The Tengis oil field, Pri-Caspian basin, USSR - A super giant of the 1980s

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 presalt 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 1,000 m and covers an area of 400 km{sup 2} . The oil column exceeds 1,500 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 associated gas. Oil production rates up to 8,000 bbl/day per well have beenmore » 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{degree} 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 H{sub 2}S, which must be considered in field development.« less