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Grain assemblages and diagenesis in organic-rich mudrocks, Late Pennsylvanian Cline Shale (Wolfcamp D), Midland Basin, Texas


Grain assemblages in the organic-rich Cline Shale in the Midland Basin are dominated by components of extrabasinal derivation (11.4 vol.% to 98.5 vol.%; average volume: 82.6%). Major extrabasinal components include K-rich clay minerals, detrital quartz, albite, K-feldspar, micas, detrital carbonate, and lithic fragments, whereas intrabasinal components mainly include biogenic siliceous allochems (sponge spicules and radiolaria), agglutinated foraminifera, Ca-phosphate peloids, clay-rich peloids, organo-mineralic aggregates, intraclasts, and biogenic carbonate allochems. Authigenic minerals are most evident as grain replacements, precipitates in large pores, displacive ankerite, pyrite framboids, and Ca-phosphate cement. Strongly localized spatial distribution of diagenetic products at micrometer to centimeter scales are observed in most siliciclastic samples, except bio-siliceous allochems-rich ones, in which abundant intergranular-pore-filling microquartz cement is observed. Compaction is evident in Cline Shale because of low porosity and generally low cement volumes. Neither textural variation nor bulk mineral composition alone is sufficient to confidently decipher the reservoir quality variation (i.e. porosity, permeability, and total organic carbon content). However, a good negative relationship between the ratio of extrabasinal grain assemblages to intrabasinal grain assemblages and reservoir properties is observed in the Cline Shale. Specifically, higher porosity, permeability, and TOC are observed in samples representing the extreme end members of intrabasinal-derived bio-silica-rich layers. Ni is recognized as a proxy for paleo-productivity and exhibits a positive relationship with intrabasinal grain content and reservoir properties (i.e. TOC, porosity, and permeability). XRF-based Ni content analysis can be a rapid and cost-effective way to delineate favorable unconventional reservoir quality (sweet spot) in the Cline Shale.