The Early Permian Patchawarra Formation was divided in the southern Cooper Basin into six regionally extensive genetic (chronostratigraphic) units (VGU 1 to VGU 6), which were separated by either high-gamma-ray shale markers representing lacustrine flooding events or regionally extensive coal seam markers. Mapping primary sediment-dispersal trends of the units indicates that deposition was initially from large bed-load fluvial systems (VGU's 3-6) that flowed along discrete structurally defined troughs between basement highs. The fluvial systems evolved into coarse-grained meanderbelts (VGU 2) as the structural troughs were infilled, and finally, lacustrine deltas (VGU 1) were deposited broadly over the region.

The Patchawarra sandstones (VGU 1, 2, and 3) are gas-productive at Toolachee field, but all are incompletely drained. VGU 1 has the lowest recovery efficiency, consistent with the lack of interconnectedness of the narrow lacustrine distributary channels. Gas recovery is the most efficient in VGU 2 and is attributed to good reservoir quality and connectivity of the channels in the coarse-grained meanderbelts. Barriers to reservoir continuity in this unit are primarily caused by postdepositional faulting. Paradoxically, good quality and high connectivity of the bed-load channel sandstones in VGU 3 have inhibited recovery efficiency because of their communication with a large interconnected aquifer. Water influx typically occurs as the reservoir is depleted by approximately 1,500 psi, and residual gas saturation following water influx was measured at 40 percent. A drilling program of infill, step-out, and exploration targets was recommended and successfully implemented (eight commercial wells from eight wells drilled) on the basis of the analysis of the fluvial architecture. Results of the drilling in relation to the reservoir heterogeneity are illustrated.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas