Cooper Basin, South Australia.

Alsop, David Barryl

Degree of Master of Science, 1990

University of Adelaide


The Toolachee, Brumby and Munkarie Fields in the southern Cooper Basin of north-east South Australia contain wet gas in the Permian Patchawarra and Epsilon Formations. The diagenetic effects on reservoir quality are important in predicting sites of good quality reservoirs and in using stimulation techniques for enhanced recovery of gas from existing wells.

The aims of the study are: to characterise chronostratigraphic facies and distribution of Permian sandstones, to determine the paragenesis of the Toolachee, Epsilon and Patchawarra Formations, to investigate the effect of diagenesis on reservoir quality, and to define and predict potential reservoir sands in the region.

A total of 1296 feet of slabbed conventional core was geologically logged. There were 128 com samples and 25 cuttings samples examined by thin section, X-Ray diffraction (XRD), and scanning electron microscope (SEM). In addition, 20 samples were studied using cathodoluminescence.

The Toolachee Gas Field is located approximately sixty kilometres south-east of Moomba, on a structural high, the Toolachee Trend. The structure is a north-oriented trend situated immediately east of the Tenappera Trough. Brumby Gas Field is approximately 20 kilometres east of the Toolachee Field and comprises eight localised highs forming a broad structure. Munkarie Gas Field is located on a fault-dependent, north-south elongate structure which is 15 kilometres south-east of the Toolachee Field.

The Toolachee and Patchawarra Formations consist mainly of fine to coarse sandstone of point bar origin and fine-grained flood plain deposits, while sediments of the Epsilon Formation accumulated in a deltaic environment.

The main diagenetic effects acting on sandstones (sublitharenites) in the study area are early quartz cementation, porosity dependence on diagenesis, microporosity associated with authigenic dickite, formation of micritic and sparry siderite and the dissolution of mineral grains to form secondary pores. Hydrocarbon migration occurred after the last phase of diagenesis.

The main influences affecting reservoir quality are a combination of environmental and diagenetic controls. The main environmental controls are grain size, sorting, and mineralogy of the detrital grains and clays. The grain size of the initial sediment is an important factor with the medium to coarse-grained rocks of the Patchawarra Formation exhibiting the best porosities and permeabilities throughout the Toolachee, Brumby and Munkarie Fields. Sorting also plays a role in reservoir quality with well-sorted sandstones (sublitharenites) in the medium grain size range often having better porosity than the more poorly-sorted sandstones in this range. Primary intergranular porosity is dominant with subordinante secondary oversized pores and microporosity in dickite.

Australian School of Petroleum



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