Greater Aneth Field, Paradox Basin, Southeastern Utah

Thomas Chidsey, Jr. and David E. Eby
Utah Geological Survey

Greater Aneth, Utah's largest oil field, was discovered in 1956 and has produced over 465 million bbls. Located in the Paradox Basin of southeastern Utah, Greater Aneth is a major stratigraphic trap. The primary reservoir is the Desert Creek zone of the shallow marine, Pennsylvanian (Desmoinesian) Paradox Formation. Carbonates in the Desert Creek are sealed by the organic-rich, overlying and underlying Gothic and Chimney Rock shales, respectively, that are also the hydrocarbon source rocks. The net reservoir thickness of the Desert Creek zone at Greater Aneth is 50 ft over a 48,260-ac area. Porosity averages 10% in interparticle, vuggy, moldic, and intercrystalline networks enhanced by fractures; permeability averages 10 mD, ranging from 3 to >100 mD. The drive mechanism is fluid expansion and solution gas; original water saturation was 24%. The initial reservoir field pressure was 2170 psi. The produced oil is a dark green, paraffinic, sour crude. The API gravity of the oil ranges from 40º to 42º. Greater Aneth field is a complex reservoir consisting of limestone (oolitic, peloidal, and skeletal grainstone and packstone, and phylloid-algal bafflestone and microbial boundstone) and finely crystalline dolomite. These lithotypes represent a variety of depositional environments (open-marine shelf, shallow-marine beach and shoal, phylloid-algal mound, low-energy restricted shelf) that produce significant reservoir heterogeneity. Several facies indicate microbial activities including sediment bindings and thrombolitic/stromatolitic structures with relief. Fractures are relatively common and there is evidence (hydrothermal dolomite and 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). Currently, there are 449 producing wells in the field. Waterflood operations are used in all four field units – the largest waterflood program in Utah. Production was greatly increased due to carbon dioxide flooding (the only one in Utah) beginning in 1985 and an extensive horizontal drilling program in the 1990s, where northwest-southeast-directed horizontal wells were drilled perpendicular to fault/fracture zones. The combination of carbon dioxide flooding and horizontal drilling, along with a better understanding of the reservoir heterogeneity, could extend the life of Greater Aneth for decades to come.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013