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Iron in the minerals calcite and dolomite of limestones and dolostones increases bulk densities and probably also decreases resistivities recorded by logging tools. Whereas dolostones alone retain porosity (intercrystalline and moldic) in the deep Anadarko basin of southwestern Oklahoma and Texas, it is particularly important to distinguish variations in the chemical compositions of dolomite (especially of iron). Bulk densities determined by measuring proportions of major minerals (X-ray diffraction) and iron content (X-ray fluorescence) permit improved estimates of true porosities and water saturations. Dolomite densities range from 2.82 g/cm3 up to 3.02 g/cm3 for strongly ferroan dolomite.
In the Hunton Group (Upper Ordovician to Lower Devonian) of the Anadarko basin, we have calculated more porosity and hydrocarbons than expected for select zones. Intervals with less than 4% apparent porosity before correction for iron actually have 4 to 10% porosity. Potentially productive intervals have been missed.
At depths exceeding 10,000 ft (3 km), ferroan dolomite and calcite in the Hunton Group carbonates are predictably associated with interbedded argillaceous zones and with the underlying marine Sylvan Shale. In the deepest of the 50 wells studied, well logs show high densities in the lowermost Hunton (above the Sylvan Shale) which can only be interpreted as the presence of iron-rich dolomite. Detailed fossil correlations by Amsden have established the relatively constant age of a single lithostratigraphic horizon locally present at the base of the Hunton Group conformably overlying the Sylvan Shale, leading us to conclude that increased temperature with depth is the predominant factor for dolomitization. The smectite-to-illite transition in the Sylvan Shale is suggested as a possible so rce of magnesium and iron. Preliminary shale analysis and stable-isotope-ratio analysis support the above conclusions.
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