In situ porosity and permeability of selected carbonate sediment: Great Bahama bank Part 1: Measurements

In situ porosity and permeability were measured on Great Bahama Bank sediments using electrical conductivity and permeability probes. Core samples were recovered at the probe measurement sites for laboratory determinations of porosity and permeability. Penetration depths of cores and probes were approximately 2.5 m subbottom. In situ porosities of the oölitic sands for depths of 0–2.5 m subbottom ranged between 36% and 50%, and at sites in the somewhat muddier oölitic sediments the porosities ranged from 42% to 61%. The in situ permeabilities ranged from 0.0032 cm/s (3.3 darcys) to 0.068 cm/s (71 darcys) at the sites where porosities were determined. Laboratory values of porosity are comparable to values obtained by in situ measurements; however, laboratory permeability values are approximately an order of magnitude lower than in situ values. The reduced permeability measured in the laboratory is attributed to disturbance of the microfabric during coring, transport, and laboratory sampling. A detailed examination of the microfabric can be found in a companion article (Bennett et al. 1990). The high in situ porosities and permeabilities of these carbonate sediments are predominantly the result of the combined effects of both the sediment grain size distribution and the microfabric. The classification scheme of Dunham (1962) for carbonate rocks (wackestone, packstone, etc.) does not always provide a clear picture of some of the crucial properties of carbonate sediments (porosity, permeability, etc.), nor does this scheme provide a realistic functional description of the behavior of these sediments when subjected to static and dynamic stresses.