WARREN, JOHN K., Curtin University, Perth, Western Australia; and PETER TINGATE and PAUL TARABBIA, NCPGG/APCRC-Thebarton Campus, Adelaide, South Australia

ABSTRACT: Geological Controls on Porosity and Permeability in Reservoir Sands, Goodwyn Field, Rankin Trend, Northern Barrow-Dampier Subbasin, Northwest Shelf, Australia

Goodwyn field is a major gas and condensate field with reserves of 4.1 tcf of gas and 254 million bbl of condensate. In addition, there is a 31-m oil rim in the southern part of the field. The field is a large tilted fault block open to the north and bounded by major faults to the south, west, and southeast. There are no significant internal faults within the field, and the various hydrocarbon-bearing reservoirs dip gently northwards at 5-6 degrees and are truncated by the Main unconformity. Progressively younger units subcrop northward beneath the Main unconformity. The Main unconformity is a relatively uniform surface over much of the block dipping northwards at 1-2 degrees, but it becomes an erosional scarp along the southern, western, and southeastern margin. Cretaceous deep-water marine sediments directly above the Main unconformity form the seal to the field.

Core from the GE unit (main reservoir sand) defines two lithofacies: a medium to coarse-grained subarkosic unit (facies 1) and a fine to medium-grained subarkosic unit (facies 2). Both facies were laid down as channeled-braid plain to coarse-grained meander-belt sands. The overlying GD unit core defines a third sand lithofacies (facies 3): a fine to medium-grained sand characterized by abundant siderite. It was deposited as fluvial point bar, shore zone, and prodelta channel sands.

Although modified by a diagenetic overprint of quartz overgrowths and kaolin cements, reservoir quality in the GE reservoir sands of Goodwyn field was established at the time of the deposition. Porosity pathways within coarse to medium grained sand sheets (facies 1 and 2) within the original depositional setting have acted as preferential conduits for ongoing fluid flow. In contrast, the finer grained and especially more clay-rich marine-associated GD sands (facies 3) have had their permeability reduced by the precipitation of authigenic quartz, kaolin, and siderite cements. Flushing associated with exposure of the Main unconformity may well exert some control on porosity and permeability levels in the reservoir sands.

Within the GE reservoir, zones with highest porosity are not a reliable indicator of the best of many excellent permeability sands. What are now the most porous reservoir sands in the GE unit are not necessarily the most permeable. The coarser grained sands have wireline-derived porosities in the range 19-23%, yet they are superior reservoirs (average k = 3760 md) than fine to medium-grained sands with porosities up to 26% (average k= 427 md). This is due in large part to the relatively larger pores in the facies 1 sands. The same amount of authigenic kaolin occurs in facies 1 sands and facies 2 sands. Because of the larger pore-throat size in facies 1 sands and a propensity to replace feldspar, as well as coat quartz grains, authigenic kaolin in facies 1 sands is a less efficient occ uder of pore throats.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.