Abstract

Carbonate rocks buried to depths of 20,000 to 30,000 feet exhibit characteristics that are diagnostic of deep-burial diagenesis. These characteristics were documented in samples of Hunton carbonates (Upper Ordovician to Lower Devonian) obtained from six deep wells drilled in the Anadarko basin of southern Oklahoma and the adjacent Texas panhandle. The rocks were sampled from depths where temperatures are about 210°C and pressures are about 2.5 kilobars. The thermal alteration of vitrinite present in well cuttings demonstrates that the maximum paleotemperatures were essentially the same as the measured, present-day, deep-burial temperatures. These conditions have subjected the carbonates to structural deformation and diagenesis approaching metamorphism. The diagnostic textures observed seem to be due, primarily, to the pressures of 20,000 to 30,000 feet of overburden, whereas, the high temperatures appear to have had little visible effect.

Carbonates with depositional grain-supported fabrics are most susceptible to modification by increased pressures and they most commonly exhibit the diagnostic deep-burial characteristics described. As the lime mud (or micrite) content becomes more abundant in the rock, it appears to have cushioned larger calcite particles from pressure effects of deep burial and there is little, if any, evidence of deep burial in the textures. Hunton carbonates subjected to deep burial have the following characteristics. Mechanical adjustments to the pressures of deep burial are commonly seen. Grainstones and micrite-poor packstones exhibit grain elongation with preferred orientation developed by movement along multiple stress-induced twin planes. Bent twin planes observed may have been caused by the stresses of deep burial and/or nearby tectonism. Cataclastic textures are present showing the development of smaller calcite crystals around the outside of or along glide planes within larger calcite-crystal particles (degrading neomorphism).

In places, original depositional fabrics have been obliterated by total recrystallization to neomorphic calcite-crystal mosaics with marble-like textures and unusual "patchy" extinction patterns. Continuing trends seen in more shallowly buried carbonates, the average grain size increases with depth due to an increase in the crystal size of micrite (aggarding neomorphism). Also increasing with depth are pressure-solution phenomena, such as abundant sutured particle contacts and abundant stylolites.

In places, the Hunton limestones are partially to completely dolomitized. Stress-induced development of twin and cleavage planes have resulted from deep burial, especially in the smaller subhedral to euhedral dolomite crystals.

Porosity in deeply buried grainstones is essentially nonexistent due to the mechanical adjustments that the particles have undergone. Fabrics less affected by deep-burial, especially dolomitized rocks show porosities of up to 26%. Moldic and intercrystalline porosity is probably enhanced by fracture porosity.