Smackover flow units in Vocation and Appleton Fields were identified, mapped, and ranked as part of an integrated reservoir characterization project. Pore categories, pore and pore throat geometries, diagenetic history, and depositional attributes were logged and resulting data were combined with core descriptions, mercury-injection capillary pressure data, and wireline log data to produce flow unit maps. Appleton and Vocation Fields produce from grainstone buildups and microbial reefs. Microbial fabrics within reefs were found to have great influence on reservoir quality. Five fabric categories and growth forms that reflect variations in water energy, sedimentation rate and substrate were identified; 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. 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. Type 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. Results of this work improve understanding of complex grainstone and microbial reef reservoirs and improve our ability to characterize and model complex reservoir architecture, pore systems and flow unit quality from pore to core to field scale.

This study examined depositional and diagenetic characteristics of the Oxfordian (Jurassic) Smackover formation in Womack Hill field, Alabama, as part of an integrated reservoir description program. In order to understand the distribution of reservoir units, this study utilized an integrated array of data from core lithological descriptions, borehole logs, core reports, thin section petrography, porosity and permeability measurements on core plugs, and mercury injection capillary pressure (MICP) measurements. These data made it possible to establish reliable measures of reservoir quality by comparing pore geometry with pore type; then determining which pore types correspond with highest porosity-permeability paired values. Pore aperture (throat) median sizes measured by mercury capillary pressures were tested for correspondence with porosity, pore type, permeability, and saturation in order to establish quality rankings for the reservoir units. This study bridges the gap between petrological and petrophysical studies, merging the data into a comprehensive model. This model, instead of mapping facies based solely on lithology, or flow units based only upon permeability, depicts petrofacies, with each petrofacies having distinct porosity, permeability, and capillary pressure ranges. These characteristics are related to specific lithologies, pore types and diagenetic processes. The resulting combinations are grouped into good reservoir quality rock, intermediate reservoir quality, or baffles, and poor quality reservoir material, or barriers. These zones were then put into a stratigraphic context, allowing for a better understanding of the effects of pore categories, original depositional texture, and diagenetic influences on the distributions of the reservoir quality zones. By incorporating these quality zones, or petrofacies units, with core lithological descriptions and petrophysical data curves, such as porosity and permeability on a cross section, a much more comprehensive model of the field emerges. While in this study, mapping these petrofacies between wells was not possible; this method resulted in the identification of several zones of potentially bypassed pay.

The Spraberry Formation (Permian, Leonardian), typically produces from siliciclastic turbidites in the Midland Basin. The Spraberry Formation at Happy Spraberry Field, Garza County Texas, however, consists of a carbonate interval about 100 feet thick. Cored intervals consist of oolitic/peloidal packstones and grainstones, rudstones and floatstones, in situ Tubiphytes bindstones, and siliciclastics. Standard petrographic analyses were done to determine pore genesis. They revealed that grain moldic, solution-enhanced intergranular, vuggy, and intraparticle pore types dominate the reservoir zones. Only some of those pore types correlate with optimum values of combined porosity and permeability, however. Those optimum combinations are associated with highest "quality" flow units in the field. To better understand the relationship between pore characteristics and petrophysical properties, pore size, shape, and abundance data were obtained with traditional petrographic methods. Repeat measurements were then made using automated image analysis equipment and Image Pro software. Statistical correlation of the two data sets indicates tentatively that image analyses can replace tedious petrographic measurement of pore geometry and pore origin. This further suggests that pore characteristics obtained from image analyses can be rapidly compared with petrophysical measurements, capillary pressure behavior, and production characteristics in complex carbonate reservoirs.

Frameless mounds are commonly called "mudmounds" because many of them contain micrite and they grew without support of skeletal organisms such as corals, stromatoporoids, chaetetids, etc. It is not quite so simple, however. "Mudmounds" – more appropriately called frameless mounds – can include many variations on the frameless theme. They may be 1) cementstone buildups; 2) mounds made of lime mud, peloids, cements, and non-framework skeletal constituents; 3) mounds made mostly of micrite precipitated around "cold" seeps, or 4) mounds made mainly of bioclastic grainstones and packstones that accumulated as piles of skeletal detritus. Frameless mounds are especially common in Mississippian (Lower Carboniferous) strata but they are also common in Upper Carboniferous beds, they dominate the Precambrian to Ordovician section, and seep mounds can occur in nearly any Cenozoic strata. This wide variety of mound types exists because the mounds can form in deep, intermediate, or shallow environments, they may have been influenced by precipitation around submarine vents – irrespective of depositional environment – or they may have been affected by early cementation plus or minus microbial carbonate production. Can such a great variety of frameless mound types have any reservoir characteristics in common? Can muddy, cement-rich mounds without intergranular porosity make reservoirs? The answers are yes to both questions. Consider that porosity forms by any of 3 end-member processes – depositional, diagenetic, and tectonic (fracture). Excluding offmound facies, depositional porosity occurs in mounds as stromatactoid vugs, as intergranular porosity in grainy beds and lenses, and as shelter cavities. Diagenetic porosity is common as dissolution cavities, or solution-enlarged depositional pores, as karst features, and as replacements such as dolomitization. Fractures are common in zones where faulting, jointing, and differential compaction have occurred. Some pore systems may include any combination of genetic pore types to produce excellent hydrocarbon reservoirs. Examples include the Lodgepole mounds in the Williston Basin, Leadville mounds in the Paradox Basin, and Chappel mounds in the Hardeman and Fort Worth Basins. Giant Caspian basin fields such as Tengiz and Karachaganak include frameless mound reservoir zones. In addition, major mineral deposits are common in frameless mounds in the Tri-State area of the USA, in Ireland, and in Africa. At the end of the day, one might conclude that frameless mounds are worth studying!

High poroperm combined values sometimes indicate high connectivity and low resistance to fluid transmissivity in carbonate reservoirs. These zones, or "flow units", may have pore systems that include any combination of 3 genetic pore classes; therefore, the geological model of the reservoir may reflect structural, depositional, diagenetic, or hybrid genetic characteristics. Pore classes within flow units can be ranked for high, intermediate, and low "quality" (ease of extracting hydrocarbons) based mainly on how reservoir rocks behave in capillary pressure runs. Extrapolating the results of the MICP runs and attendant Hg withdrawal efficiency calculations to field scale depends on finding "tags" that link MICP behavior to objective rock properties such as pore geometry, pore aperture size, and pore genesis. Those rock properties can be placed in a geo-history model of depositional properties, burial diagenesis, and tectonism. A test case has been studied at Happy Spraberry field, Garza County, Texas, where about 100 feet of oolitic-peloidal packstones and grainstones, and skeletal rudstones, bindstones, and floatstones compose the reservoir. Each rock type has relatively distinctive pore properties that reflect their geological history, e.g., leached depositional porosity on probable, subtle paleotopographic highs and matrix or cement-reduced pores off topography. Principal pore types in this field are grain-molds, vugs, solution-enlarged intergranular pores, and matrix or cement reduced pores. Flow unit boundaries are loosely facies-selective and were defined by comparing poroperm values from core analyses with pore properties from thin section petrography. The results define correspondences between genetic pore classes and poroperm values ("pore facies"). Pore facies are ranked for "quality" on their capillary pressure character, median pore throat size, and mercury withdrawal efficiency. Pore facies overlays on facies maps indentified spatial distribution of quality-ranked pore facies within flow units.