ABSTRACT: Diagenetic Reactions in Sandstones of the Red Wash Oil Field, Uinta Basin, Utah

R. B. Wanty, J. K. Pitman, T. D. Fouch

Oil reservoirs in the Red Wash field consist of lacustrine and fluvial sublitharenites deposited along the margin of ancient Lake Uinta during the Paleocene and Eocene. These rocks are now near their historic maximum burial depth and temperature. Major and trace-element chemistry and stable isotopy of the diagenetic phases reflect the timing, extent, and character of diagenetic reactions. Thermodynamic models of these reactions were constructed to simulate the effect of diagenesis on reservoir properties and groundwater chemical evolution in the Red Wash field.

Diagenetic reactions have produced multiple generations of calcite cement, with lesser amounts of early quartz overgrowths, dolomite, and later anhydrite. Multiple episodes of calcite precipitation are distinguished on the basis of their trace-element concentrations. For example, early calcite is iron poor (Fe/Ca atomic ratio < 0.05), but later calcite is iron rich (Fe/Ca atomic ratios to 0.2). Iron zoning in the calcites documents the varying iron concentration of the diagenetic solutions with time. The distribution of trace elements in individual grains gives clues to the rate at which calcite precipitated or dissolved in these rocks. Variation in carbon and oxygen isotopes (^dgrC^13_calcite from -10.5 to +1^pmil PDB; ^dgrO^18_calcite from 14 to -6^pmil PDB) from closely spaced samples indicates multiple sources for these elements, perhaps from more than one fluid source.

Analyses of formation-water samples in the Red Wash field indicate that these waters are presently slightly oversaturated with respect to calcite and dolomite. Thus, the cementation reactions described here may be continuing.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990