Genesis Of Unique Carbonate Fans In A Pre-Salt Reservoir Analogue, Indio Mountains, West Texas

Hydrocarbon exploration in pre-salt sediments of rift basins on the South Atlantic Margin have proven to be successful major oil plays. Located on the western margin of the rift, the Brazilian pre-salt reservoirs of the Santos, Campos, and Espirito Santo Basins are hosted by lacustrine/marine-influenced carbonates that comprise laterally extensive, laminated microbialite facies. The Indio Mountains, located in West Texas, provide well-exposed outcrops of Lower Cretaceous marine and non-marine rift basin strata that can be considered a reservoir-analog to the South Atlantic Margin pre-salt facies. Of particular interest for hydrocarbon exploration are unique carbonate features that have been discovered in the Lower Cretaceous Yucca Formation at the Indio Mountains. The carbonate features include a laterally continuous bed of septarian nodules and radial carbonate fans that are associated with fault zones. Field observations show that syndepositional faulting took place during the deposition of the lake sediments and fluvial deposits. The lakes of the Upper Yucca Formation were exposed to strong evaporation, and were supplied with water by rivers and groundwater discharge, as well as by fluids entering the lakes via fault systems. These conditions maintained alkaline lake conditions which in concert with oxidation of biogenic methane at the seabed favored carbonate precipitation. The presence of radial carbonate fans indicates the presence of syndepositional faults that could affect the interconnectedness of otherwise compartmentalized reservoir blocks, whereas the clay-rich lake sediments point the strong possibility that thin lake beds, which may not be resolvable by seismic methods could act as impermeable seals, causing compartmentalization. As such, the lacustrine sediments form the Upper Yucca Formation are the key to so far untapped information about the geologic history of pre-salt sedimentary deposits, and their propensity to become hydrocarbon reservoirs