No abstract

No abstract

Six evaporite–carbonate sequences are recognized in the terminal Neoproterozoic–Early Cambrian Ara Group in the subsurface of Oman. Individual sequences consist of a lower, evaporitic part that formed mainly during a lowstand systems tract. Overlying platform carbonates contain minor amounts of evaporites and represent transgressive and highstand systems tracts. Detailed sedimentological and geochemical investigation of the evaporites allowed reconstruction of the depositional environment, source of brines and basin evolution. At the beginning of the evaporative phase (prograding succession), a shallow‐water carbonate ramp gradually evolved into a series of shallow sulphate and halite salinas. Minor amounts of highly soluble salts locally record the last stage of basin desiccation. This gradual increase in salinity contrasts sharply with the ensuing retrograding succession in which two corrosion surfaces separate shallow‐water halite from shallow‐water sulphate, and shallow‐water sulphate from relatively deeper water carbonate respectively. These surfaces record repeated flooding of the basin, dissolution of evaporites and stepwise reduction in salinity. Final flooding led to submergence of the basin and the establishment of an open‐water carbonate ramp. Marine fossils in carbonates and bromine geochemistry of halite indicate a dominantly marine origin for the brines. The Ara Group sequences represent a time of relatively stable arid climate in a tectonically active basin. Strong subsidence allowed accommodation of evaporites with a cumulative thickness of several kilometres, while tectonic barriers simultaneously provided the required restricted conditions. Subsidence allowed evaporites to blanket basinal and platform areas. The study suggests a deep‐basin/shallow‐water model for the evaporites.

Pervasive early‐ to late‐stage dolomitization of Lower Ordovician Ellenburger Group carbonates in the deep Permian Basin of west Texas and southeastern New Mexico is recorded in core samples having present‐day burial depths of 1.5–7.0 km. Seven dolomite‐rock textures are recognized and classified according to crystal‐size distribution and crystal‐boundary shape. Unimodal and polymodal planar‐s (subhedral) mosaic dolomite is the most widespread type, and it replaced allochems and matrix or occurs as void‐filling cement. Planar‐e (euhedral) dolomite crystals line pore spaces and/or fractures, or form mosaics of medium to coarse euhedral crystals. This kind of occurrence relates to significant intercrystalline porosity. Non‐planar‐a (anhedral) dolomite replaced a precursor limestone/dolostone only in zones that are characterized by original high porosity and permeability. Non‐planar dolomite cement (saddle dolomite) is the latest generation and is responsible for occlusion of fractures and pore space. Dolomitization is closely associated with the development of secondary porosity; dolomitization pre‐and post‐dates dissolution and corrosion and no secondary porosity generation is present in the associated limestones. The most common porosity types are non‐fabric selective moldic and vuggy porosity and intercrystalline porosity. Up to 12% effective porosity is recorded in the deep (6477 m) Delaware basin. These porous zones are characterized by late‐diagenetic coarse‐crystalline dolomite, whereas the non‐porous intervals are composed of dense mosaics of early‐diagenetic dolomites. The distribution of dolomite rock textures indicates that porous zones were preserved as limestone until late in the diagenetic history, and were then subjected to late‐stage dolomitization in a deep burial environment, resulting in coarse‐crystalline porous dolomites. In addition to karst horizons at the top of the Ellenburger Group, exploration for Ellenburger Group reservoirs should consider the presence of such porous zones within other Ellenburger Group dolomites.