ABSTRACT

A detailed study of relative clay mineral abundance and distribution in Lower Tuscaloosa cores from the North Hustler field area of southwest Mississippi provides insight into the original clay mineral composition of these rocks and their subsequent diagenetic history. Data produced by this study show that the relative percentages of authigenic clays often change across major lithofacies boundaries as a function of: Major changes in the depositional environment; differences in the energies involved during sedimentation that resulted in substantial changes in textures; mineralogical compositions; and biota, as well as wholesale changes in the chemistry of the waters of sedimentation, in cases where continental to marine contacts or transitions occur. In such cases, differences in the effects produced by progressive diagenesis of the formation are to be anticipated. An important conclusion from this study was that Lower Tuscaloosa clays may act as sensitive indicators of even subtle geologic change. Changes within lithofacies across very sharp discontinuities (such as those which separate stacked fluvial bars) can also produce shifts in clay mineral proportions; with or without textural changes across the contact. These shifts may reflect important aspects of reservoir heterogeneity. Even siltstone-siltstone contacts are highlighted by dramatic shifts in the proportions of the clays, (for example, across the contact between marine interdistributary bay deposits and those of a lower fluvial flood plain). The study also established that carbonate-cemented "tight streaks" are highlighted as much (or more) by shifts in clay mineral proportions than by the expected decrease in porosity.