In the southeastern part of the North Caspian depression the sub-salt Paleozoics have been penetrated by the drill in the 2-6 km depth interval and in the stratigraphic range from the Lower Permian Artinskian to the Upper Devonian Frasnian rocks. Development of the main structural features such as Karaton-Prorva zone of highs where Tengiz field is located led to substantial differences in environment of deposition in inner and outer parts of the depression. There were numerous breaks in deposition, particularly the pre-Permian hiatus in which much of the Carboniferous section was lost to erosion. The reality of this erosional loss is confirmed by discordance in level of maturity of the organic matter in the Carboniferous and Permian rocks. Characteristic of the Lower Permian sub-salt Artinskian sediments is absence of any relationship between degree of maturation of its organic matter and the maximum temperature of heating of these rocks to their present depth of occurrence. For example, at the crest of one large high where the Artinskian is at depths of 2200-3100 m, vitrinite reflectance is 0.48-0.54 o/oo, and paleo-tempersture was less that 90-100 degrees C. However, a similar level of maturation is found in the north of Karaton- Prorva zone of highs in spite of their occurrence at depths of 4.0-5.0 km. The organic matter of the Lower and Middle Carboniferous rocks that sub-crop on the pre-Permian surface exhibits substantially greater matura- tion. In direct proximity to the contact vitrinite reflectance is 0.65-0.80 o/oo. The stratigraphic discordance is thus acccompanied by a large cataclastic discordance. The jump in paleo-temperature at this contact is 30-50 degrees C. The Carboniferous sediments of this region experienced their maximum heating in pre-Permian time. This appears also to have been the main stage of pool formation. The deep erosion of the Carboniferous during Early Permian time contributed to oxidation of some of the oil. Permian-Mesozoic subsidence, compensated in this region by 2.5-3.5 km of Kungurian salt, contributed to outflow of heat from the underlying sediments, not providing a repetition of the earlier heating of Paleozoic rocks. Oil accumulations here in the Lower Permian are secondary, formed by migration from lower-occurring Carboniferous and Devonian beds. Analysis of the paleo-geothermy of the study region must take into account its position on the border of the East European craton and the Turan platform. The North Ustyurt syneklize of the Turan platform and the southeast part of the North Caspian depression were part of this same basin during the Mesozoic, when 3.0-3.5 km of Jurassic-Cretaceous sediments were deposited. The Paleozoic sediments in these regions of downwarping were subjected to higher temperatures, which smoothed out the maturation discordances and subjected the Carboniferous rocks to a new process of maturation. The Kungurian salt seal was already in place by now and was capable of retaining new shots of hydrocarbons. (From Volkova and others, 1991, two paleo-temperature maps; digested in Petroleum Geology, vol. 27, no. 9-10). Copyright 2000 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Back issues are available at http://geocities.com/internetgeology Inquiries concerning the journal Petroleum Geology may be made by telephone at (703) 759-4487 or by mail at 914 Leigh Mill Road, Great Falls VA 22066.