Replacement Geometry and Fabrics of the Smackover (Jurassic) Dolomite, Southern Alabama

ABSTRACT The intensity of dolomitization and associated reservoir development in the Smackover Formation, Conecuh Ridge-Wiggins Arch areas, is not predictable by comparison with time-temperature index methods or porosity-depth curves. Major alteration patterns and associated dolomite fabrics reflect the early paleotographic settings, where high fluid flux and massive replacement are associated with Late Jurassic basement highs. This large-scale replacement is most important in the lowermost Smackover section overlying the Norphlet sands from Chunchula Field going into the Manila Embayment. Its heterogeneous nature of dolomite reservoir development is evident in Chunchula Field, where local pods of permeable dolomite cut across sedimentary facies boundaries. Irregular distribution results from at least three separate processes: (1) general decrease of dolomite replacement going into the embayment northeast of the field; (2) the later addition of iron-rich dolomite cement to permeable replacement dolomite areas prior to oil migration; and (3) the inheritance of an array of original carbonate lithologies, changing vertically and laterally and differing in their textural response to the dolomite replacement process. This pattern of changing original lithologies and the diagenetic pinchout of permeable dolomite is also present in the Barnett-Vocation field areas. Distribution of dense versus permeable dolomite was mapped in Vocation as reflecting the present-day structural crest, but later drilling demonstrated the dolomite permeability barrier to be oriented to the trend of general thinning of the Smackover. Patterns of permeability pinch-outs in dolomite do not necessarily reflect changing geochemistry of dolomitizing fluids, but are a response to an original lithological change to more muddy carbonates. These muddy sediments differ in number and sizes of nucleation sites, differ in their original pore configuration, and generally result in a dense dolomite in the Alabama Smackover. Predicting the orientation of a dolomite permeability barrier where there is little well control can be facilitated by mapping the regional thickness variations; the extension of impermeable dolomite trends is generally parallel to isopach trends.