Dolomites Formed Under Deep Burial Conditions: Hunton Group Carbonate Rocks (Upper Ordovician to Lower Devonian) in Deep Anadarko Basin of Oklahoma and Texas: ABSTRACT

Abstract

Petrographic and geochemical study of cores and cuttings from 25 boreholes ranging in depth from near surface to 30,000 ft (9.1 km) of the Hunton Group (Upper Ordovician to Lower Devonian), in the deep Anadarko basin of Oklahoma and the Texas panhandle, shows progressive burial diagenesis with increased depth. Limestone conformably overlying shale has been diagenetically altered to dolomite, commonly ferroan, chiefly below current depths of 10,000 ft (3.0 km). The dolomite occurs as finely disseminated, 10 µm and larger rhombic crystals, and is most abundant near the base of the Hunton Group, particularly where an oolite unit overlies the thick marine Sylvan Shale inferred to be the chief source of Fe2+ and Mg2+ ions. Dolomite crystals are euhedral above about 10,000 ft (3 km). Below 10,000 ft, more complete dolomitization of the oolite produced hypidiotopic and xenotopic textures. Fluids associated with hydrocarbon migration (following dolomitization) dissolved the nonreplaced calcite, thereby creating intercrystalline and moldic porosity. X-ray diffraction verifies a trend of higher dolomite concentrations in the same oolite horizon with increasing depth. Oolite samples from outcrop lack dolomite (100% CaCO3); cores from 9,200 ft (2.8 km) are about 25% dolomite; and cores and cuttings from 15,000 ft (4.6 km) and below are +85% dolomite. Radioisotope-induced x-ray fluorescence shows that dolomites below 10,000 ft (3 km) are iron enriched relative to both nondolomitized oolite and dolomites of surface origin. We therefore conclude that dolomite has formed under deep burial conditions. End_of_Article - Last_Page 1317------------