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Thirty-six shale samples ranging in depth from 1,454 to 13,430 ft (443 to 4,093 m) from Shell Oil Co. No. 1 Dixie Mortgage Loan well and 33 shale samples ranging in depth from 2,183 to 13,632 ft (665 to 4,155 m) from Shell Oil/Delhi-Taylor Oil Corp. No. 3 A. A. McAllen well were examined by X-ray techniques to determine the mineralogic parameters of the geopressured zone in the Vicksburg Fairway. Both wells have the same weight-percent trends with depth for the mineralogy: quartz, calcite, total clay, and potassium feldspar are constant; plagioclase feldspar gradually increases; kaolinite increases; discrete illite decreases; total mixed-layer illite-smectite (I/S) decreases; illite in mixed-layer I/S increases; and smectite in mixed-layer I/S decreases. Chlorite is found only in the geopressured zone of each well.
Significant diagenetic changes begin at calculated equilibrium temperatures of 58 to 69°C. The most important change is the transformation of smectite to illite within the mixed-layer I/S phase which occurs according to the reaction suggested by Boles and Franks (1979) with Al3+ acting as an immobile component. The source of K+ for this reaction is discrete illite. The breakdown of discrete illite results in two other changes with depth: the formation of kaolinite; and the increase of plagioclase feldspar which is due to reaction with Na+ and Ca2+ provided by the smectite to illite transformation.
The Boles and Franks model is compatible with a steady supply of original mixed-layer I/S during the depositional history of the McAllen Ranch area. The constant content with depth of calcite, quartz, and potassium feldspar indicates that limited material, if any, is supplied by the shales to surrounding sands. The ions generated by changes within the clay minerals are involved in further clay mineral reactions as outlined above. In addition, magnesium and iron are involved in forming chlorite within the shales.
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