Characterizing and Evaluating Tar-Sand Reservoirs with Scanning Electron Microscope--an Example from Sunnyside, Utah

Richard M. Pollastro, Christopher J. Schenk

The presence of authigenic clays and other minerals in tar sand reservoirs presents problems during stimulation and recovery of bitumen. Therefore, it is important to characterize the authigenic mineralogy, morphology of pores, and surfaces of framework grains. Such reservoir characterizations provide insight into the subsequent fluid-flow properties, water-rock chemical interactions, and mechanical stability of reservoir pore systems so that effective and efficient in-situ recovery methods can be implemented.

The scanning electron microscope (SEM), combined with energy-dispersive x-ray analysis (EDX), optical petrography, and x-ray powder diffraction (XRD), was used to characterize bitumen-impregnated rocks from the Sunnyside, Utah, area. The bitumen is hosted mostly in lenticular, fluvial, and lacustrine sandstones of the Tertiary Green River and Colton Formations. Samples from core and outcrop were analyzed by SEM before and after extraction of the bitumen with chloroform; XRD and EDX were used after bitumen extraction.

SEM analysis reveals that the surfaces of grains making up the pore systems of these bitumen-saturated rocks are extremely complex in morphology and mineralogy. Framework grains are commonly coated with a variety of authigenic clay minerals, including nearly pure smectite, illite, illite/smectite, and chlorite. Kaolinite, however, occurs in booklets or vermicules as a pore-filling or replacement phase. Other minerals found as surface growths or cements include zeolites (analcime, chabazite, erionite) and carbonates (calcite, dolomite, ankerite).

The diversity in grain morphology and authigenic mineralogy revealed through the SEM demonstrates that the bitumen-impregnated rocks near Sunnyside are a complex reservoir system. Therefore, complications are expected using conventional in-situ recovery methods for these tar sands.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.