PRECAMBRIAN MICROBIALITE OIL AND GAS RESERVOIRS, SULTANATE OF OMAN: EXPLORING THE LIMITS OF COMPARATIVE SEDIMENTOLOGY

The Precambrian intra-salt carbonate-silicilyte petroleum system, South Oman Salt Basin, is an enigmatic oil and gas play that is unique in being characterized by the oldest commercially viable reservoir/source rocks in the world. Comparative sedimentology, based on modern analogs, provides only general constraints on depositional models for reservoir rocks. Several specific aspects of these reservoir rocks (and other Precambrian carbonates) highlight significant differences with Holocene sediments: 1) in the absence of bioturbation in rocks of this age all facies are laminated; 2) textures associated with the activity or microorganisms, or inorganic processes, dominate nearly every facies except those affected by strong currents; 3) the mineralogy, geochemistry, and sedimentology of some facies may be controlled by processes not currently active or common in modern marine environments. Challenging problems are posed by the sedimentology of the reservoir rocks, which include broken slabs of carbonate and “silicilyte”. Carbonates once formed several isolated platforms and are differentiated into shelf and basin facies. However, many facies are finely laminated, usually with textures consistent with the former activity of microorganisms, yet these laminated sediments extend across all water depths represented by the platform. The Holocene record of finely-laminated, microbially-influenced carbonates would suggest an intertidal to supratidal origin for these facies, but their broader context and suites of associated structures indicate a largely subtidal origin for these significant reservoir facies. Silicilytes are organic-rich rocks consisting of 80-90% by weight microcrystalline quartz of uniform crystal size (2-3 microns). The remainder of the silicilyte has a sheet-like pore network (up to 20% porosity), filled with kerogen and mobile hydrocarbon. The silicilyte facies has been controversial for several reasons: what water depth did it accumulate in; what was the source of silica in a world without diatoms or radiolarians; could it be a microbial biochemical precipitate or is it purely inorganic; is it a self-sourcing reservoir? This facies may be unique in earth history, with analogs closer to the silica-rich facies of Archean banded iron formations than to modern sediments.