Online Journal for E&P Geoscientists

Al Rajaibi, Ibrahim M.^*1; Hollis, Cathy ²; Macquaker, Joe ³
(1) Exploration, Petroleum Development Oman, Muscat, Oman. (2) School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, United Kingdom. (3) Department of Earth Sciences, Memorial, University of Newfoundland, Newfoundland, NF, Canada.

The Precambrian-Cambrian Athel Silicilyte is an enigmatic chert unit of up to 390 m thick found as slabs (each slab ≈ 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 used 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.

Three main silica-rich facies reflecting variability in detrital contents were recognised in the Athel Silicilyte. They exhibit wavy discontinuous lamination, suggesting a microbial origin, and predominately composed of connected-networks of microcrystalline quartz (80 wt. %; 1-5 µm). The detrital contents increase towards the Upper Athel Silicilyte and towards the basin margins (3-30 wt. %). The homogeneity, loose packing of detrital grains and preservation of 390 m thick laminated fabric suggest that the Athel Silicilyte precipitated syndepositionally. The Athel Silicilyte shows strong enrichments of redox elements, positive Ce anomalies, and small framboidal pyrites (< 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. The ability of dissolved silica to form hydrogen bonds with the functional groups 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. Consequently, dissolved silica was concentrated in microbial layers, resulting in silica nucleation and polymerisation.