Carbonates and Source Rocks


It was recognized by Kendall et al (2007) that Holocene cyanobacterial mats and lime muds provide links to Middle Eastern carbonate source rock potential. They indicated that carbonate reservoirs ranging in age from Permian to Tertiary contain most of the 675 Bbbl of Arabian Gulf hydrocarbon reserves. They proposed that 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 and should be a topic future research (Shinn and Kendall, 2011).

Whitings are numerous and often widespread in the Arabian Gulf (Wells and Illing, 1964; De Groot, 1965). Even their origins remain controversial, as with the whitings on the Great Bahama Bank (Shinn et al., 1988; Robbins and Blackwelder, 1992; Robbins et al., 1996). Are they bottom muds stirred into suspension by fish, or are they, as is proposed here, precipitated directly from seawater? Certainly the Arabian Gulf has few aragonite-precipitating algae, and in shallow water often the bottom consists of rock and/or carbonate sand and coral reefs (Houbolt, 1957; Shinn, 1969; Kendall et al., 2007). Water depths in the open Gulf are more than double that on the Great Bahama Bank. It is proposed here that mud and cyanobacteria are being quickly sequestered into the sedimentary section in the axial trough of the Gulf (Houbolt, 1957) and extensive tidal flats that rim it (Birdwell et al 1991; Kendall, et al, 2002; Schreiber et al, 2001). Kendall et al., (2007) proposed that short-lived isotopes in the Bahama banks support the instantaneous character of this whiting precipitation.

Regional Whiting: Southern Arabian Gulf (NASA satellite scene) with extensive ‘Whiting’ (lime suspension) stretching from Saudi Arabia and Qatar to offshore the northern UAE over water often deeper than ten to twenty meters. Winds over 25 mph carry dusts out to this location that act as nutrients to carbonate and organic blooms produced at this location.

Source rock analysis of the Gulf carbonate mud/cyanobacterial deposits demonstrates that these sediments are future source beds for hydrocarbons. What is happening in the Arabian Gulf is supported by the fact that in the Bahamas 25% of the 1.3 million metric tons of the whitings precipitated and suspended each year is organic matter, dropping to 1.8% of the surface sediment. Kendall et al (2007) note that the Bahamian Bank whitings and associated organic matter covering more limited areas is swept off the bank into deep water. At the same time cores through Neogene western platform slope sediments preserve 1% TOC up to 4%. For this reason Kendall et al (2007) believe that Cyanobacteria may contribute more to the occurrence of hydrocarbons in the carbonate section than was previously thought. In fact the authors argued that organic matter associated with whiting blooms is dispersed in the lime muds of the ancient Arabian Gulf section and may have generated large volumes of its oil. They propose that the cyanobacterial membranes would liquefy at low threshold temperatures. They indicate that short time interval bursts 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. Kendall et al (2007) concluded that the whitings of the modern Arabian Gulf are the analogs of and key to the origin of the vast petroleum reserves of this region.

Power Point Summary
A click on the Icon accesses a lecture on "
Holocene Cyanobacterial Mats & Lime Muds: Links to Middle East Carbonate Source Rock Potential" that summarizes how carbonate muds and cyanobacteria sequestered in axial trough of Gulf and the extensive tidal flats that rim it are potential sources for future hydrocarbons and analogs for Middle Eastern Mesozoic carbonate hydrocarbon fields.


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