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^PSFacies and Reservoir Quality Variations in the Late Visean to Bashkirian Outer Platform, Rim, and Flank of the Tengiz Buildup, Pricaspian Basin, Kazakhstan*

J.F. Collins¹, J.A.M. Kenter², P.M. Harris³, G. Kuanysheva⁴, D.J. Fischer⁴, and K.L. Steffen¹

Search and Discovery Article #20052 (2008)

Posted May 30, 2008

*Adapted from poster presentation at AAPG Annual Convention, Houston, Texas, April 9-12, 2006.

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 ExxonMobil Development Company, Houston, Texas

² Vrije University, Amsterdam (currently ETC, Chevron, Voorburg, Netherlands ([email protected]))

³ Chevron Energy Technology Company, San Ramon, California ([email protected])

⁴ TengizChevroil, Atyrau, Kazakhstan

Abstract

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.

Selected Figures

	(A) Location map. (B) Tengiz field on an isolated carbonate buildup that grew on a regional pre-Middle Devonian structural high. Dashed line shows position of seismic profile. (C) Seismic profile across the Tengiz buildup that grew more or less uninterrupted from Middle Devonian to early Bashkirian. The total thickness of carbonate is about 1.2 seconds TWT. The deepest borehole to date bottomed in the middle Famennian at 6032 m subsea, more than 2100 m below the top of the reservoir and 700 m below the Famennian SSB.
	Age relationships, shown in cross-section, between the rim and the platform, was established from biostratigraphy of core and cuttings; they demonstrate that progradation and microbial boundstone production occurred largely during the Serpukhovian.
	Matrix porosity-permeability relationships, which are shown graphically, indicate that large portions of Unit 1 form distinct reservoirs (central platform, transitional, reservoir zone, and rim (flank)). The outer platform, rim, and flank have lower porosity due to facies and bitumen cement, and higher permeability scatter due to enhanced dissolution compared to the central platform.
	Paragenesis.