Comparison Of Rock-Type-Derived Reservoir Properties With Estimates From Standard Petrography.
Honours Degree, 1999
University of Adelaide
The Cooper Basin, containing Permo-Triassic sediments of fluvial, lacustrine, local glacio-fluvial and rare paraglacial aeolian environments, is characterised by predominantly low porosity, low permeability sandstone reservoirs for oil and gas. Despite this, it is one of Australia?s main producers of hydrocarbons. Rock Typing provides a fundamental description of potential reservoir rocks using any available sample, including cuttings. Samples are grouped into reservoir types. Rock Typing can assess marginally economic zones determined from petrophysical data to support decisions on perforation and drill stem or production testing of zones of interest.
This thesis aims to calibrate the semi-quantitative permeability assessment from Rock Typing within the Cooper Basin, with traditional petrographic methods. Three important economic formations within the Gidgealpa Group were utilised for this study: the Tirrawarra Sandstone, and the Patchawarra and Toolachee Formations.
Thirty-five core samples from across the South Australian region of the Cooper Basin, in Pondrinie, Kanowana, Tirrawarra, Big Lake and Dullingari fields were taken next to the site of core-plugs which had measured permeability and porosity values. All the samples were examined by thin section, x-ray diffraction, and scanning electron microscope. A portion of some sample was turned into artificial cuttings, and Rock Typed using the standard procedures within Santos. Three Rock Types are identified: I, II and III with Rock Type I being further classified into four classes (IA-ID).
Rock Typing has been found to give representative results of porosity, permeability and grain size estimates. Variations occur between the results of quartz cement and clay estimation derived from Rock Typing and thin section analysis. Rock Typing overestimates quartz cement in Types II and III as it includes fractured grains that occur in more compacted rocks. Rock Typing and thin section analyses estimate different types of clay, with Rock Typing viewing grain coating clays while thin sections mainly estimate altered rock fragment percentages.
Rock Typing is a feasible and accurate method of estimating porosity and permeability. Further work to improve the estimation of quartz cement and clays would improve the accuracy of Rock Typing. Rock Typing should be used in conjunction with other methods, such as petrophysical or scanning electron microscopy where possible.
Core comparators and aggregate photomicrographs have been produced to assist in the development of standard procedures and training for those undertaking rock typing within Santos.