ABSTRACT: The Tengis Oil Field, Pri-Caspian Basin, USSR--A Super Giant of the 1980s

Yuri A. Petzoukha

The super giant Tengiz oil field, located near the southeastern edge of the Pri-Caspian basin, is one of the most important new discoveries in the world made in the last decade. The field is 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 led 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 km². The oil column exceeds 1500 m without having encountered an oil-water contact. The total original oil and gas in place of the Tengiz field are estimated at 25 billion bbl of oil and 46 tcf of ssociated gas. Oil production rates up to 8000 bbl/day per well have been experienced.

The reservoir rocks, which range in age 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 #91000©1990 AAPG Conference-Giant Oil and Gas Fields of the Decade 1978-1988 Conference, Stavanger, Norway, September 9-12, 1990