Dolomites formed under conditions of deep burial: Hunton Group carbonate rocks (Upper Ordovician to Lower Devonian) in the deep Anadarko Basin of Oklahoma and Texas
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Progressive burial diagenesis of Hunton Group (Upper Ordovician to Lower Devonian) rocks of the deep Anadarko Basin of Oklahoma and the Texas Panhandle is evident from petrographic and geochemical study of cores and cuttings from more than 25 boreholes up to 30,000 ft deep. Limestone of the Hunton Group, which originated as shallow shelf carbonates, has been replaced, chiefly below present depths of about 10,000 ft, by dolomite that is commonly ferroan and is associated with shale. This diagenetic dolomite is inferred to have formed under deep-burial conditions.
The dolomite occurs as finely disseminated, 10μm and larger rhombic crystals, and is most abundant near the base of the Hunton Group, particularly where an oolite unit (the Keel Formation) overlies the thick marine Sylvan Shale that is inferred to be the chief source of Fe2+ and Mg2+ ions. Ferroan dolomite also occurs where clay minerals are abundant in the Middle Hunton Group. In shallow wells, dolomite crystals are euhedral. Below 10,000 ft(3.0 km), where dolomitization of the oolite has been more complete, hypidiotopic and xenotopic textures result. Hydrocarbon-associated fluids are inferred to have dissolved the calcite that was not replaced, and to have created intercrystalline and moldic porosity.
X-ray diffraction verifies a trend of higher dolomite concentrations with increasing depth in the same oolite horizon. For example, oolite samples from outcrop lack dolomite and are 100% CaCO3; cores from 9,200 ft (2.8 km) are about 25% dolomite; and cores from 15,000 ft (4.6 km) and below are more than 85% dolomite. Radioisotope-induced x-ray fluorescence shows that dolomites below 10,000 ft(3.0 km) are iron-enriched relative to both non-dolomitized oolite and dolomites of surface origin. Where the Hunton Group and the Sylvan Shale are buried below 10,000 ft(3.0 km), well logs show high densities in the lowermost Hunton (above the Sylvan Shale) which can be interpreted as the occurrence of Fe2+- rich dolomite. Stable isotope ratios suggest a higher temperature of origin for burial dolomites than for dolomites of surface origin. Formation waters recovered from within the shale and carbonate are greatly depleted in Mg2+ ions compared to normal marine waters.
General restriction of this shale-associated ferroan dolomite to strate that are currently buried below 10,000 ft (3.0 km) in the Anadarko basin supports other lines of evidence for the belief that previously deeply buried strate can be recognized even if the strata have been subsequently uplifted. The transformation of smectite to illite in the Sylvan Shale (conformably underlying the oolite) is suggested as a possible source of Mg2+ and Fe2+. Smectite/illite ratio decreases with increasing depth in the Sylvan Shale. Furthermore, Sylvan Shale below 10,000 ft(3.0 km) is depleted in iron by 70% relative to Sylvan Shale less than 10,000 ft(3.0 km) deep, suggesting that under deep conditions shales have given up their iron (and magnesium), presumably to the overlying carbonates.
KeywordsShale Dolomite Dolomitization Ooids Lower Devonian
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