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Kenter, Jeroen A.M.1, Paul M (Mitch) Harris2, Giovanna Della Porta1, Dennis Fischer3, L. James Weber4 
(1) Vrije Universiteit, Amsterdam, Netherlands 
(2) ChevronTexaco Energy Technology Company, San Ramon, CA
(3) TengizChevroil, Atyrau, Kazakhstan 
(4) ExxonMobil Exploraton Company, Houston, TX

ABSTRACT: Microbial and Cement Boundstone Dominated Flanks (and Reservoirs) of an Isolated Carbonate Platform

Tengiz field in western Kazakhstan field produces oil from an isolated carbonate platform and its flanks. The Late Visean and Serpukhovian is characterized by several kilometers of platform progradation, followed by drowning and termination of the platform in the early Bashkirian. Platform to basin relief approached 1.5 km. One of the uncertainties of the Tengiz buildup was the nature of the high rising Late Visean to Serpukhovian flanks or slopes (0-700 m below platform break) that comprise some 25% of the reservoir volume. The understanding of the character and spatial distribution of lithofacies in the slope accelerated when an equivalent outcrop analog in northern Spain was studied. 
Upper to middle slope lithofacies are dominated by: 1) massive "cabbage-shaped" to "wave laminated" micrite-cement boundstone with dominant marine cements (25-60%) in primary cm-dm irregular-shaped voids with minor skeletal grains; 2) micrite-cement boundstone with minor "growth" structures, few primary voids, minor cement (<25%) and abundant platform-derived skeletal grains and; 3) mosaic breccia of in-place collapsed boundstone. Microfilamental fossils are abundant and interpreted as cyanobacterial species. It is most likely that microbes have played a role in the genesis of the peloidal and accretionary fabrics and/or mediated the growth of marine cements. 
Remarkable observations to both Tengiz and Spain are the rates of in-situ boundstone growth (and as result progradation rates) that equal those of coralgal reef systems, the asymmetric distribution not related to paleo wind directions, and the contrasts with the Bahamian high stand shedding depositional model.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.