ESTIMATION OF ANCIENT SUBSURFACE TEMPERATURES WITH THE APATITE FISSION TRACKS IN JIYUAN DEPRESSION, NORTH CHINA BASIN

By measuring apatite fission tracks,the paleo geotemperature gradients in the Mesozoic and Tertiary in the Jiyuan Depression,North China Basin were estimated.The annealing temperatures of the apatite fission tracks were 67-74℃ and 114-125℃ corresponding to the Tertiary samples and Mesozoic samples respectively.Based on the anneal modeling method,the calculated subsurface temperature gradients were 2.9℃/100m in the Mesozoic,and 3.1℃/100m in the Tertiary respectively.The different temperature gradients between the Mesozoic and Tertiary were resulted from different tectonic movements in the studied area,where a great lake depre ssion(with lower temperature gradient)was formed in Mesozoic,and become a rift(with higher temperature gradient)in Tertiary.These temperature gradients were applied successfully in assessment of thermal evolution of organic matters of source rocks,and in estimation of generated petroleum quantity in the Jiyuan Depression.Therefore,estimation of paleo geotemperatures with apatite fission tracks is an useful tool in rebuilding basin thermal evolution,even in the basin with different tectonic natures.