Thermal regime and petroleum systems in Junggar basin, northwest China

The conversion of organic matter into oil and gas depends upon temperature and time, and the study of thermal maturation of oil-source rocks in a petr oleum system is of importance for formation of a petroleum system. Based on the temperature data from 196 wells and thermal conductivity measurements of 90 core samples, altogether 35 heat flow data are obtained in the Jungga r basin. The results show that the Junggar basin is a relatively "cold" bas in at present, with a mean temperature gradient and heat flow of 21 degrees C/km and 42mW/m(2), respectively. Thermal history reconstructed from vitrin ite reflectance data indicates that the Paleozoic formations experienced th eir maximum paleotemperature during the Permian and Triassic at higher pale oheat flow of about 85 mW/m(2) and that the basin then cooled down to the p resent low heat flow. The high paleoheat flow can be attributed to the Carb oniferous to Permian rifting. The thermal evolution has a quite important e ffect on the formation and evolution of the petroleum systems in the Jungga r basin, i.e. the Permian and the Jurassic systems. The Jurassic petroleum system is quite limited in space for the cooled thermal regime during the M eso-Cenozoic and the source rocks of the Middle-Lower Jurassic entered the oil window only along the North Tianshan foreland region, where the Jurassi c is buried to the depth of 5-7 km at present. In contrast, the Middle-Lowe r Permian source rocks have experienced oil and gas generation in late Perm ian to Triassic, and the Permian petroleum system was formed prior to the T riassic when the upper Paleozoic fort-nations reached their maximum paleote mperature due to higher paleoheat flow. (C) 2001 Elsevier Science B.V. All rights reserved.