Abstract: Stratigraphic Controls on Facies Characteristics and Petrophysical Attributes in Fluvial Channel Sandstones

Timothy A. Cross, Yohan Kusumanegara

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 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 mud- stones 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 the pore volume, and all pore volumes were fully filled with dead, waxy il.

Three genetic sequences are exposed in quarry highwalls. Each begins with a basal erosional surface on the wettest coastal plain or deepest lake facies of the underlying cycle. This is succeeded by amalgamated or isolated channelbelt sandstones capped by floodplain mudstones and/or shallow lake sandstones and mudstones. This facies succession records accumulation of each genetic sequence during increasing accommodation. At a larger scale, all genetic units are recording increasing accommodation, because channelbelt sandstones of the lowest unit are more amalgamated and cannibalized than those of the second unit. The upper genetic unit contains only isolated channel sandstones and more floodplain and lacustrine mudstones.

Porosity and permeability changes followed exactly the changes in degree of preservation of bedforms within channel sandstones that record these changes in accommodation. Channel sours in the amalgamated channel sandstones are permeability barriers, and porosity and permeability of trough-cross stratified sandstones decrease upward to the base of the next channel scour. This pattern occurs at all scales and for all facies. 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 p rmeability that they would be more efficient reservoirs.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France