INTRODUCING THE LARGEST SINGLE OIL FIELD (GREATER ANETH, SOUTHEASTERN UTAH) COLLECTION OF CARBONATE CORES IN THE ROCKY MOUNTAINS— TOOLS FOR EDUCATION AND RESEARCH

The Utah Core Research Center (UCRC) has added to its inventory a significant collection of carbonate cores (as well as thin sections and other formerly proprietary data) taken from wells in Utah’s largest oil field, Greater Aneth, in the Paradox Basin. Greater Aneth has produced over 483 million barrels of oil and 441 billion cubic feet of gas from the shallow marine Pennsylvanian (Desmoinesian) Paradox Formation. Limestone and finely crystalline dolomite reservoir rocks are sealed by organic-rich, overlying and underlying shale beds, that are also the source of hydrocarbons in this enormous stratigraphic trap.

The new collection consists of cores from 127 wells totaling about 7.4 km. These cores display a wide variety of characteristics that are critical for understanding carbonate rocks—lithofacies, diagenetic events, petrophysical properties, and sequence stratigraphy (flooding surfaces, stacking patterns, cyclicity, systems tracts). The Aneth cores reveal complex packages of carbonate rocks consisting of (1) oolitic, peloidal, and skeletal grainstone and packstone, (2) phylloid-algal bafflestone, (3) microbial boundstone, and (4) deeper water, crinoid-bearing wackestone and mudstone. These lithotypes are the products of diverse depositional environments including shallow-marine beach and shoal, algal mound, low-energy restricted shelf, open-marine shelf, etc., that produce significant heterogeneity within the Aneth cores. Fractures are relatively common and there is evidence (i.e., hydrothermal dolomite, stylolite swarms, and local brecciation) of minor but important faults that may affect fluid flow. Porosity includes interparticle, shelter, intraparticle, vuggy, moldic, and intercrystalline pore networks, often enhanced by fractures. The original carbonate fabrics are commonly overprinted by dolomitization, early marine cementation, dissolution, and late, post-burial compaction and calcitic or anhydritic filling.

The Aneth core collection is now permanently preserved and publicly available at the UCRC for detailed studies by students, professors, and research organizations, as well as oil companies. The carbonate characteristics of the Paradox Formation observed in the Aneth cores provide outstanding teaching tools for geology students.