Examination of core and surface samples from several nonmarine sandstones of the Upper Cretaceous Mesaverde Group in the southeastern part of the Uinta basin, Utah, reveals extensive diagenetic alterations. Diagenesis has significantly altered the porosity characteristics of the sandstones and is thought to have a definite influence on porosity log calculations and on the interpretation of various other geophysical logs, sometimes leading to erroneous interpretations of reservoir characteristics.

Leaching of rock fragments and carbonates--calcite, dolomite, and ankerite--has resulted in the development of significant amounts of secondary porosity which probably provide the favorable reservoir properties of the sandstones. Formation of authigenic pore-filling kaolinite, and illite, which replaces rock fragments and which lines pores, has produced microporosity, greatly increased pore surface area, and has created a tortuous pore network. These properties dramatically alter pore geometry and, thus, fluid-flow characteristics which significantly alter the response of resistivity logs and interpretation of sonic and neutron logs. Consequently, erroneous values may be obtained for calculated porosity and water saturation. Interpretation of mercury-injection curves is also influence by authigenesis.

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