El Paso Geological Society

Caprocks are an assemblage of lithologies that can be found on top, and more rarely, in flanking positions on salt diapirs (Jackson et al., 2013). The lithologies commonly found begin with anhydrite (CaSO4), which is located closest to the salt, followed by gypsum (CaSO4•2H2O) and then, occasionally, carbonate dominated lithologies referred to as carbonate caprock (Labrado et al., 2019). The accumulation of anhydrite and gypsum is caused by the dissolution of salt (NaCl) in the top of a salt diapir, with the less soluble sulfate salts preserved. Carbonate caprocks result from a subsequent process that replaces gypsum with carbonate, the latter being supplied by the oxidation of constituents from oil and gas that is coupled to the consumption of sulfate by sulfate-reducing microbes. In some cases, such as many salt domes in the Gulf Coast, the carbonate caprock is associated with large native sulfur deposits (Long, 1992). Predominantly, the carbonate caprock is calcitic. Dolomitic caprock has been reported for the Spindletop salt dome, Beaumont, Texas, (challenged by Barton and Paxson, 1925), for caprock in Southern Australia (Kernen et al., 2019), and at the Gypsum Valley salt wall, Colorado (Lerer, 2017; Poe et al., 2018).

Accessory minerals in anhydrite, gypsum and carbonate caprock have great potential to help fill the gaps in the understanding of caprock formation. Their presence or absence serve as indicators for geochemical conditions, such as pH, alkalinity, oxidation state, or presence of reactive constituents like sulfide or O2. More than 18 accessory minerals have been reported from salt from Gulf Coast diapirs, which originates from the Jurassic Louann Salt (Taylor, 1937; Simmons, 1988; for a compilation, see attached table). Not counting numerous sulfide minerals (Saunders and Thomas, 1996), 10 accessory minerals have been reported for anhydrite and gypsum caprock from Sulphur Salt Dome, Louisiana (Goldman, 1952). For carbonate caprock, we know from thin sections that quartz, sulfide minerals, native sulfur, clays, and organic matter are present. It is unclear if the accessory minerals in the carbonate caprock are identical to those from the sulfate-salt caprock or salt diapir, have undergone alteration, or are entirely new. For the case that these minerals are identical to those from precursor caprock, it has not been assessed if their relative concentration has increased or decreased, or whether some minerals have vanished during the transformation from sulfate to carbonate caprock. We even lack a comprehensive inventory of accessory minerals in carbonate caprock, and there is no reliably tested sampling protocol for the extraction of the various phases.