Abstract: Dolomitization of an Eocene Lacustrine Carbonate Oil Reservoir, Eagle Springs Field, East-Central Nevada

WAN, MAUREEN, Colorado School of Mines, Dept. of Geology and Geological Engineering, Golden, CO

Eagle Springs Field in Nye County is the first oil-producing field in Nevada. The field produces oil from the Sheep Pass Formation, a Cretaceous- Eocene lacustrine carbonate deposit, thought to be partly stratigraphically time-equivalent to the Green River Formation. Variable production rates among wells suggest a heterogeneous reservoir complicated by faulting and diagenesis.

Detailed sedimentologic, diagenetic, and geochemical studies of the Sheep Pass Fm. at Eagle Springs oil field have been undertaken to obtain an understanding of dolomitization and porosity evolution of the reservoir lithologies. Since no core (only cuttings) is available at Eagle Springs, two stratigraphic sections from nearby Sheep Pass outcrops have been sampled in detail. Standard petrography performed on field samples indicates that reservoir lithologies are dominantly peloid-ostracod-mollusc-charophyte dolowackestones/mudstones with vuggy/moldic porosities and recrystallized textures. Future work involves using well log analyses techniques to characterize the subsurface. X-ray diffraction analyses of bulk samples taken from cuttings would be used to verify the lithologic interpretations.

Similarities in d¹⁸O and d¹³C values between cuttings and outcrop samples imply parallel diagenetic trends. Both show a trend toward more depleted stable isotope values, suggesting dolomitization and/or dolomite recrystallization with increasing burial temperatures. Isotopically depleted calcite that fills late stage fractures post-dates dolomitization; however, early stages of Sheep Pass diagenesis have yet to be resolved.

AAPG Search and Discovery Article #90931©1998 AAPG Foundation Grants-in-Aid