The Tirrawarra Sandstone is the primary producing zone at Tirrawarra field in the Cooper Basin of South Australia, where it contains 146 million barrels (MMbbl) of unrecovered oil. Because previous reservoir models had failed to predict reservoir performance accurately and development drilling had yielded poor results, we undertook this study in order to construct a three-dimensional descriptive model of the Tirrawarra Sandstone reservoir, within which we calculated the volumes and residency of original and remaining oil in place. This depositional-facies-based flow-unit model resulted in the identification of an additional 36 MMbbl of oil in place and proved crucial to our understanding of past reservoir behavior and to our identifying opportunities for incremental development. A comprehensive subsurface data base, including 3,000 ft of core from 31 wells, was used to develop the flow-unit model. The Carboniferous-Permian Tirrawarra Sandstone reservoir, which was deposited in a proglacial intracratonic setting, is 90 to 180 ft thick, and it comprises four main depositional facies that record progradation of several fluvial-deltaic systems across a lacustrine basin. A braid-delta system, the core of the reservoir, is dominated by bed-load fluvial channel-fill sandstone and is bounded by more lithologically heterogeneous facies. Prodelta, delta-front, distributary-channel, and channel-mouth-bar facies underlie the braid-delta facies, which are truncated and overlain by braid-plain facies. The Tirrawarra Sandstone is primarily a fine- to medium-grained sublitharenite, although each facies has distinct petrographic characteristics. Texture and detrital composition, both inherited from the depositional environment, determined the magnitude of porosity loss during diagenesis. Depositional environment is the primary control on reservoir properties and productivity at Tirrawarra field. Each facies has distinctive petrophysical properties and displays characteristic geometries and internal stratification patterns. We defined reservoir flow units within each facies on the basis of stratigraphic position and petrophysical properties. Stratigraphic analysis, by means of well log and core data, established flow-unit architecture and delineated flow barriers. Petrographic and petrophysical analyses quantified flow-unit-specific properties such as porosity, permeability, and hydrocarbon saturation. We then used geological and petrophysical data to construct the three-dimensional model, which enhanced visualization of reservoir heterogeneities, performed volumetric calculations, and served asa basis for developing strategies to optimize recovery. The model identified undeveloped pay in all major flow units and revealed untapped reservoir compartments in the north part of the field. Geologically targeted infill wells and recompletions will improve recoveryefficiency in existing enhanced-oil-recovery (EOR) patterns, and the northern reservoir compartments will be accessed by new EOR patterns.
Hamlin, H. S., Dutton, S. P., Seggie, R. J., Tyler, Noel, and Yeh, J. S., 1995, Flow-Unit Characterization and Recovery Optimization of a Braid-Delta Sandstone Reservoir, Tirrawarra Oil Field, South Australia: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 232, 44 p.