Sequence Stratigraphy and Reservoir Delineation of the Middle Pennsylvanian (Desmoinesian), Paradox Basin and Aneth Field, Southwestern Usa
Authors: Weber L. James , Sarg J. F. (Rick) , Wright Frank M. 
Published: November 2010

A sequence stratigraphic framework has been established for the Middle Pennsylvanian (Desmoinesian) section in southeastern Utah using: 1) surface exposures at Honaker Trail, Raplee Anticline, and Eight Foot Rapids located 25 to 40 miles (40-64 km) west of the Aneth field, 2) core and well logs in SE Utah, S W Colorado, NW New Mexico, and NE Arizona, and 3) regional seismic. Bounding discontinuities (sequence boundaries and maximum flooding surfaces) have been correlated over several thousand square miles in the Four Corners region. Systems tracts of 3rd-order composite sequences (0.5-5.0 m.y.) are comprised of 4th-order sequences (0.1-0.5 m.y.) and 5th-order depositional cycles or parasequences (0.01-0.1 m.y.). Nineteen discrete and mappable high-frequency depositional cycles are recognized within three fourth-order depositional sequences of the Desert Creek and lower Ismay section (Middle Desmoinesian) at the McElmo Creek Unit of the Aneth field, southeastern Utah. These simple sequences stack into parts of two third-order sequence sets.

Facies analysis of 12,000 feet (3,660 m) of core was tied into the chronostratigraphic framework to constrain correlation of high-frequency depositional cycles (parasequences). Facies mapping within parasequences permitted 1) prediction of porous and permeable facies and 2) characterization of variability in reservoir pore systems. Syndepositional dolomitization and dissolution in peritidal facies, at shoal crests of parasequences and at sequence boundaries caused modification of reservoir character.

Since discovery of the Aneth field (1956), 370 million barrels of oil (∼ 1.3 billion barrels original oil in place) have been produced from Middle Pennsylvanian carbonates of the Ismay and Desert Creek intervals. Stratified reservoirs occur within lowstand, transgressive, and highstand systems tracts. Siltstone, dolostone, and evaporites form lowstand wedges that were deposited 150 feet below the crest of the Aneth carbonate platform. Porous dolomudstone and dolowackestone are productive where they onlap and pinch out against the Aneth carbonate platform and are isolated from reservoirs on the platform. Within transgressive systems tracts, lagoonal/tidal flat dolomudstone/wackestone compose parasequences and display intercrystalline and solution-enhanced secondary porosity. Core analysis and production/performance data indicate that significant fluid pathways are developed in dolomudstone deposited on the carbonate platform on paleodepositional highs. In the lower Desert Creek, initial parasequences of the highstand systems tract represent a time of mound building and platform development as a result of coalescing biologic communities of phylloid algae. Interparticle and shelter porosity dominate. Subsequent parasequences within the lower Desert Creek highstand systems tract are composed of skeletal and nonskeletal wackestone to grainstone. Porosity is developed on paleodepositional highs at the top of parasequences where shoal water facies have preserved primary pore systems that are secondarily enhanced by leaching of less stable carbonate minerals by meteoric water. Reservoirs dominated by primary pore systems provide the best long term production and account for the majority of oil produced in McElmo Creek. In the upper Desert Creek highstand systems tract, ooid/peloid grainstones aggrade and prograde to fill available depositional space. Hydrocarbons are produced on the platform and along the platform to basin margin from carbonate sand sheets and allochthonous debris aprons. Grainstone debris aprons may also be deposited during early lowstand conditions of the lower Ismay sequence. On the platform meteoric diagenesis resulted in the formation of oomoldic porosity in ooid grainstone deposits beneath the upper Desert Creek sequence boundary. Within moldic pore systems storage capacity is favorable, but permeability is low, generally less than 1 md. Facies composed dominantly of moldic porosity in the absence of significant primary porosity are poor reservoirs.