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

By

J.A.M. Kenter1, P.M. Harris2, and J.F. Collins3

 

Search and Discovery Article #20048 (2008)

Posted May 20, 2008

 

*Adapted from oral presentation at AAPG and AAPG European region Energy Conference & Exhibition, Athens, Greece, November 18-21, 2007.

Click to view list of articles adapted from presentations by P.M. (Mitch) Harris or by his co-workers and him at AAPG meetings from 2000 to 2008.

 

1 Chevron Energy Technology Company, San Ramon, California, U.S.A.; currently ETC, Chevron, Voorburg, Netherlands ([email protected])

2 ETC, Chevron, San Ramon, California, U.S.A. ([email protected])

3 ExxonMobil Development Company, Houston, Texas, U.S.A.

 

Abstract

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.

 

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Tengiz Field - Background

  • Produces oil from an isolated Devonian-Carboniferous carbonate platform (aerial extent of >110 km2).
  • "Giant"— one of the world's 10 largest oil fields with 6-13.5 billion barrels of reserves.
  • More than 115 wells.
  • Highest rate wells in the platform margin and slope in fractured carbonates with low (<6%) matrix porosity; platform wells higher porosity (up to 18%), but matrix permeability is typically low (<10 md).

 

Reservoir Characterization

Reservoir Characterization
Allochthonous slope facies: fractures + matrix porosity
Central platform: matrix porosity
Outer platform: fractures + matrix porosity
Autochthonous slope facies: fractured microbial boundstone

 

Summary

  • Tengiz reservoir quality: a product of primary facies and diagenetic modification with particular spatial trends.
  • Platform: cyclic depositional system overprinted by calcite cementation and bitumen, matrix controlled, minor fractures.
  • Rim-flank:
    • Deeper outer platform and allochtonous flank: matrix cementation (calcite & bitumen) and corrosion, minor fracturing.
    • Autochthonous flank: minor matrix, mostly fractures and corrosion.
  • Spatial reservoir quality prediction linking diagenesis and petrophysics through pore network modeling

 

Reference

Jones, Gareth D., and Yitian Xiao, 2006, Geothermal convection in the Tengiz carbonate platform, Kazakhstan: Reactive transport models of diagenesis and reservoir quality: AAPG Bulletin, v. 90, p. 1251-1272.

 

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