Abstract: Kinds of Pores in Cenozoic Reefs of Indonesian Archipelago
Gerald M. Friedman
Cenozoic reefs of Oligocene and Miocene ages are present here and there along the Indonesian Archipelago between Sumatra and Irian Jaya, a distance of about 4,500 km. These reefs are both on surface and in the subsurface; in the subsurface they form major reservoirs for oil and gas. On surface the reefs are devoid of pores, the depositional fabric of reef core has been micritized, primary pores have been obliterated, and secondary pores did not develop. In the subsurface almost all primary porosity has vanished; initial cryptocrystalline high-magnesian calcite, now low-magnesian calcite, filled intraskeletal and interskeletal pore space when the reefs were still submergent. A part of their postdepositional history involved subaerial emergence, and freshwater diagenetic changes or dolomitization together with freshwater leaching created secondary porosity. Kinds of secondary pores include those related to (1) dissolution of aragonitic particles or framework builders, mostly corals and mollusks (crittermolds); (2) solution-enlarged crittermolds, some of which formed vugs and even solution channels; (3) in dolomitized reefs dissolution of nondolomitized carbonate created intercrystalline pore space; and (4) fracture-related pores. In places, late-stage cements occluded all or parts of the secondary pores. Thus calcite, as a filling, healed many microfractures. In some reefs pressure solution yielded stylolites and eliminated any existing pore space; in other reefs stylolites are rare or absent. Calcite crystals, composing echinoderm fragments, commonly are twinned suggesting strain.
Vertical sequences reveal former paleowater tables. Porous reef zones, as products of vadose diagenesis, are interbedded with less porous zones of phreatic origin. Such cycles reflect sea-level fluctuations or tectonic changes which caused movement of the freshwater table. Rapid rises in sea level, perhaps reflecting tectonic pulses, may explain the preservation of the pore space in former vadose zones.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC