--> Peripheral grain replacements by clays control matrix permeability in the Codell Sandstone, northeastern Colorado

AAPG Rocky Mountain Section Meeting

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Peripheral grain replacements by clays control matrix permeability in the Codell Sandstone, northeastern Colorado


The Codell Sandstone is a hydrocarbon-bearing, tight-sand unit (permeability <0.1 md) that remains an active target for unconventional hydrocarbon production in the DJ Basin. In northeastern Colorado, the intergranular drainage network within this clay-rich sandstone is poorly understood, with the lack of correlation between permeability and facies suggesting a strong diagenetic control. Core samples from the Wattenberg and Redtail areas were used to identify which diagenetic processes where most important in developing a connected pore network within the Codell sands. A paragenetic sequence containing eleven features was defined using thin-section petrography and electron microprobe mineralogical phase mapping. Skeletonized flow paths across each thin section were also defined from epifluorescence imaging of micropores. Cumulative flow-path lengths positively co-vary with permeability, indicating that the skeletonized flow paths capture the features that control permeability. The longest flow paths in high permeability (≥0.09 md) samples follow micropores created by peripheral clay (illite, chlorite, and kaolinite) replacement of framework grains. Micropores associated with intergranular clay masses (detrital and especially authigenic replacements of feldspars and chert) associate with shorter flow path that dominate in low permeability (≤ 0.01 md) samples, and feed the longer paths along peripherally replaced grain in mid to high permeability samples. While all types of observed cements and mechanical compaction had a negative impact on primary porosity and permeability, the development of long contacts between mechanically juxtaposed grains and the formation of quartz overgrowths eventually became beneficial to the drainage system. 51 Wyoming Geological Association – September 15-18, 2019 Those processes increased the connected surface area of adjacent quartz grains, and thus the continuity of the subsequent microporosity along mineral surfaces developed with peripheral grain replacements. All of these observations indicate that peripheral grain replacements by clays was the most important diagenetic event in creating the most efficient drainage networks that now facilitate the movement of hydrocarbons at the core-plug scale.