ABSTRACT: The Tengiz Oil Field, Pri-Caspian Basin, USSR, A Super Giant of the '80s

N. N. Lisovsky, G. N. Gogonenkov, Y. A. Petzuukha

The super-giant Tengiz oil field, located near the southeastern edge of the Pri-Caspian basin is one of the most important new discoveries made in the last decade. The Tengiz oil field occurs in a desert climate on the edge of the Caspian Sea and experiences extreme temperature variations along with other harsh environmental conditions.

The structure was first identified by seismic surveys in 1975 when it became possible to map the base of thick Permian salt. The first exploration well was drilled on the top of the pre-salt structure in 1979 and encountered important oil-bearing carbonates. The drilling of additional appraisal wells and the acquisition of three-dimensional seismic with improved seismic velocities contributed to the development of the Tengiz seismic-geologic model and confirmed the presence of a super-giant oil accumulation. The amplitude of the structure is approximately 1000 m and covers an area of 400 mi². The oil column exceeds 1500 m without having encountered an oil-water contact. Proven plus probable reserves are estimated to exceed 18 billion bbl. Oil production rates up to 8000 bbl day per well have been experienced.

The reservoir rocks, which range from Upper Devonian to Middle Carboniferous, are comprised of biohermal and nodular limestones and characterized by variable porosity and permeability. The best reservoir quality is attributed to widespread fractures, vugs, and solution cavities that supplement the primary porosity. Recrystallization and healed fractures with substantial amounts of solid bitumen are observed throughout the reservoir.

The reservoir pressure is abnormally high. The oil gravity is approximately 45° API and the sulfur content is low. There is no gas cap on the Tengiz field, and the oil is highly undersaturated. The associated gas contains a high percentage of H₂S, which must be considered in field development.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990