BALANCED CROSS SECTION AND RESTORATION CONSTRAINTS ON HYDROCARBON MIGRATION IN THE CENTRAL UTAH THRUST BELT: CHALLENGES FOLLOWING THE COVENANT FIELD DISCOVERY

The 2004 discovery of Covenant oil field in central Utah is spurring regional exploration much like discoveries in the Utah-Wyoming thrust belt in 1975 and the Canadian thrust belt in 1957. Both earlier discoveries were immediately followed by regional confirmation, whereas current successful play extension in central Utah is limited to the Covenant area. Where Utah-Wyoming and Canadian thrust belt exploration were bolstered by robust, well-connected hydrocarbon systems and good surface or seismic targeting, the central Utah play is hampered by a complex hydrocarbon system, large extensional modification of buried thrust structures, and difficult seismic imaging. These limiting factors are evaluated by sequential restoration of a 215 km-long balanced cross section across central Utah. Attendant thermal models for the Pavant, Paxton, and Gunnison-Salina thrust sheets indicate that Paleozoic source rocks passed beyond oil generation during mid Cretaceous foreland basin burial, well before latest Cretaceous-Paleocene folding along the productive southern Gunnison-Salina trend. The Covenant structure displays only minor modification by late normal faulting, whereas the northern Gunnison-Salina trend is profoundly extended, as are the more western Paxton and Pavant thrust sheets. Low-velocity Cenozoic grabens complicate seismic imaging in these areas. Published hydrocarbon system scenarios rely on initial eastward migration and trapping of oil in the western Colorado Plateau, followed by Cenozoic westward remigration to Covenant field. The regional restoration indicates that remigration may have been aided by uplift associated with Sevier Desert extension, which reversed the dip beneath the thrust belt from westward to eastward during the late Cenozoic. However, the under-filled oil column at Covenant field and lack of prior success along adjacent anticlinal trends may imply that remigration along complex pathways is inefficient in filling classic anticlinal traps in this highly extended thrust belt.