Detailed mineralogical characterizations, using X-ray diffraction and scanning electron microscopy of sand-shale and carbonate-shale-bentonite sequences, of some classic low permeability hydrocarbon reservoirs reveal basic clay-mineral relationships that are helpful in evaluating the extent of diagenesis and petroleum resource potential.

Gas-potential units of the El Paso Natural Gas 1 Wagon Wheel and BELCO 3-38 (formerly INEXCO 1-A WASP) wells, Green River basin, Wyoming, contain shales composed primarily of altered detrital clay suites of various origins and compositions that are consistently different than authigenic clays formed within adjacent sandstones. Commonly, discrete illite is abundant in the shales as a major detrital component; the percentage of illite is lower in the sandstones. Chlorite typically comprises a high percentage of the clay-size (< 2µm) material of sandstones and is primarily authigenic, however, little chlorite (< 8%) is inherent in the shales. Authigenic interstratified illite-smectite (I/S) formed in relatively clean sandstones is typically less illitic and more restricted in omposition range than I/S clay of various origins within shales or shale-laminated sandstones.

Therefore, because of these primary differences in clay mineral assemblages between sandstones and shales from these units, log interpretations of low permeability sand reservoirs should not be extrapolated from the log response of adjacent shales. The composition of I/S clay from relatively cleaner sands also may give a better indication of diagenetic "minimums."

Indigenous gas and oil are produced from low-permeability chalk beds of the Upper Cretaceous Niobrara Formation within the Denver basin of eastern Colorado and western Kansas. Clay-mineral studies of I/S clay in insoluble residues of shaly chalk sampled throughout the basin show that the starting composition of the I/S clay in these strata is highly variable due to a complex mixing of I/S clays from many different sources at the time of deposition. However, the composition of I/S clay from thin bentonites in these strata is quite consistent throughout a section from any one specific location and/or depth, and these clays become progressively more illitic and ordered with increasing temperature due to increased burial. At approximately 60% illite layers in I/S, the I/S of bentonites pr ceed from random (R = 0) to short-range ordered (R = 1) interstratification. I/S clay formed in thin, discrete bentonite beds within Niobrara strata, where original starting compositions were nearly fixed and uniform and there was little or no contribution from detrital clays, is the best indicator of the extent of diagenetic reactions. The presence (or absence) of regular interstratified clay minerals in thin bentonite beds of the Niobrara can, therefore, be used as relative geothermometers for constructing predictable petroleum resource maturation maps of the Denver basin and adjacent areas.

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