--> Preliminary Reservoir Geochemistry Study of Oils and Bitumens from the Tengiz Field, Kazakhstan, CIS, by B. A. Patterson, B. T. Robertson, and J. Dahl; #90986 (1994).

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Abstract: Preliminary Reservoir Geochemistry Study of Oils and Bitumens from the Tengiz Field, Kazakhstan, CIS

Brooks A. Patterson, B. T. Robertson, J. Dahl

The super-giant Tengiz field produces high gravity oil from a Late Devonian to Middle Carboniferous carbonate platform. To aid in reservoir management, Tengizchevroil authorized a reservoir geochemistry study of oils, solid bitumens, and source rocks.

Early generation of asphaltene rich oil from basinal Carboniferous carbonates initially charged the upper platform. Later charges of light oil, co-sourced from Carboniferous carbonates and Permian shales, resulted in precipitation of asphalt derived bitumen which was subsequently thermally altered. The solid bitumen is widespread and has reduced reservoir quality.

The produced oils are similar in physical properties and hydrocarbon composition. However, differences in the quantity of C35+ n-alkane precipitate indicate that the oils are not stable at surface conditions. Oil instability results in variations in pour point and high temperature C40+ gas chromatographic fingerprints but does not affect carbon isotopes, C15+ gas chromatography, and biomarker analysis.

Nearly identical C8 to C34 gas chromatographic signatures suggest that the oil is homogenized. Subtle differences in biomarker source parameters indicate that wells in the central part of the field produce oils predominantly sourced from the Carboniferous whereas wells on the north flank of the structure produce co-sourced Carboniferous and Permian oils. Variations in biomarker maturity parameters in north flank oils support contribution of the less mature Permian source. The addition of less mature oil to the low permeability (approx. 0.5 mD) Tengiz reservoir has resulted in a present day oil pool at compositional disequilibrium.

Despite the harsh ambient conditions and high hydrogen sulfide concentrations which have hindered collection, storage, and shipment of representative oil samples, the preliminary geochemical data have proven beneficial and will be integrated into Tengizchevroil's future development program.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994