Mapping and ranking flow units in reef and shoal reservoirs associated with paleohighs: upper Jurassic (Oxfordian) Smackover Formation, Appleton and Vocation Fields, Escambia and Monroe Counties, Alabama

Abstract

Flow units in the Oxfordian Smackover Formation at Vocation and Appleton Fields were identified, mapped, and ranked as part of an integrated reservoir characterization project. Pore categories by origin, pore and pore throat geometries, pore-scale diagenetic history, and core-scale depositional attributes were logged with conventional petrographic and lithological methods. Resulting data were combined with core descriptions, mercury-injection capillary pressure data, and wireline log data to produce flow unit maps at field scale. Appleton and Vocation Fields produce from grainstone buildups and microbial reefs. Classification of microbial fabrics within reefs was found to have significant influence on pore facies and flow unit quality rankings and ultimately on reservoir quality in these fields in particular and in southwest Alabama in general. Microbial reefs are composed of five fabric categories and growth forms that reflect variations in water energy, sedimentation rate and substrate. They include Type I layered thrombolite with characteristic mm/cm-scale crypts, Type II reticulate and "chaotic" thrombolite, Type III dendroidal thrombolites, Type IV isolated stromatolitic crusts , and Type V oncoidal packstone/grainstones that grew on soft to firm substrates in high-energy conditions. Types I, II, and III buildups are the most productive reservoirs. Of these, Type III buildups contain the highest quality reservoir rocks, which consist of extensively dolomitized reticulate and dendritic fabrics that have well-connected intercrystalline and vuggy porosity. Types IV and V microbialites are poor reservoir rocks because Type IV reefs are rarely in communication with the bulk of the reservoir and Type V oncoids exhibit separate vug porosity with low to moderate permeability. Results of this work have improved our understanding of complex grainstone and microbial reef reservoirs. In so doing, the results have improved our ability to characterize and model complex reservoir architecture, pore systems and flow unit quality from pore to core to field scale. This study should enable more accurate and economical development of these fields and of others like them.