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We have studied in outcrop and in the subsurface the reservoir characteristics and diagenesis of the Early Jurassic, eolian, Navajo (Nugget) Sandstone in Utah and southwestern Wyoming. Principal topics include: texture, framework constituents, intergranular and intragranular cements, matrix, directional porosity, porosity types, and hydraulic conductivity.
Present characteristics of the sandstone are related to variations in dune and interdune environments, fluctuating ground-water chemistry, and postdepositional environments. Diagenesis, in which an open-system diagenetic model is considered, superimposes and is influenced by these variations and events.
The sandstone was deposited principally in dune and interdune settings. Surface and/or near-surface cementation, mostly by grain overgrowth and calcite from ground-water solutions, consolidated the sediment. Thereafter, the succession was uplifted, eroded, and buried by Middle Jurassic formations. During the Late Jurassic through Eocene, the sandstone was affected by compressional folding and thrust faulting in central and northern Utah and southwestern Wyoming. During the Tertiary, extensional deformation occurred.
Petrographic criteria in thin sections indicate that secondary leached porosity generally is common to dominant in surface, near-surface, and possibly in deep subsurface sequences. Variations in porosity and volume of cements with depth exist and are correlative in all areas studied. Porosity changes with depth, as well as the volume of cements, are the result of downward-decreasing surface leaching of intergranular and intragranular cements, creation of secondary porosity, effects of initial stratification types, and facies changes.
Compaction of the sandstone due to mechanical rearrangement of detrital grains and intergranular pressure solution played little or no role in modification of porosity or hydraulic conductivity. Likely, they account for not more than 5% of the porosity reduction. Greatest porosity reduction (16%) was by cementation. Present mean total porosity is 19%, ranging from 3 to 35%.
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