Micro-facies characterization of the Cane Creek Shale, Paradox Basin, Utah: implications of diagenetic controls on reservoir quality

Abstract

The fine-grained Pennsylvanian Cane Creek Shale of the Paradox Formation, Paradox Basin, Utah exhibits relatively thin cyclic interbeds of contrasting lithologies including fine sandstone to siltstone, organic-rich carbonate and dolomitic mudstone, and evaporites. As such, it provides a unique opportunity to evaluate micro-facies and the fluid storage capacity potential of mixed fine-grained systems. Micro-facies descriptions were performed using core and petrographic analyses, including scanning electron microscopy with energy dispersive spectroscopy. The study leverages core-based porosity and permeability data, as well as previous core description. These data are used together to explore mineralogical depositional and diagenetic controls on porosity and permeability. Twelve micro-facies were described based on sedimentological textures, grain size, lithology, and mineralogy categorized into three groups: 1) sandstone to siltstone, 2) mudstone, and 3) evaporitic micro-facies. Sandstone to siltstone micro-facies exhibit variable porosity and permeability. Porosity connectivity is dependent on the degree of authigenic cementation and compaction that negatively impacts porosity. Mudstone micro-facies show less variability in porosity and permeability; and are characterized by carbonate content, organic-matter, and clay nanopores. The development of early carbonate grains maintains porosity in mudstone and dolomitic siltstone micro-facies. Evaporitic micro-facies represented by evaporitic sabkha-like conditions are characterized by pore-reducing anhydrite and halite cement. A diagenetic paragenetic sequence was developed to assess the timing and impact of mineralogy on reservoir quality. Early (eogenetic), Middle (mesogenetic), and Late (telogenetic) stages correspond with diagenetic stages and known tectonic basin events. Syndepositional dolomite and diagenetic illite/smectite authigenic cements are the main controls on reservoir quality and have opposing effects related to porosity and permeability by preserving early pore space or reducing porosity and permeability.