Geology and Oil Occurrence, Southeast Turkey

Abstract

Southeast Turkey lies in the northern part of the Middle Eastern basin which has produced some of the world's greatest oil fields. The present structural framework is similar to the rest of the basin and the large anticlinal traps which contain the oil in Iraq and Iran continue into Turkey. However, the great wealth of source, reservoir, and cap rocks which are found in the producing parts of Iraq and Iran are to a large degree lacking in southeast Turkey. This is due to the following conditions. 1. A major erosional interval in Upper Jurassic-Early Cretaceous time has stripped away great volumes of sediment from central southeast Turkey restricting pre-Cretaceous prospects to small areas. This uplift has also provided ample opportunity for the escape to the surface of any oil reservoired in pre-Cretaceous time. 2. The thick Lower and Middle Cretaceous basinal sediments which, according to some authorities, are the source of most of the oil in northern Iraq do not occur in southeast Turkey. Here shelfal carbonates predominate. 3. Upper Tertiary evaporites which are the cap for most of the oil in Iraq and Iran have been largely removed from southeast Turkey by Upper Tertiary and Recent erosion exposing potential Tertiary and Mesozoic reservoirs on the surface. Further, the widespread occurrence of fresh water along with asphalt and heavy oil shows in subsurface reservoirs suggests the flushing and dissipation of oil, probably resulting from this lack of Upper Tertiary cap rock. The three existing small oil fields of southeast Turkey produce from Upper Cretaceous limestones. Overlying and interfingering shales are the probable source rock. Two of these fields, Raman and Kahta, produce from a widespread Cretaceous carbonate zone underlying thick Upper Cretaceous-Paleocene shales. The heavy oil is associated with fresh water and is believed to be the residue of much larger accumulations, now largely flushed away. The Garzan reservoir is a lenticular limestone completely enclosed in Cretaceous shale and contains lighter oil associated with salt water. Future oil discoveries should be made in southeast Turkey, but will probably be small by Middle Eastern standards. The best prospects are in unflushed Upper Cretaceous-Lower Tertiary lenticular limestones enclosed by impervious shale as at Garzan.