Abstract Little is known about the effect of thrusting on lithological and petrophysical properties of reservoir sandstone. Here we use field observations, probe permeability measurements and thin-section analysis along ten transects from the Muddy Mountain thrust contact downwards into the underlying Jurassic Aztec Sandstone to evaluate the nature and extent of petrophysical and microstructural changes caused by the thrusting. The results reveal a decimetre- to metre-thick low-permeable (≤50 mD) and indurated (0–3% porosity) zone immediately beneath the thrust contact in which dominant microscale processes, in decreasing order of importance, are (1) cataclasis with local fault gouge formation; (2) pressure solution; and (3) very limited cementation. From this narrow zone the petrophysical and microstructural effect of the thrusting decreases gradually downwards into a friable, highly porous ( c. 25%) and permeable (≤2 D) sandstone some 50–150 m below the thrust, in which strain is localized into deformation band populations. In general, the petrophysical properties of the sandstone as a result of overthrusting reveal little impact in overall primary reservoir quality below some tens of metres into the footwall, except for the relatively minor baffling effect of deformation bands.

Abstract: The Vedder (Oligocene) and Kreyenhagen (Eocene) sandstones at the Greeley oil field consist of arkosic to subarkosic arenites and wackes deposited in shallow marine environments. Burial depths of the Vedder sandstones exceed 3150 m and the reservoir temperature is 124°C. The Kreyenhagen sandstones are buried to greater than 3920 m and the reservoir temperature is estimated to be c. 135°C. These sandstones are currently at or very near their deepest burial depths. The textural relationships of the diagenetic minerals suggest syndepositional formation of glauconite, phosphate and pyrite, followed by early precipitation of pore-lining clay coatings and carbonate cements along with framework-grain fracturing and possibly dissolution. With increasing burial, dissolution of the framework grains continued, accompanied by the albitization of feldspars, the formation of K-feldspar and quartz overgrowths, the precipitation of kaolinite and other clays and possibly the precipitation of late carbonate cements. Finally, hydrocarbon migration and the formation of pyrite occurred during late diagenesis. Porosity preservation and reservoir quality are primarily the result of plagioclase dissolution occurring as the strata approached their current burial depths. Mass balance calculations indicate the significant export of aluminium out of the sands. Thus secondary porosity produced by plagioclase dissolution has replaced the primary porosity destroyed by compaction, and now accounts for the majority of the porosity in these rocks.