Porosity?permeability relationships in Miocene carbonate platforms and slopes seaward of the Great Barrier Reef, Australia (ODP Leg 194, Marion Plateau)

This paper reports a series of 700 porosity–permeability analyses and supporting petrographic and sedimentologic descriptions from Early to Late Miocene carbonate strata cored on the Marion Plateau, offshore from north‐eastern Australia, during Ocean Drilling Program (ODP) Leg 194. The samples analysed are not only mainly coarse bioclastic limestones and dolomitized equivalents from platform‐top facies, but also include 79 plugs from deeper‐water slope to hemipelagic drift facies. Outstanding characteristics of this data set are the wide ranges of porosity and permeability in both limestones and dolostones, the large degree of short‐range heterogeneity typical of these strata, and the better porosity–permeability correlation of dolostones than limestones. The platforms have experienced widely varying calcite cementation, dolomitization and dissolution but show little clear evidence of meteoric diagenesis, suggesting that subaerial exposure may have played little role in porosity–permeability evolution. Permeability‐for‐given‐porosity is controlled by grain size and calcite cement content in grainstones and by occurrence of larger shelter pores and vugs in mud‐rich samples. Dolomitization tends to reduce the variation of permeability‐for‐given‐porosity by recrystallizing mud matrix to form intercrystalline macroporosity that connects vugs and moulds to become integrated with the effective pore system. As a result, there are no differences in permeability–porosity trends for different dolostone textures, whether dominated by intercrystalline, vuggy, or preserved intergranular pore types. Two platform‐top sites separated by only 5 km display a major lateral variation in dolostone porosity–permeability characteristics within the youngest dolostone units. This difference is interpreted as reflecting a relatively ‘windward’ (current‐facing) setting of the site with the overall higher permeability‐for‐given‐porosity (Site 1199) that led to less muddy depositional facies, greater cementation, and lesser grain dissolution. Pore‐geometry parameters measured by petrographic image analysis confirm that the ‘windward’ dolostones have pores that are both larger and less intricate than dolostones comprising the more current‐protected location