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^PSLate Visean to Bashkirian Platform Cyclicity in the Central Tengiz Buildup (Pricaspian Basin): Depositional Evolution and Reservoir Development*

J.A.M. Kenter¹, P.M. Harris², L.J. Weber³, J.F. Collins³, M. Stalinski⁴, G. Kuanysheva⁴, and D.J. Fischer⁴

Search and Discovery Article #20051 (2008)

Posted May 25, 2008

*Adapted from oral 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 Chevron Energy Technology Company, Amsterdam, Netherlands ([email protected])

² Chevron Energy Technology Company, San Ramon, California ([email protected])

³ ExxonMobil Development Company, Houston, Texas

⁴ TengizChevroil, Atyrau, Kazakhstan

Abstract

Cores, logs, and more than 4000 core plugs from four deep wells penetrating upper Viséan-Serpukhovian and Bashkirian supersequences in the central platform portion of the supergiant Tengiz reservoir of western Kazakhstan provide a coherent model of the lithofacies distribution and reservoir development.

Depositional cycles, several to 10’s of meters in thickness, are made up of a succession of lithofacies generally overlying a sharp base with evidence for subaerial exposure. Tight peloidal mudstone and ash beds occur in association with sequence boundaries, reflecting low-energy conditions in deeper platform areas at lowstand and during initial flooding. Beds with brachiopods are shallow low-energy and signal initial open-marine influence. The succeeding crinoid-dominated intervals represent maximum marine flooding, and the overlying skeletal-peloidal to coated grain and ooid/algal grainstone a highstand shoaling phase. Cycles are generally easy to correlate laterally over several km’s distance; identification of the cycle boundaries is indicated by gamma-ray spikes (ash beds).

The present-day distribution of reservoir rock types in the central platform was mostly determined by late burial diagenetic modification of an earlier reservoir system that was controlled by highly cyclic depositional and early diagenetic processes. Late burial diagenesis increased porosity by dissolution and reduced porosity through pyrobitumen cementation. Pyrobitumen increases towards the outer platform and near the base of cycles; this suggests that the first fill of hydrocarbons migrated through the flanks laterally into the platform cycles. Late burial diagenesis “flattened” the initial vertical, nearly cyclic, porosity away from the central platform and generally obscured the relationship between pore types and permeability behavior.

Selected Figures

	Location map (left); map of top reservoir (= top Bashkirian), contour interval of 100 m (right). Line shows location of seismic profile (lower), which shows general depositional areas (central platform, outer platform, rim-slope or flank) and key reservoir zones (RZ).
	Cross-section showing major stratigraphic surfaces and reservoir zonation scheme. Stratigraphic surfaces, in ascending order, are: Tournaisian (Tour_MFS); Early Visean (Evis_SSB); Late Visean (Lvis1_MFS), (Lvis_SSB), and (Lvis13_csb); Serpukhovian (Serp_SSB); and Bashkirian (Bash_SSB). Reservoir zones bracketed by these surfaces are: Visean D, Visean C, Visean B, Visean A, Serp, and Bash.
	K-Phi plots and porosity histograms suggest different behavior between northern central platform wells (T-220 and T-5246) and southern central platform wells (T-6246 and T-6846) with generally lower mean porosities and better behaved permeability-porosity relationships in the southern wells and bimodal porosity distributions in the northern wells.
	Paragenetic sequence for the central platform.