NEW EXPLORATION TECHNIQUES IN THE ANALYSIS OF DIAGENETICALLY COMPLEX RESERVOIR SANDSTONES, SYDNEY BASIN, NSW

Abstract

The Sydney Basin, despite numerous encouraging shows of both free oil and gas from coal and petroleum exploration drilling, remains unproductive of commercial hydrocarbons. Reservoir potential has historically been the primary concern, owing to widespread distribution throughout the sequence of lithic, diagenetically- altered, clay- rich sandstones. This study aimed at defining areas of acceptable reservoir quality by careful examination of stratigraphic, depositional and diagenetic controls.

Interpretation and extrapolation of reservoir distribution, attributes and quality were carried out within a genetic stratigraphic framework. Stratigraphic packages of widespread correlatability that were deposited during discrete episodes of basin filling provide the basis for delineation of component depositional systems and for further mapping of framework sandstone facies and associated mud rocks.

The availability of numerous, continuous drill cores from existing coal bores and limited petroleum exploration wells provided an opportunity to directly quantify porosity and permeability. A visual method of estimating permeability was applied by comparison of the drill cores with a standard set of cores of known permeability. The comparison was made on fresh, dry rock surfaces with the aid of a binocular microscope at 20 × magnification. Reliability of the visual estimates was then assessed by laboratory measurement of a large representative sample set.

Lithofacies maps of genetic stratigraphic packages define sand- body trends and allow interpretative extrapolation of reservoir facies tracts which, when integrated with the visually- estimated and laboratory- derived reservoir quality data, enabled mapping of regional permeability distribution and thickness.

The principal conclusions of the study are that reservoirs with sufficient porosity, permeability and volume for conventional oil and gas production exist within the Sydney Basin. Best reservoir quality occurs in quartzose sandstones of the Narrabeen Group in the southwestern part of the basin. Potential reservoir sandstones are up to 20 m thick, have permeabilities in the 10- 1000 md range and porosity between 10 and 18 per cent. Calibration and testing of the visual estimation technique allowed accurate and efficient continuous recording and mapping of porosity and permeability, and this technique may have much wider application for the petroleum industry.