--> --> Petrographic, Halokinetic, and Structural Analysis of Permian-Age Carbonate Caprock at Gypsum Valley Salt Wall, Paradox Basin, Colorado

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Petrographic, Halokinetic, and Structural Analysis of Permian-Age Carbonate Caprock at Gypsum Valley Salt Wall, Paradox Basin, Colorado

Abstract

Diagenetic carbonate caprock forms by altering gypsum/anhydrite caprock with sulfate-reducing bacteria present in hydrocarbons, and may generate prolific reservoirs, local seals, or major drilling hazards if unpredicted in diaper-flanking positions. This study provides a comprehensive evaluation of diaper-proximal trap architecture of a well-exposed lateral carbonate caprock assemblage along the southern margin of the Gypsum Valley Salt Wall, Paradox Basin, Colorado.

Detailed mapping, stratigraphy, and petrographic data were collected and analyzed from Lower Permian strata flanking the southeastern end of the Gypsum Valley Salt Wall. Near vertical, locally mappable units (previously interpreted as Lower Permian Honaker Trail Formation) are reinterpreted as lateral carbonate caprock. This is based on consistent location at the diapir-stratal interface, lack of interbedding with adjacent stratigraphy, and presence of non-cyclic, non-fossiliferous, laminated or brecciated, microcrystalline carbonate facies. Presence of caprock-derived clasts within the adjacent Lower Permian Cutler Formation indicate that the lateral caprock is Lower Permian. Because the vertically-oriented lateral caprock is parallel to bedding in the halo-kinetically deformed Cutler strata, the lateral caprock must have formed horizontally atop the diapir and subsequently halokinetically rotated to a diaper-flanking position along with Cutler strata. Local faults confined to the lateral caprock and flanking Cutler strata mark distinct offset events prior to deposition of adjacent onlapping strata. This is likely the result of passive diapirism, in concert with drape folding mechanics.

Results from our work: (1) predict that Cutler-age lateral caprock may be present in the subsurface at the Cutler/diapir contact at Gypsum Valley Salt Wall, and possibly at other Paradox Basin salt walls; (2) suggest that near-diapir trap elements may be compartmentalized by syn-halokinetic deformation faulting; (3) indicate that hydrocarbon generation and migration in the Paradox Basin started by Early Permian time; and (4) may be used as an outcrop analog to aid in pre-drill prediction of lateral carbonate caprock in other salt basins.