Holocene Cyanobacterial Mats and Lime Muds: Links to Middle East Carbonate Source Rock Potential

Kendall, Christopher G. St. C.¹, Gene Shinn², Xavier Janson³ (1) Department of Geological Sciences, University of South Carolina, Columbia, SC (2) University of South Florida, St. Petersburg, FL (3) The University of Texas at Austin, Austin, TX

Carbonate reservoirs ranging in age from Permian to Tertiary contain most of the 675 Bbbl of Arabian Gulf hydrocarbon reserves. Two major Holocene organic sources serve as probable models: whitings that turn part of the Arabian Gulf milky white; and cyanobacteria forming mats on intertidal areas. The mud and cyanobacteria is quickly sequestered into the sedimentary section in the axial trough of the Gulf and extensive tidal flats that rim it. Short-lived isotopes in the Bahama banks support the instantaneous character of whiting precipitation. Source rock analysis of the Gulf carbonate mud/cyanobacterial deposits demonstrates that these sediments are future source beds for hydrocarbons. 25% of the 1.3 million metric tons precipitated and suspended each year in the Bahamas is organic matter, dropping to 1.8% of the surface sediment. The Bahamian Bank whitings and associated organic matter covering more limited areas is swept off the bank into deep water. Cores through Neogene western platform slope sediments preserve 1% TOC up to 4%. Cyanobacteria may contribute more hydrocarbons than previously thought. Organic matter associated with whiting blooms is believed to be dispersed in the lime muds of the ancient Arabian Gulf section and may have generated large volumes of its oil. Cyanobacterial membranes liquefy at low threshold temperatures. A short time interval burst of oil generation could produce transient overpressures liberating oil by micro fracturing and in some cases long-range migration. Rapid accumulation of large volumes of oil in a short time-span would provide the collective buoyancy necessary to drive large-scale migration. We propose that whitings of the modern Arabian Gulf are the key to the origin of the vast petroleum reserves of this region.