Abstract

Analysis of water chemistry and pressure data from the Queensland sector of the Cooper and Eromanga Basins indicates that two regional fluid flow systems are operating within these basins. In the Eromanga Basin (Jurassic-Cretaceous) a southwestward and descending open flow system is evident, driven by the topographically induced Great Artesian Basin hydrodynamic regime. In the underlying Cooper Basin (Upper Carboniferous-Triassic) a closed system is reflected by overpressuring and ascending flow out of each of the major Permian troughs. Where the two systems interact and equilibrate, mixing of formation waters from Permian and Jurassic reservoirs is apparent in water chemistry data and potentiometric surface minima can be identified in vertical pressure profiles.

The water chemistry and pressure data have been used to identify hydraulic baffles within the Permian and Jurassic sequences. These barriers are likely to provide effective lateral or vertical seals and, once identified, can be targeted in regional mapping projects to identify potential stratigraphic traps. In addition to the potential exploration applications, regional hydrodynamic databases are also valuable in petrophysical evaluations, by providing estimates of water resistivity in areas with poor well control, and for estimating field limits when the only available pressure data are from hydrocarbon columns within crestal wells.

Introduction

Exploration programmes in the Cooper and Eromanga Basins have historically relied upon 2D, and more recently 3D, seismic data for the identification of potential structural traps. As exploration programmes in these basins mature and reserve additions from conventional plays reach a plateau, complementary or alternative techniques are required to identify new exploration plays and non-conventional traps and to provide insights into the petroleum systems operating within these basins. Hydrodynamics is one such technique by which water pressure and chemistry data can be used to identify flow barriers, which may reflect hydrocarbon traps, and flow conduits, which are likely migration fairways and areas of elevated seal risk. The technique provides a tool for evaluating the relative effectiveness of inter-formational and intra-formational sealing lithologies, which can then be applied in exploration risk assessment and particularly in the ranking of stratigraphic leads and prospects.

This paper summarises results of a review of water chemistry and pressure data from the most commercially significant reservoirs in the Eromanga and Cooper Basins in Southwest Queensland (Figs. 1 & 2). Water analyses are available for samples collected during drillstem tests or field production; these data have been screened to eliminate samples contaminated with drilling or completion fluids. Aquifer pressure measurements are available from drill stem test data or wireline (RFT or MDT) data; the pressure data have been screened to eliminate anomalies that are the result of hydrocarbon effects or sampling errors (supercharging, seal failures etc). The data have been mapped both areally and stratigraphically to identify directions and magnitude of fluid flow within and between major flow units.

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