Gypsum, Calcite, and Dolomite Caprock Fabrics and Geochemistry from the Gypsum Valley Salt Diapir, Paradox Basin, Southwestern Colorado

摘要

Caprocks are found on top of or in a lateral position relative to salt diapirs. The caprocks predominantly comprise sulfate minerals such as anhydrite or gypsum, which presumably accreted during the dissolution of salt from evaporite sediments. In some cases, the sulfate minerals are replaced by carbonate minerals, referred to as 'carbonate caprock'. Carbonate caprocks associated with salt diapirs are important because they can act as reservoirs or conduits for hydrocarbons, and because they can be easily misidentified as carbonate lithologies belonging to the stratigraphy of the sedimentary sequences adjacent to the diapir, which can jeopardize the accurate interpretation of seismic profiles. Today's understanding of caprocks is based on observations from salt diapirs found along the US Gulf Coast salt diapir province, home to the famous Spindletop oil field; whereas carbonate caprocks from another salt diapir province in the United States, the Paradox Basin, have been hardly recognized, and in many cases misinterpreted as part of the 'normal stratigraphy'.;At one of the salt walls exposed in Gypsum Valley, Paradox Basin in Southwestern Colorado, abundant and well-exposed carbonate caprock was found in an area referred to as Mary Jane Draw. The carbonate and adjacent gypsum caprock display an amazing richness in lithologies, including micritic calcitic and dolomitic caprocks that are silicified to various degrees, and display an astonishing diversity in caprock fabrics, including massive, brecciated, zebraic, and finely laminated benches. In order to enable caprock identification, and to facilitate communication between caprock researchers, this richness in fabrics calls for a comprehensive caprock classification scheme, and first steps towards such unified nomenclature are reported in this study.;Light carbon isotope signatures of dolomitic and calcitic caprock indicate that hydrocarbon oxidation likely contributed to the formation of either carbonate caprock lithology; whereas the oxygen isotope signature did not provide conclusive evidence for calcite or dolomite as primary or secondary mineral phase. However, the relatively higher abundance of micritic dolomite compared to calcite indicates that dolomite, or a dolomite-precursor, must have been an early mineral phase. The oxidation of the hydrocarbons was likely tied to microbial sulfate reduction. However, all of the classical signatures for this process, such as pyrite, native sulfur or heavy sulfur and oxygen isotope signatures of sulfate associated with carbonate caprock are absent. The only geochemical fingerprint for microbial sulfate reduction is a high content of isotopically light sulfur in organic matter extracted from the carbonate caprock.;These observations can be explained by a carbonate caprock formation at the Gypsum Valley salt diapir as the result of a multi-stage process involving oxidation of hydrocarbons coupled to microbial sulfate reduction in a system that was open to fluid flow. The fluid flow provided magnesium and silica, enabling the precipitation of primary dolomite and silicification, while removing sulfide and sulfate. Waxing and waning supply with fluids may also have triggered phase transitions between anhydrite and gypsum, causing rock deformation, which explains the amazing richness and diversity in caprock fabrics found in Gypsum Valley. As such, the carbonate caprock at Gypsum Valley constitutes the 'open-system' end member of geochemical settings that are conducive to carbonate caprock formation.