Depositional Environments
Ratcliffe core lithofacies were described from three wells in order to determine lithology, textures, skeletal and non-skeletal allochems, cements, porosity types and oil shows. Ratcliffe sediments gradually filled the Charles basin with off-lapping or progradational wedges of carbonates and evaporites. From base to top, the Ratcliffe is characterized by shallowing-upward and salinity-restricting paracycles
Facies 1. Black to dark gray, shaley lime mudstones with sparse crinoid skeletal fragments were deposited in open marine environments during base level rise. These sediments represent the deepest Ratcliffe despositional environments in the study area. This facies commonly displays an increased gamma-ray response on electrical logs.
Facies 2. Open marine sediments are dark gray to black and contain crinoids, bryozoans, brachiopods, rugose and tabulate corals, and sparse mollusks. These lime wackestones and packstones were deposited in normal marine environments. Minor oscillations in bathymetry produced intertonguing of these sediments with restricted subtidal sediments.
Facies 3. Brown, burrow-mottled sediments locally contain normal marine fauna, but were probably deposited in environments that were slightly to highly stressed by elevated salinity and/or low oxygen levels. Thalassinoides, Rhizocorallium, Chondrites, and Planolites trace fossils are common, and are part of the Cruziana ichnofacies which has been described along shallow shelf environments. Burrowed sediments are slightly to completely dolomitized. It is inferred that burrowing infauna probably increased sediment transmissibility and made these shallow subtidal sediments susceptible to seepage dolomitization from overlying gypsum (anhydrite) beds.
Facies 4. Light brown to brown, algal and skeletal lime mudstones and wackestones were deposited in protected-shelf and lower-shoal environments. These sediments are sparsely burrowed, slightly dolomitic, and generally not reservoirs. Algal fragments are broken and coated Ortonella fragments, and skeletal fragments are ostracods.
Facies 5. Upper shoal environments commonly cap burrowed mottled sediments. Shoal sediments are peloidal and algal mudstones to packstones. Textural variations are related to differences in depositional energy and relative bathymetry across ancient structural noses. Shoal sediments were probably deposited in very shallow subaqueous environments based on the presence of the Codiacean alga Ortonella. This alga characteristically is nodular with well developed radiating microtubules. Peloidal grains indicate reworking of muddy substrates and algal fragments in swash zones and possibly intertidal environments. Coated grains are sparse. Original porosity within grain-rich beds is commonly occluded by calcite spar cements and secondary anhydrite. Because of early cementation and sparse dolomitization, the shoal facies in the study area is generally not a reservoir.
Although not cored in the project area, laminated beds commonly underlie and immediately overlie sabkha sediments. These intertidal beds are dolomites with millimeter-scale, flat to wavy laminations. This facies was deposited in environments where the trapping and binding activities of cyanobacteria were common. Desiccation features are sparse, and in situ fossils are missing.
Anhydrite caps the shallowing-upward paracycles. The presence of anhydrite (gypsum) indicates that restriction accompanied progradation and basin filling. Displacive nodular and enterolithic anhydrites are common, and it is inferred that deposition of gypsum was in coastal sabkhas which were both intermittently wet and desiccated.