Formation And Diagenesis of Encrinite Facies in Hydrocarbon Plays: Predicting Which Encrinite Types Have The Highest Reservoir Potential

Phelps, William T.¹ (1) Riverside Community College, Riverside, CA

Encrinites (rocks containing at least 50% pelmatozoan debris by volume) are a common yet underappreciated reservoir facies. Encrinites occur throughout Paleozoic and Mesozoic carbonates but are ubiquitous in Mississippian sediments. This is reflected in the large number of Mississippian hydrocarbon producing lithologies around the world that include encrinites.

The Great Basin of the western United States offers an excellent laboratory for examining the variety and formation of encrinites. In particular, the Mississippian Joana Limestone of Nevada and Utah provides the material for describing the reservoir potential and characteristics of eight encrinite types that exhibit differences in grain size, faunal content, bedforms and depositional environment. Examples of encrinites from hydrocarbon plays in other locations and of various ages, provides the basis for the prediction and description of the encrinite types most likely to be reservoir facies. In addition, the typical diagenetic pathways for encrinites are examined and a general model for the development of porous and permeable encrinites is developed.

This research suggests the most important depositional factors for producing encrinites with reservoir potential are grain size, mud content and percent pelmatozoan grains. Of these, pelmatozoan gain content is most critical because the grains enhance the formation of porosity occluding syntaxial rim cement. The principal post depositional factor is the degree of dolomitization. Due to compositional and diagenetic characteristics of the encrinites examined, we find three have high probability of serving as reservoir rock, two have moderate, two have low and one has a variable degree of reservoir potential.