Organic maturation, primary migration, and clay mineralogy of selected Permian Basin shales



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Texas Tech University


The organic composition and maturity, and clay mineralogy of Woodford, Atoka, Canyon, and Wolfcamp shales of the Permian Basin were investigated with conventional and fluorescence microscopy, x-ray diffraction, and laser fluorescence spectroscopy. These shales were found to be mature source rocks with respect to the generation of liquid hydrocarbons and were of organic facies B and BC (oil and oil/gas prone, respectively) at the onset of catagenesis. An important exception is immature Woodford Shale in the northern region of the Midland Basin. Remaining hydrocarbon potential in mamre shale is gas or oil/gas. Maceral compositions favor the liptinites, mainly Tasmanales alginite and liptodetrinite.

The clay mineral assemblages of Woodford and Canyon shales consist predominandy of discrete illite and accessory iUitic I/S and chlorite. In Canyon shales, kaolinite may be locally abundant. Discrete smectíte and smectite-dominated l/S are not observed in either shale sequence. Analytical electron microscopy reveals tíie detrital origin of the iUite and chlorite. These clays are interpreted to represent the influx of material firom weathered basement rocks in their respective hinterlands.

Primary hydrocarbon migration from these shales is complex. In Canyon clastics, primordial oUs diffuse to sand laminae from adjacent shale. In thickly bedded shales, the continuous hydrocarbon phase is more conspicuous. An iinusual variety of exsudatinite was found to accumulate along bedding plane fracmres and exhibit a strong blue to blue-green fluorescence. Time-resolved fluorescence data indicate that several microenvironments, each characterized by unique ranges of fluorescence Ufetimes, may be detected.



Wolfcamp Formation (Tex. and N.M.), Shale, Permian Basin (Tex.)