--> --> Abstract: Pore Characteristics in Microbial Carbonate Reservoirs, by Wayne M. Ahr, Ernest A. Mancini, and William C. Parcell; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Pore Characteristics in Microbial Carbonate Reservoirs

Wayne M. Ahr1; Ernest A. Mancini1; William C. Parcell2

(1) Geology, Texas A&M, College Station, TX.

(2) Geology, Wichita State University, Wichita, KS.

Microbial carbonates are biogeochemical precipitates formed in situ directly or indirectly by benthic microorganisms. Their depositional pore characteristics are determined by biogeochemical growth fabrics constructed from microbial building blocks instead of grains or crystals as in detrital limestones and crystalline dolostones. Diagenetic alteration or fracturing may create hybrid pore types, but unless the depositional fabric is destroyed, petrophysical characteristics of microbialites depend on building blocks and their corresponding growth fabrics. The term “boundstone” is insufficient to differentiate between varieties of depositional pore geometry constructed within or around peloids, peloid clusters ("clots"), filaments, "shrubs", stromatolitic laminae, spherulites, or fossil microbes per se. Differences in depositional pore characteristics between microbial and detrital or crystalline fabrics can impact estimates of oil in place, recovery efficiency, behavior in secondary recovery, and reservoir models. For example, depositional fabrics made of peloidal building blocks occur in modern microbial buildups at Cuatro Cienegas, Mexico. Similarities exist between the Cuatro Cienegas depositional pore characteristics and those in the Smackover (Jurassic) microbial reservoir at Little Cedar Creek field, Alabama. Modern depositional fabrics in Shark Bay microbialites exhibit fenestral and interlaminar pore types similar to some of the Cretaceous microbialites in the Atlantic off Brazil, where thrombolitic and spherulitic fabrics dictate reservoir pore characteristics. Because microbial carbonates are in situ biogeochemical accumulations such as reefs, stromatolites, thrombolites, and leiolites, the identification of petrophysical rock types and their corresponding flow units requires different approaches than those used to characterize reservoirs with interparticle porosity in detrital and crystalline carbonate or terrigenous sandstone reservoirs where particle size rather than biogenic fabric is the essential parameter. One alternative approach defines petrophysical rock types by genetic pore type instead of depositional facies.