From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Bureau Seminar, September 22, 2006

UAE Algal mats and lime muds: links to
Middle East carbonate source rock potential

Dr. Christopher Kendall

Professor of Geological Sciences and Marine Science at the University of South Carolina
The Distinguished Visiting Scientist at the Jackson School of Geosciences at the University of Texas for the Academic Year 2006-7

Abstract:

Whitings – Source of Black Gold?
The in-place hydrocarbon reserves of 675 Bbbl within the Arabian Gulf are predominantly in carbonate reservoirs that range in age from the Permian to Tertiary. Two dominant components of the Holocene sediments of the Arabian Gulf include cyanobacteria (as algal mats and other occurrences) and lime mud, matching that from the underlying ancient sedimentary section. Today the lime mud and associated cyanobacteria are thought to form as instantaneous precipitates out of the water column that turn the otherwise clear tropical water milky white (hence the name “whitings”). The aragonite mud and cyanobacteria thus formed is sequestered into the sedimentary section of the axial trough of the Gulf and in the tidal flats that rim the basin. Source rock analysis of the carbonate mud/cyanobacterial deposits demonstrates that these sediments are future source beds for hydrocarbons.

Analysis of Holocene organic productivity data from Bahamian whitings demonstrates that as much as 25% of the 1.3 million metric tons precipitated each year is organic matter.  Similarly Bahamian cores down to 1000m on the western slope of the platform preserve 1% TOC (up to 4%).  This in turn suggests that these whitings-derived sediments may represent early stages of source rock formation.

It is proposed here that much of the hydrocarbons trapped in the Middle East were sourced from dispersed organic matter associated with whiting blooms of the geologic past. It has been shown recently that the contribution of cyanobacteria to oils is more than previously thought. The simple chemical composition and structure of cyanobacterial membranes liquefy at a threshold temperature. It has been argued that maturation does not require a time and temperature integral.  Thus the low percentages of organic matter dispersed in the lime muds of the ancient Arabian Gulf section would be expected to generate a large volume of oil over a short time. This matches the published argument that a burst of oil generation would produce transient overpressures liberating oil by micro-fracturing and even long-range migration because of creation of fractures. This rapid accumulation of large volumes of oil in a short time would also provide the collective buoyancy necessary to drive large-scale migration.  Thus it is possible that whitings of the modern Arabian Gulf are the key to the origin of the vast petroleum reserves found here.

References for this presentation

Collister, James, Robert Ehrlich, Frank Mango, and Glenn Johnson (2004), Modification of the petroleum system concept: Origins of alkanes and isoprenoids in crude oils AAPG Bulletin, v. 88, no. 5, p. 587–611

Kramer, P.A., Swart, P.K., De Carlo, E.H., and Schovsbo, N.H., 2000. Overview of interstitial fluid and sediment geochemistry, Sites 1003-1007 (Bahamas Transect). In Swart, P.K., Eberli, G.P., Malone, M.J., and Sarg, J.F. (Eds.), Proc. ODP, Sci. Results, 166, 179-195

Robbins, L.L. and Blackwelder, P.L. 1992, Biochemical and ultrastructural evidence for the origin of whitings: A biologically induced calcium carbonate precipitation mechanism: Geology, v. 20, p. 464-468.

Robbins, L. L., Tao, Y., Evans, C. A., 1997, Temporal and spatial distribution of whitings on Great Bahama Bank and a new lime mud budget: Geology , v. 25, p. 947-950.

Shinn, E.A., Steinen, R.P., Lidz, B.H., and Swart, P.K., 1989, Whitings, a sedimentologic dilemma: Jour. Sed. Petrol., v. 59, p. 147-161.