AMTHOR, JOACHIM E.¹, NEIL L. FREWIN¹, and KARL RAMSEYER^2
1Petroleum Development Oman, Muscat, Sultanate of Oman
²University of Bern, Switzerland

Abstract: Enigmatic Origin of a Late Pre-Cambrian Laminated Chert: Have Bacteria Done It?

The intra-salt Athel Formation (Vendian Ara Group) silicilyte is a unique source and reservoir rock found in the South Oman Salt Basin, where slabs of this laminated chert are entrapped in salt domes at 4 to 5 km depth. The play is characterized by light oil, hard overpressures, and a high-porosity, low-permeability silica matrix rich in organic matter.

Because of its unusual nature, its age, and its economic importance, the origin of the silicilyte is much debated, but remains still enigmatic.

The Athel Formation silicilyte typically is organic-rich, finely laminated and consists to 80-90 % of micro-crystalline quartz with a modal crystal-size of 2-3 microns. Commonly, the silica crystals form aggregate masses with high amounts of intercrystalline microporosity. The organic matter comprises about 7-8 % of the bulk rock volume. It is not a kerogen by definition since it is totally soluble in organic solvents. No identifiable biogenic remains have been found so far in the silicilyte.

The textural and chemical characteristics of the Athel Formation silicilyte suggest formation in a reducing, probably anoxic environment below wave base. A model with a stratified water column is proposed, in which the oxic surface waters represented the site of carbonate deposition, and the deeper water below the thermocline the site of silica precipitation. The uniform crystal size distribution could well be the result of a rapid nucleation of silica gel by inorganic means. The large volumes of silica required for silicilyte formation demand a source of reasonable magnitude for silica, i.e., seawater. All of the likely inorganic processes of silica deposition require a silica concentration in seawater during Athel times in excess of that in present-day oceans, which is plausible. Silica precipitation may well have been mediated by sulphate-reducing bacteria (chemoautotrophs), hence a bio-chemical origin of the silicilyte is advocated.

Such a model of bacterial-mediated silica precipitation is consistent with i) the available isotope data from Ara Group carbonates and sulphates, ii) biomarker analyses which indicate that Athel Formation oils are characterised by a relative abundance of specific biomarkers derived from chemo-autotrophic bacteria, iii) petrographic data showing abundant rodand sphere-shaped bodies of likely bacterial origin in the quartz matrix, and iv) abundance of cyanobacterial remains and framestone fabric in time-equivalent Ara carbonates.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas