Publication Name: The Sedimentary Basins of WA
Authors: Glynn Ellis, Alan M. Tait and Peter J. Gibson
Publication Volume: 1
Date Published: July 1994
Number of Pages: 28
Reference Type: Book Section
Abstract:
Carbonate concretions in the mid-Cretaceous Gearle Siltstone and Beedagong Claystone form unusual hydrocarbon reservoirs in the Barrow Island Oil Field in the Carnarvon Basin of Western Australia. Concretions recovered from cores vary in size, lithology and distribution. One group, described herein as cementstones, have considerable vuggy and fracture porosity, display evidence of extensive development and are suspected of forming the hydrocarbon reservoirs. Lithological and geochemical studies indicate that the concretions formed in a shallow subsurface zone. The carbonate was provided by the degradation of organic material by sulphate-reducing and methanogenic bacteria. Concretions are most abundant within clay-rich (high gamma-ray) sediments suggesting that slow or non-deposition stabilised the diagenetic processes required for concretionary carbonate precipitation.Carbon and oxygen stable-isotope compositions of the cementstones and material mass balance considerations
indicate that the migration of aliphatic acid anions (principally acetate) and dissolved carbon dioxide in oilfield waters from deeper stratigraphic units is responsible for the development of the cementstones. Introduction of these 'fluids' to the early diagenetic zones at shallow depths promoted bacterial activity and increased alkalinity, inducing extensive calcite cement precipitation. These fluids migrated up fractures and faults and towards structural highs within the Gearle Siltstone and Beedagong Claystone. This influence on the formation of concretionary carbonates is reflected in their areal distribution, where extensive development is restricted to the crest of the Barrow Anticline and adjacent to the Barrow Fault.