Facies and Reservoir Quality Variations in the Late Visean to Bashkirian Outer Platform, Rim, and Flank of the Tengiz Buildup, Precaspian Basin, Kazakhstan

Joel F. Collins¹, Jeroen A. M. Kenter², Paul M. Harris³, G. Kuanysheva⁴, Dennis J. Fischer⁴, and Kelley L. Steffen^1
1 ExxonMobil Development Company, Houston, TX
² Vrije University, Amsterdam
³ Chevron Energy Technology Co, San Ramon, CA
⁴ TengizChevroil, Atyrau, Kazakhstan

Tengiz field is an isolated carbonate buildup in the southeastern PriCaspian basin, containing a succession of shallow-water platforms ranging in age from Late Famennian to Early Bashkirian. Platform backstepping from Tournaisian through late Visean resulted in approximately 800 m. of bathymetric relief above the Famennian platform. Serpukhovian progradation of up to 2 km. formed a depositional wedge around the older platforms referred to as the Serpukhovian rim and flank.

Rim and flank facies include lower slope boundstone breccia mixed with laminated mudstone, volcanic ash, and platform-derived skeletal packstone to grainstone; middle slope poorly-bedded to massive boundstone breccia with sub-types based on clast composition, size, and packing; and upper slope in-situ microbial boundstone. This boundstone represents the dominant source of clasts in the middle and lower slope breccias. Upper slope boundstones are overlain by platform facies consisting of coarse algal, skeletal, coated grain, and ooid packstone to grainstone. Periodic large-scale failure of the rim during both Serpukhovian and Bashkirian time resulted in a high degree of lateral facies discontinuity.

Solution-enlarged fractures, large vugs, and lost circulation zones formed by mainly late diagenetic processes produced a high-permeability, well-connected reservoir in the rim and flank. This later diagenetic overprint, accompanied by bitumen formation, extended upward into overlying Serpukhovian and Bashkirian platform facies, and inward into adjacent late Visean platforms, where it substantially altered platform reservoir properties that remained after early diagenesis related to cyclic depositional processes.