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.