The Tale of 3 Megaflaps, Paradox Salt Basin, Utah and Colorado

Abstract

Three outcropping megaflaps (steep, stratal panels that extend several kms up the flanks of salt diapirs) in the Paradox Basin were examined for thickness, structure, lithofacies, and timing of megaflap rotation. The 3 megaflaps are on the south sides of Fisher Valley (FV), Sinbad Valley (SV), and Gypsum Valley (GV) diapirs; FV and SV are proximal to the Uncompahgre Uplift source area, while GV is distal. The megaflaps contain basal, parallel strata formed by the Pennsylvanian Honaker Trail (HT) Fm, which is unconformably overlain by growth strata of the Permian Cutler Fm that displays progressively gentler dips. Regionally, the HT contains cyclic marine carbonates interlayered with marine and fluvial siliciclastics that thicken and deepen into the proximal Paradox Basin foredeep; in the minibasins adjacent to the 3 megaflaps, HT has thick (737-1200m) deep water facies. In contrast, the HT on the 3 megaflaps has thin (117-380m) shallow water facies representing thin roof strata deposited over inflated Pennsylvanian Paradox Fm salt pillows. The overlying growth panel of Cutler Fm has been previously shown to young with distance from the Uncompahgre and here is interpreted to have initiated diachronous, depositional-load induced drape fold rotation of the megaflaps. Although all 3 megaflaps are composed primarily of HT Fm, their thicknesses and lithofacies are variable: FV (182m; 92% coarse-grained siliciclastics and 8% carbonates), SV (380m, 94% coarse-grained siliciclastics and 6% carbonates), and GV (117m, 40% fine-grained siliciclastics and 60% carbonates, with the addition of Paradox Fm dolomite and calcareous mudstone). GV has abundant dead oil throughout its megaflap HT panel, but particularly within its carbonates, whereas FV and SV have only minor dead oil in their siliciclastics. We conclude, based on the thickness and lithologic differences, that these 3 megaflaps would behave significantly differently with respect to hydrocarbon play elements despite their similar origins. Such variations should be carefully assessed when risking megaflap associated plays.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018