The Nature of Dolomite Fabrics: Implications for Permian Hugoton Field and Jurassic Smackover Reservoir Models

Abstract

The complexity of predicting hydrocarbon recovery in dolomitized carbonates is often increased by two oversimplified assumptions. First, pore volumes are considered to reflect a supposed volume increase association with dolomite replacement. Second, the inheritance of earlier fabric and pore volume is not incorporated. In dolomitized rocks from the Lower Permian of Hugoton field, Kansas, and the Jurassic Smackover Formation, Alabama and Florida, a volume-for-volume replacement model dependent on previous fabrics has resulted in an improved prediction of reservoir quality distribution. Predictive mapping of dolomite reservoir types begins with a stratigraphic model of original lithologic distribution and often associated previous diagenetic pore modifications. When the replacement dolomite pattern(s) is superimposed, distribution of different dolomite reservoir types results. In the Smackover Formation, isotopic data indicate possible dolomitizing conditions, but dolomite rhomb size and pore structure vary within similar geochemical overprints to reflect original fabric control. Capillary pressure data from the Chase Group of Hugoton field also support a fabric approach to reservoir modeling. Equivalent nondolomitized and dolomitized carbonates do not systematically differ in pore volume. Dolomite fabrics increased in pore size and continuity, indicating a recrystallization of the previous structure. Grouping mercury data of dolomitized rocks by original lithology (reflecting earlier grainmore » and pore size distributions) results in distinct rock characterization values. These different population values can then be extrapolated three-dimensionally based on the boundaries of original lithologic distribution.« less