Sedimentology and Reservoir Characteristics of a High-Latitude, Mixed Carbonate-Siliciclastic System: Middle Permian Beekeeper Formation, Perth Basin, Australia

The Middle Permian Beekeeper Formation serves as the key gas reservoir in the Woodada Field of the onshore Northern Perth Basin, Western Australia. The formation, restricted to the subsurface, formed in a high-latitude inland sea that occupied a narrow rift basin. Well productivity is difficult to predict due to significant heterogeneity in reservoir character. This study aims to improve understanding of the distribution of reservoir facies through examination of materials recovered in core. Facies disposition indicates a ramp-like depositional profile that was situated basinward of a sandy coastal zone. The ramp was inhabited by a low-diversity heterozoan assemblage dominated by bryozoans, brachiopods, and crinoids. Inner ramp deposits are composed of fine to coarse skeletal packstones, grainstones and rudstones consisting of crinoid, bryozoan, and brachiopod fragments. Mid ramp facies are mainly bryozoan-rich wackestones to floatstones. Outer-ramp facies are more argillaceous and often dominated by robust bryozoa. Argillaceous facies grade laterally into poorly fossiliferous siltstones interpreted to represent deeper water deposition. Sharp-based interbeds of bioturbated quartz sandstone are interpreted as the products of storms that impacted sandy shoreface environments and swept material offshore. Petrographic observations reveal that although early cementation in the marine environment was minimal, primary porosity is low in calcareous facies. In mud-poor facies, skeletal grains are commonly tightly packed along microstylolitic contacts. In muddier facies, primary porosity is occluded by micrite and argillaceous mud. Any residual primary porosity was infilled by blocky calcite cement during later stages of diagenesis. Gas production is related to the development of secondary fracture porosity and, to a lesser degree, dolomitized limestone and dolomite-cemented sandstone. Fractures are well-developed in carbonate-rich inner and middle ramp facies. By contrast, dolomitization was both fabric-selective and facies-specific. These relationships suggest that the distribution of producing intervals is best predicted in a context of depositional environment and expected facies distributions. In this regard, comparable high-latitude Permian carbonates that outcrop elsewhere in Australia may provide useful analogs for the Beekeeper Formation and other reservoirs developed in the deposits of cool-water, mixed carbonate-siliciclastic systems.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California