桂北南丹-河池构造带晚白垩世岩石圈伸展作用及其对油气成藏条件的影响

在广西南丹大厂超大型多金属矿床的中泥盆统礁灰岩中发育大量的天然沥青,表明这里曾经存在过大厂古油藏。大厂古油藏东部车河花岗岩属于过铝质高钾钙碱性岩系,花岗岩的轻稀土富集,具有显著的负Eu异常,富集高场强元素和大离子亲石元素,该花岗岩具有壳幔混源的特点。对花岗岩中锆石进行的激光探针等离子体质谱U-Pb定年获得了(86.27±0.68)Ma的加权平均年龄。结合区域地质分析认为,晚白垩世中期南丹-河池一带处于强烈的岩石圈伸展状态,伸展背景下花岗质岩浆底辟作用一方面使先存古油藏受到剥蚀,破坏了古油藏的保存条件;另一方面提高了该地区的地温梯度,使古生界烃源岩过成熟。晚白垩世中期的岩石圈伸展作用强度大,范围广,对古生界油气系统的破坏作用比较严重。对南盘江盆地东部古生界油气系统的油气勘探应该着眼于由残存封盖层封闭的次生油气藏。

Late Cretaceous lithospheric extension in the Nandan-Hechi tectonic zone of northern Guangxi Province and its influence on hydrocarbon accumulation conditions

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.