Abstract:

Great amount of natural bitumen was developed in the Middle Devonian reef limestone of the Dachang giant polymetallic ore deposit in Nandan,Guangxi province.This reveals that there is a Dachang fossil oil pool here.The Chehe granite in the eastern part of the Dachang fossil oil pool belongs to peraluminous calc-alkalic rock series with high content of kalium.This type of granite,rich in light rare earth,is considered to originate both from the crust and mantle.It shows significant negative anomaly of Eu,and is rich in high field strength elements and large-ion lithophile elements.The weighted average age of(86.27?0.68)Ma is obtained through the U-Pb dating of laser probe plasma mass spectrum for zircon in granite.Combined with the analysis of regional geology,it is inferred that the Nandan-Hechi area experienced strong lithospheric extension in the mid-Late Cretaceous.On one hand,the diapirism of granitic magma under extensional setting caused denudation and destroyed the preservation conditions of the fossil pool.On the other hand,it increased the geothermal gradient and made the Paleozoic source rocks over mature.The strong and widely-spread lithospheric extension in the mid-Late Cretaceous made severe damage to the Paleozoic petroleum systems.Therefore,secondary oil/gas pools sealed by residual cap rocks should be the target for hydrocarbon exploration in the Paleozoic hydrocarbon systems of eastern Nanpanjiang Basin.

Key words: granite, extension, fossil oil pool, secondary oil/gas pool, mid-Late Cretaceous, Nandan-Hechi tectonic zone, northern Guangxi province