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Tengiz field, western Kazahstan. 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 the platform break) that comprise some 25% of the reservoir volume. Detailed observations and analog studies in northern Spain confirmed the presence of a “deep” microbial cement “reef” that extends from sea level down to more than 300 m below sea level.