WAN, MAUREEN, JOHN D. HUMPHREY, and NEIL F. HURLEY
Dept. of Geology and Geological Engineering, Colorado School of Mines, Golden, CO

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

Eagle Springs field in Nye County, Nevada produces oil from the Sheep Pass Fm., a Cretaceous-Eocene lacustrine carbonate deposit, thought to be stratigraphically time-equivalent to the Green River Fm. Variable production rates among wells suggest a heterogeneous reservoir complicated by faulting and diagenesis. Specifically, some of the complexity arises from variable degrees of dolomitization.

Detailed sedimentologic, diagenetic, and geochemical studies of the Sheep Pass Fm. have been undertaken to help understand dolomitization of the reservoir lithologies. Standard petrography performed on outcrop samples and subsurface indicates that reservoir lithologies are dominantly peloidal, ostracod-, mollusc-, charophyte-bearing dolowackestones and dolomudstones with vuggy/moldic porosities and recrystallized textures. Mineralogy is predominantly dolomite, with subordinate calcite, quartz and mixed calcite/dolomite.

Similarities in d¹⁸O and d¹³C values between cuttings and outcrop samples imply parallel diagenetic trends. Stable isotope data also indicate that reservoir quality of the Sheep Pass Fm. developed during progressive, but relatively shallow, burial diagenesis and dolomitization.

Well log analysis has been used to understand dolomite distribution. Cross plots of density, neutron, sonic and photoelectric logs differentiate interbedded limestones from dolostones. Roughly 25% of the vertical section is dolomite, which is commonly the lithology of the perforated intervals.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas