Why Modelers Need to Look at the Rocks! – Examples from Greater Aneth Field, Paradox Basin, Utah

Chidsey, Thomas C.¹, David E. Eby², Michael D. Laine¹, and J. Thomas Dempster^1
1Utah Geological Survey, Salt Lake City, UT
²Eby Petrography & Consulting, Inc, Littleton, CO

     Greater Aneth field, Utah's largest oil producer, has produced over 440 million bbls. The Aneth Unit, in the northwestern part of the field, was selected for a combined enhanced oil recovery (EOR) and CO₂ sequestration demonstration project; the projected production increase is 15,000 BOPD. Located in the Paradox Basin, Greater Aneth is a major stratigraphic trap. The primary reservoir is the Desert Creek zone sealed by the overlying Gothic shale, both within the Pennsylvanian Paradox Formation. Past well-log interpretations and published cross sections divide the Desert Creek into several correlatable reservoir subzones and units, as well as a few seals, across the field. However, caution is urged when using this type of information alone to generate reservoir models for EOR and CO₂ sequestration at Aneth and other fields.
     Examination of available slabbed conventional cores from Aneth Unit wells reveals a more complex reservoir consisting of limestone (oolitic, peloidal, and skeletal grainstone and packstone, and algal boundstone/bafflestone) and finely crystalline dolomite. These lithotypes represent a variety of depositional environments (openmarine shelf, shallow-marine beach and shoal, algal mound, lowenergy restricted shelf) that produce reservoir heterogeneity beyond what is determined from well logs. Fractures in cores are relatively common and there is evidence (hydrothermal [saddle] dolomite, brecciation) of minor but important faults that may affect fluid flow. Cores reveal additional potential seals within the Desert Creek (mudstone and very fine grained sandstone units). Finally, several units containing the bryozoan Chaetetes have good well-log porosity, but core observations show the porosity is ineffective.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah