Paul M. Harris¹, Raymond A. Garber¹, Jeff L. Warner²

(1) Chevron Petroleum Technology Company, Houston, TX
(2) LA Petrophysical, La Habra Heights, CA

ABSTRACT: Characterizing Tengiz Platform Deposits - Core and Log Data from a Key Well

Tengiz well T-220, because of a great amount of continuous core (450 m), is invaluable from the perspective of understanding the nature of deposition, stratigraphy, diagenesis, and reservoir quality for the interior portion of this Carboniferous isolated carbonate platform.

The upper contact of the Bashkirian is significant as it represents the demise of the shallow water platform, a significant time gap, and a major regional change from carbonate to siliciclastic deposition. Three Bashkirian sequences of similar grainstone-dominated deposits form a composite sequence. The Bashkirian - Serpukhovian boundary represents a major change in the style of deposition and equates with a time gap. Four Serpukhovian sequences of more skeletal-rich environments form another composite sequence. Three composite sequences within the Late Visean contain grainstone-packstone cycles that are similar to those of the Serpukhovian. The difference in character of the cycles between the Late Visean-Serpukhovian and the Bashkirian reflects changes in the nature of sea-level variations and climate.

Porosity is lost by calcite cementation and minor compaction. The variety of open primary and secondary pore types is a principal reason for the permeability variation that is observed within the well. Cores are dominantly limestone with only scattered partial dolomitization and silicification, generally associated with near-by tuffs or subaerial exposure. Samples have near constant 15% porosity with 1-10 md permeability; thin intervals of poorer reservoir quality correspond to sequence and cycle boundaries. Bitumen occurs in short intervals, almost always above sequence or cycle boundaries, is more abundant in rocks with low porosity, and fills 5-75% of potential porosity.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado