--> Identification of Lateral Carbonate Caprock Flanking Paradox Basin Salt Walls, Utah and Colorado

AAPG ACE 2018

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Identification of Lateral Carbonate Caprock Flanking Paradox Basin Salt Walls, Utah and Colorado

Abstract

Lateral carbonate caprock (LCC), previously mapped as marine limestone of the Pennsylvanian Honaker Trail Formation, has been identified at several salt walls in the Paradox Basin, Utah and Colorado. Using outcrop and isotopic studies of LCC from Gypsum Valley (GV) and Castle Valley (CV) salt walls, we propose a set of lithologic, stratigraphic, and isotopic criteria for identification of LCC in the Paradox Basin. Identification of LCC is important because it is often a very porous and permeable rock type at the salt-sediment interface that may act as reservoir or fluid migration pathway, influencing salt wall related traps.

Caprock assemblages consist of a vertically zoned sequence of, in ascending order: anhydrite, gypsum, and occasionally calcite and/or dolomite. Carbonate caprock forms in two stages. First, undersaturated water preferentially dissolves halite, and the less soluble components of diapirs (i.e. anhydrite) are concentrated at the crest. With continued dissolution, insolubles are underplated to the base of older caprock. Second, sulfate-reducing bacteria consume hydrocarbons that migrate into the caprock mediating the replacement of sulfate minerals by carbonate minerals. The diagenetic processes that produce caprock generates a variety of distinctive gypsic and carbonate fabrics. In the Paradox Basin, homogeneous, banded, brecciated, and porphyritic fabrics are recognized. The process of underplating forms layering that superficially resembles bedding in the caprock. In the Paradox Basin, the orientation of LCC layering parallels that of the adjacent halokinetically drape-folded stratigraphy, which contains fluvial channel conglomerates bearing caprock-derived clasts. Locally adjacent to the CV there is LCC associated with the Permian fluvial Cutler Fm. and the Triassic fluvial Moenkopi Fm. at GV the LCC is associated with the Triassic fluvial Chinle Fm. From these relationships, we interpret the Paradox Basin LCC formed in a crestal position during distinct periods of time, a “caprock event”, and were subsequently rotated into a lateral position as a result of halokinetic drape folding. LCC can be distinguished from neighboring depositional carbonates by the recognition of caprock fabrics, the lack of fossils and sedimentary structures, the stratigraphic relationships associated with the outboard stratigraphy, and its carbon isotope signature that reflects the contribution of isotopically light carbon from the oxidation of hydrocarbons.