Facies and Reservoir Quality of the Tengiz Isolated Platform, Pricaspian Basin, Kazakhstan

Jeroen Kenter¹, Paul (Mitch) Harris², and Joel Collins^3
1Chevron Energy Technology Company, Voorburg, Netherlands
²Chevron Energy Technology Company, San Ramon, CA
³ExxonMobil Development Company, Houston, TX

Tengiz field is an isolated carbonate buildup in the southeastern Pricaspian basin, containing a Late Famennian to Early Bashkirian platform succession. Platform backstepping resulted in approximately 800 m (2625 ft) of relief above the Famennian platform, followed by up to 2 km (1.2 miles) of Serpukhovian progradation.
The upper Visean, Serpukhovian, and Bashkirian form the main hydrocarbon-bearing interval at Tengiz. Visean and Serpukhovian platform cycles, several to 10's of meters thick, are laterally continuous and have predictable facies. In contrast, icehouse-driven, m-scale Bashkirian platform cycles show significant lateral facies heterogeneity. The distribution of reservoir rock types in platform facies is determined by burial diagenetic modification of an earlier reservoir system that included meteoric alteration and porosity enhancement below major sequence boundaries, and reduced dissolution along higher-order sequence boundaries associated with the presence of volcanic ash. The burial diagenetic overprint included corrosion and cementation phases followed by bitumen emplacement and associated corrosion.
The Serpukhovian progradational margin (rim) consists of in-situ upper slope microbial boundstone, and middle and lower slope breccias containing microbial boundstone clasts. Periodic rim failure during both Serpukhovian and Bashkirian time resulted in a high degree of lateral facies discontinuity. Solution-enlarged fractures, large vugs, and lost circulation zones enhanced mainly during late diagenesis form a high-permeability, well-connected reservoir in the rim and flank. This diagenetic overprint is associated with the presence of bitumen, and extends upward into overlying Serpukhovian and Bashkirian platform facies and inward into adjacent late Visean platforms, where it has substantially altered reservoir properties that remained after early diagenesis related to cyclic depositional processes.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece