Tarry residues within porous zones in the upper 15 m of the erosionally truncated Pipe Creek Jr. reef attest to it being a fossil oil reservoir. The radially arranged, steeply dipping (25 to 40°) flank beds of this large (1.4 km diameter) limestone buildup are composed of crinoidal grainstone, packstone, and wackestone with minor stromatoporoid and coral boundstone. Interparticle, intraparticle and shelter pores of all sizes accounted for depositional porosity of 60 to 80%. Syndepositional submarine and marine phreatic cementation, including palisade and monocrystalline syntaxial overgrowth-cements, reduced depositional porosities to 5% or less. The abundance of hardgrounds, isopachous palisade-cemented grainstones, and lithoclasts indicate the syndepositional genesi of these cements. Syndepositional and younger fractures, some extending at least 215 m laterally, cut through the tightly cemented reef. Sedimentary dikes resulted as these fractures were filled with syndepositional reef sediment and later with Devonian quartzarenites. The tightly cemented dikes form permeability barriers that may inhibit lateral fluid flow altering reservoir quality. Dolomite selectively replaced the finer sediment and partly replaced many skeletal grains in zones within the reef-interreef transition and along the present unconformity. Leaching of the remaining calcite in dolomitized skeletal grains produced localized porous zones (up to 15%). Meteoric phreatic sparry calcite cementation further reduced reef porosity to under 2%. Further quarrying of this beautifully p eserved limestone reef will insure its value to geologists studying reef facies, diagenesis, and porosity evolution for many decades.

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