Comparison of Pore-Filling Material in Some Pennsylvanian and Cretaceous Reservoir and Nonreservoir Sandstones: ABSTRACT

SEM and thin-section study of core samples from producing sandstones and associated fine-grained facies from several stratigraphic intervals of Pennsylvanian and Cretaceous age in the Rocky Mountain region show different processes of pore-size reduction relating to: (1) original pore-fluid composition; (2) grain size and texture; (3) depositional environment; (4) timing of diagenesis; and (5) depth of burial. The sandstones studied and depositional environments are: Tyler Sandstone, southwestern North Dakota, shoreline (barrier island); J sandstone, northeastern Colorado, distributary channel and channel margin; Frontier Sandstone, northeastern Wyoming, offshore marine bar; Almond Sandstone, southwestern Wyoming, shoreline (barrier island or tidal channel); Terry and Hygiene sandstones, northeastern Colorado, offshore marine bar; and Raton Formation, southeastern Colorado, fluvial channel and channel margin. All the reservoir sandstones were deposited by relatively high-energy transport processes in fluvial, brackish, or marine depositional environments, and represent initially clean sandstones with relatively good porosity and permeability. Pore space in five of the six reservoir sandstones has been initially reduced by precipitated kaolinite or chlorite. The Tyler Sandstone, a nearshore sandstone which underwent early near-surface diagenesis, exhibits reduced porosity related to precipitation of caliche. Associated fine-grained, nonreservoir facies show a pore-filling composition that differs from the reservoir facies in both the marine and nonmarine sequences. End_of_Article - Last_Page 550------------