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The AAPG/Datapages Combined Publications Database

Southeast Asia Petroleum Exploration Society (SEAPEX)

Abstract


Proceedings of the South East Asia Petroleum Exploration Society Volume VI, 1983
Pages 35-40

Reefs and Porosity: Examples from the Indonesian Archipelago

Gerald M. Friedman

Abstract

Modern reefs from the Java Sea, like reefs from other parts of the world (such as the Red Sea or the Australian Great Barrier Reef), illustrate that reefs are not inherently porous; in fact, as long as reefs remain submerged cryptocrystalline cement precipitates within millimeters to centimeters of the surfaces of reefs and occludes all possible porosity. This cement, which has the appearance of lithified lime mud or micrite, forms in even the highest of high-energy environments. As bioerosion converts the solid colonies of reef organisms into skeletal particles that are cemented rapidly beneath the surfaces of the reef by cryptocrystalline cement the unwary geologist is tempted to term reefrock biomicrite, wackestone, and packstone (cf. Friedman, G.M. and Sanders, J.E., 1978, Principles of Sedimentology, New York, John Wiley & Sons, espec. p. 175). How often have I heard geologists complain that the “reef core is lithified lime mud.” Such a misconceived interpretation relegates reefrock to a low-energy setting, whereas, in fact, it forms in high-energy environments. Reefrock remaining submerged closes its pores shut. Thus when reefs become part of the subsurface record primary porosity has vanished.

Acquisition of porosity among Cenozoic reefs of Indonesia involved post-depositional subaerial emergence and freshwater diagenetic changes or dolomitization together with freshwater leaching. One of the processes creating secondary pores in the emergent vadose zone is “chalkification,” that is the reefrock weathered subaerially to a friable carbonate sediment. Such “chalkified” friable reefrock can be compared to that of subaerially exposed reefs in Caribbean islands, which are sufficiently soft to be removed by shovel, yet preserve the details of their former fabric as hard, solid reefrock. Other kinds of porosity acquired during subaerial emergence include those related to (1) dissolution of aragonitic particles or framework builders, mostly corals and molluscs (crittermolds), (2) solution-enlarged crittermolds, some of which formed vugs and even solution channels, (3) in dolomitized reefs, dissolution of nondolomitized carbonate created intercrystalline or moldic porosity, and (4) fracture-related pores (Friedman, 1977). Late-stage cements occluded much of this newly created porosity. As an example calcite, as a filling, healed many microfractures.


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