Origin and variability of the late Precambrian-Cambrian Athel Silicilyte, South Oman Salt Basin

The Precambrian-Cambrian Athel Silicilyte is an enigmatic chert unit of up to 390 m thick found as slabs (each slab typically 2 × 6 km across) entrapped within salt domes at a depth of 4-5 km in the South Oman Salt Basin. This formation is a prolific self-charged reservoir with high porosity (up to 34 %) and high oil saturation (80 %). Despite its economic value, the origin and the variability of this formation are not fully understood. This study therefore aims to investigate the variability and the origin (silica source and precipitation mechanism) of the Athel Silicilyte. Data obtained from core, wireline log and petrographical analysis were employed to establish the vertical and the lateral variability and, with the assistance of geochemical data, the likely source and precipitation mechanism of silica was determined.The Athel Silicilyte is only present in the deepest parts of the South Oman Salt Basin within the fault-bounded Athel Basin, bounded by two silt-rich mudstone units. Six lithofacies were recognised in the Athel Silicilyte that reflect variability in detrital material contents (three silica-rich facies), sediment remobilisation (slumped and brecciated lithofacies) and diagenetic modification (carbonate-bearing lithofacies), with the silica-rich facies being the most abundant (> 97 % of the total thickness). The Athel Silicilyte exhibits wavy discontinuous lamination and it is predominately (silica-rich facies average = 80 wt. %) composed of connected-networks of microcrystalline quartz (1-5 µm). Other constituent components are detrital quartz (3 wt. %), illite (10 wt. %), pyrite (4 wt. %) and organic carbon (TOC = 3 wt. %). The detrital contents increase towards the Upper Athel Silicilyte and towards the basin margins (ranging from 3 to 30 wt. %). The wavy laminations are interpreted to have a microbial origin. The homogeneity, loose packing of detrital grains and preservation of 390 m thick laminated fabric suggest that the Athel Silicilyte precipitated syndepositionally in microbial layers during low detrital input. The intense detrital input during bounding mudstones sedimentation inhibited Athel Silicilyte precipitation as a result of the rapid burial of microbial layers. The Athel Silicilyte shows strong enrichments of redox-sensitive elements (U, V and Mo), Mn-depletion, positive Ce anomalies, and small framboidal pyrites (3.6-4.0 µm), suggesting that the water column was euxinic during precipitation.Based on the petrographical evidence for the Athel Silicilyte forming as a syndepositional precipitate alongside seawater-like rare earth element (REE) characteristics, silica is interpreted to have been sourced directly from seawater. Mass balance calculations support this interpretation, indicating that silica-rich Precambrian seawater provided the significant silica mass in the Athel Silicilyte. The ability of dissolved silica to form hydrogen bonds with the functional groups (e.g. carboxyl and hydroxyl) in microbial layers was the key for Athel Silicilyte precipitation. The formation of hydrogen bonds was made possible under euxinic conditions, where the pH values were probably lower (< 7) than for the normal seawater as a result of HS- and H+ production by sulphate reducing bacteria and HS- oxidation at the redox boundary by sulphur oxidising bacteria. Consequently, dissolved silica was concentrated in microbial layer microenvironemnts, resulting in silica nucleation and polymerisation.EThOS - Electronic Theses Online ServicePetroleum Development of Oman (PDO)GBUnited Kingdo