Abstract
Previous studies of fluvial, alluvial fan and lacustrine strata have documented changes in stratigraphic architecture and facies that occur as a function of accommodation. One of the most pervasive change of this type is the degree to which original geomorphic elements, such as bedforms, bars and other macroforms, are preserved. In high accommodation conditions, a greater diversity and greater diversity and greater proportion of original bedforms are preserved in continental strata. The same continental environment will be represented by lower facies diversity, and more amalgamated and cannibalized bedforms under conditions of low accommodation. This study sought to determine whether these predictable changes in facies would be accompanied by changes in porosity and permeability, and whether very subtle changes in facies would have marked changes in porosity and permeability. We also wished to identify how subtle variations in facies would affect flow of hydrocarbons through reservoirs. To this end, we conducted stratigraphic, facies and petrophysical analyses of fluvial channelbelt sandstones and associated floodplain and lacustrine mudstones in an exhumed oil reservoir of Tertiary age in the Uinta basin, Utah (USA). Through measurements, we determined that visual estimates of the intensity of surface oil staining were an accurate proxy measurement of themore » pore volume, and all pore volumes were fully filled with dead, waxy oil. Three genetic sequences are exposed in quarry highwalls. Contrary to intuition, the most porous and permeable sandstones characteristic of the lowest unit would not make the most efficient reservoirs because permeability variations are so great they would be difficult to sweep effectively. Channelbelt sandstones of the intermediate accommodation have less variation in permeability, and yet retain sufficient porosity and permeability that they would be more efficient reservoirs.« less
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