Abstract

The Codell Sandstone is a hydrocarbon-bearing, tight sand (permeability <0.1 mD) that is an active target for unconventional hydrocarbon production in the Denver-Julesburg Basin. In northeastern Colorado, the intergranular microporous drainage network within this clay-rich sandstone is poorly understood, with a strong diagenetic control suggested by the lack of correlation between permeability and depositional facies. Core samples from the Wattenberg Field and Redtail areas in Weld County were used to identify which diagenetic processes were most important in developing a connected pore network. Thirteen diagenetic features were defined using thin-section petrography and electron microprobe mineralogical phase mapping, and skeletonized flow paths were delineated by epifluorescence imaging. Quartz overgrowths, mechanical compaction, and clay cements (illite, chlorite, and kaolinite) are better developed in the laminated facies than the burrowed facies. Authigenic calcite and pyrite, and dissolution of framework grains are equally developed in both types of facies. Cumulative 2D flow-path lengths positively co-vary with permeability, indicating that the skeletonized paths capture the features that control permeability. The longest flow paths in high permeability (≥0.09 mD) samples follow micropores created along the periphery of framework grains where the discontinuous quartz overgrowths abut clays. Micropores within intergranular clay masses (detrital, pore-filling cements, and authigenic replacements) associate with shorter flow paths that dominate in low permeability (≤ 0.01 mD) samples and feed the longer paths in high permeability samples. While compaction and all types of cements had a negative impact on the original pore network, the development of long contacts between quartz overgrowths and mechanically juxtaposed grains eventually became beneficial to the drainage system. The increased surface area along those contacts increased the continuity of the flow paths developed along grain surfaces. All observations indicate that the minute quartz overgrowths, and the high authigenic rugosity they created along grain boundaries, were a key diagenetic event in creating the most efficient drainage networks that now facilitate the movement of hydrocarbons at the core-plug scale.