Peripheral Clay Replacements as the Critical Diagenetic Feature Controlling Matrix Permeability in the Codell Sandstone, Northeastern Colorad

The Codell Sandstone Member of the Late Cretaceous Carlile Formation is a hydrocarbon-bearing, tight-sand unit and is an active target for unconventional hydrocarbon production in the DJ Basin. The Type 2 sandstone of the Codell Member in northeastern Colorado is a ~30-ft thick, heavily bioturbated, low permeability (<0.1 md) argillaceous sandstone with subordinate amounts of cross-laminated sandstones. The intergranular drainage network within this "tight" sandstone is poorly understood, with the lack of correlation between permeability and lithofacies suggesting a strong diagenetic control. This study focuses on the diagenesis of the Type 2 Codell sands in the Wattenberg and Redtail fields to better comprehend which processes played a role in developing a connected pore network through this clay-rich rock. Five lithofacies were defined using 7 cores, and a paragenetic sequence including 11 features was assembled from thin-section petrographic analysis and electron microprobe mineralogical phase mapping. Quartz cementation, mechanical compaction, precipitation of authigenic clays, and peripheral clay replacements of framework grains are better developed in the laminated lithofacies. Epifluorescence imaging of micropores impregnated with rhodamine dye, coupled with analysis using ImageJ-FIJI, revealed skeletonized flow paths through the clay-rich sands. Cumulative flow-path lengths positively co-vary with permeability, indicating that the skeletonized flow paths capture key aspects of features that control permeability. Imaging of the longest flow paths suggests that peripheral clay replacement of framework grains by illite, chlorite, and kaolinite was the most important diagenetic feature in creating an efficient drainage network. The microporous network formed along grain boundaries created extensive flow paths characteristic of high permeability samples. Micropores associated with intergranular clay masses were also observed, and this network was utilized for short distances when connecting between peripherally replaced grains. While cementation had a negative impact on primary porosity, the development of quartz cements became beneficial to the drainage system by extending the peripheral micropore network between adjacent grains. Therefore, pore connectivity was most improved during intermediate and deep burial, when pore-filling clays began to replace framework grains and cements and create the connected pore network that facilitates the movement of hydrocarbons from storage in micropores and nanopores to induced fractures.