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 Dempster1
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