Abstract:
Analyses of porosity and permeability are examined from cores drilled in two Miocene carbonate platforms cored by ODP Leg 194, seaward from the Great Barrier Reef, including one site in the Northern Marion Platform (NMP; mostly preserved as limestone) and two sites in the Southern Marion Platform (SMP; mostly dolomitized). The majority of plug samples are from coarse bioclastic facies and their dolomitized equivalents. Dolomitization probably occurred by circulation of normal to slightly modified seawater. Increasing fabric destruction at greater depth in the SMP may reflect overprinting of multiple dolomitization episodes in older strata, perhaps related to successive cycles of sea-level fluctuation. Both limestones and dolostones show similarly wide variation in porosity and permeability, reflecting extreme metre-scale heterogeneity of lithologies and diagenetic responses. The limestones show poorer porosity-permeability correlation, with generally lower permeability for given porosity compared with the dolostones, but with similar maximum permeabilities. Despite wide textural diversity, the dolostones cluster along a single trend that parallels the ideal relationship described by the Kozeny equation and characteristic of well-sorted sandstones. The low permeability-for-given-porosity of many limestones is explained by the fine grain size of some samples and, in other cases, by isolation of macropores behind mud matrix. Pore systems in the dolostones, however, tend to be dominated by interparticle macroporosiity, consisting of either relict intergranular pores (grainstones with fabric-preserving dolomitization) or intercrystalline pores (fabric-destructive dolostones and packstones with fabric-preserving dolomitization). Although many dolostones have vuggy pore systems, the vugs appear to be effective for fluid flow because they are connected by the enclosing system of interparticle macropores.