ABSTRACT

Although deeply buried (18,000 -> 20,000 feet) eolian and reworked marine Norphlet arkose and subarkose in Mississippi, Alabama, and Florida have been intensely studied by several workers, fundamental questions remain regarding diagenetic controls on reservoir quality and the origin of porosity. In spite of a regionally uniform framework composition of quartz, albite, and potassium feldspar, the diagenetic character of the unit is variable on a scale ranging from individual laminations to single hydrocarbon-producing fields to areas encompassing several fields or offshore blocks. The presence or absence of clay minerals in various forms clearly is a dominant control on porosity-permeability trends. In deep reservoirs in Mobile Bay and offshore Alabama and Florida petrographic evidence for dissolution of pervasive authigenic carbonate and/or evaporite minerals to produce high secondary porosity values is equivocal or absent. Although evidence exists for some secondary porosity, much porosity appears to be relict primary porosity. On a regional scale, including both onshore and offshore areas, sandstones with radial, authigenic chlorite coats consistently have high porosity and permeability. In Mobile Bay and offshore Alabama, the distribution of this form of chlorite may be controlled by the presence of precursor clay/iron-oxide grain coats. The occurrence of these coats likely is related to environment of deposition. For example, reworked marine sands, which constitute much of the "tight zone" in the upper Norphlet in Mobile Bay and offshore Alabama, were deposited in an environment hostile to formation or preservation of syndepositional grain coats, as also may have been the case in some subenvironments of the eolian system.

Although chlorite is the dominant authigenic clay mineral in offshore areas, illite is dominant in onshore areas. Sandstones with authigenic illite fibers and flakes typically have permeability values lower, by an order of magnitude or more, than those with authigenic chlorite. In general, chlorite is dominant in eastern areas of Norphlet exploration and production and illite is dominant in western areas. It is unlikely that provenance variations are of sufficient magnitude to account for the distribution of the two clay minerals, particularly since illite or chlorite may dominate in different wells within the same onshore field. Variations in types of authigenic carbonate minerals and in degree and type of feldspar alteration suggest that differences in fluid composition and fluid-flow paths relative to sub-basins, structural highs, salt structures, fault systems, and depositional texture are important. The occurrence of authigenic magnesium-rich chlorite, zeolites, and an unusual ferroan magnesite cement indicates that fluids associated with evaporites and variations in the composition of evaporites or residual fluids may be responsible for variation in diagenetic character of the Norphlet. Evaporites associated with these fluids include the underlying Louann salt and evaporite systems within the Norphlet.