Upper Cretaceous Shannon Sandstone reservoirs, Powder River basin, Wyoming; evidence for organic acid diagenesis?

Shallow Shannon sandstone reservoirs show minor diagenesis characterized by skeletal plagioclase, unaltered K-feldspar, 40- 60% expandable glauconite-smectite, rare quartz or albite overgrowths that are small and unetched, and secondary porosity due to dissolution of early calcite cement partly filled with authigenic kaolinite and Fe-oxide. In contrast, deep Shannon sandstone reservoirs show significant diagenesis characterized by abundant, large, etched quartz and albite overgrowths, skeletalized or albitized K-feldspar, albitized plagioclase, grain- rimming chlorite, ferroan carbonates and secondary porosity due to dissolution of 10-20% expandable glauconite-smectite. Geohistory analysis suggests that shallow reservoirs never exceeded 75 degrees C, a T too low for kerogen to release organic acids. In contrast, deep reservoirs attained T up to 110-120 degrees C, the T range of maximum release of organic acids. The different diagenetic and different T profiles are asserted to be cause and effect.