Late Visean to Bashkirian Platform Cyclicity in the Central Tengiz Buildup (Precaspian Basin): Depositional Evolution and Reservoir Development

Jeroen A.M. Kenter¹, Paul M. Harris², L.J. (Jim) Weber³, Joel F. Collins⁴, M. Stalinski⁵, G. Kuanysheva⁵, and Dennis J. Fischer^5
1 Vrije Universiteit, Amsterdam, Netherlands
² Chevron Petroleum Technology Company, Houston, TX
³ ExxonMobil Exploration Company, Houston, TX
⁴ ExxonMobil Development Company, Houston, TX
⁵ TengizChevroil, Atyrau, Kazakhstan

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. 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; cycle boundaries are 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 which 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.