Application of Sequence Stratigraphic Concepts to Reservoir Characterization in an Intracratonic Basin Setting: Toolachee Field, Cooper Basin, Australia

M. H. Holtz, D. S. Hamilton, J. S. Yeh, E. F. Tadiar, D. P. Anthony, M. Chamalaun

Toolachee field (Cooper Basin, Australia) produces gas from fluvial-lacustrine deposits of the Lower Permian Patchawarra Formation. It has produced >393 Bcf of gas from 38 wells and is mature. Toolachee field is strongly compartmentalized by multiple, heterogeneous, complexly faulted sandstone reservoirs, thus providing opportunities for reserve growth.

To unravel this complex reservoir architecture, we first identified the finest scale genetic units for mapping reservoir sandstone distribution. In defining genetic units, three types of operational boundaries were used: extensive, high gamma-ray shale markers representative of lacustrine flooding events, extensive coal seams that accumulated during prolonged episodes of sediment supply shut-off, and surfaces that mark dramatic changes in depositional style between successive units, thus marking major episodes of basin reorganization. Sandstone distribution and log facies analysis within each genetic unit indicate that the lower Patchawarra Formation was deposited in bed-load fluvial systems and the upper Patchawarra, in lacustrine deltas.

The second step was to establish fluid flow trends within the reservoir from cumulative gas and water production, resistivity of connate and produced waters, gas-water contacts maps, and reservoir pressure depletion history. Discontinuities in fluid flow trends, reconciled when integrated with sandstone distribution, suggest the presence of numerous northeast-trending normal faults with displacements that are ~ 50 ft and below, or very near, the resolution limits of existing seismic data. Presence of faults is only demonstrated by recognizing missing section in well bores, which is made possible by the high resolution of the stratigraphic framework. Identifying the faulting pattern completed the picture of reservoir architecture from which compartments were defined, providing opportun ties for reserve growth.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995