In situ porosity and permeability of selected carbonate sediment: Great Bahama bank Part 2: Microfabric

Selected oölitic sediments from the Great Bahama Bank were studied to assess (1) the role of microfabric in determining porosity and permeability, and (2) particle packing relationships, i. e., grain support versus matrix support. Scanning electron microscopy and light microscopy revealed that the sediment consists of oöids, which are the major constituents of the sand‐size fraction, supported by a matrix composed predominantly of aragonite needles. The supporting matrix of aragonite needle clusters, which constitutes only about 10–20% of the total sediment dry weight, is the microstructural characteristic that increases the porosity and lowers the wet bulk density compared to a grain‐supported microfabric characteristic of clean sands. The presence of a fine‐grained matrix reduces the permeability of these sediments relative to clean sands. The influence of the microfabric is clearly reflected in the mass physical and depositional (particle packing) properties of the sediment. Laboratory values of porosity are comparable to values obtained by in situ measurements; however, laboratory permeability data are an order of magnitude lower than values obtained from in situ measurements. Lower permeabilities measured in the laboratory are attributed to disturbance of the microfabric during coring and transport to shore‐based laboratories. This conclusion is supported by permeability data obtained on a completely remolded sample. Remolding resulted in a decrease in permeability by two orders of magnitude, despite increased porosity due to addition of water. The disparity between in situ data and laboratory data is considered reasonable in light of the fragility of the microfabric. Thus, physical property data and textural interpretations reported in the literature from laboratory measurements for coarse‐grained carbonate sediments obtained by conventional sampling techniques are highly suspect.