碳酸盐岩团簇同位素约束下的川东地区二叠系热演化

Thermal evolution of the Permian in the eastern Sichuan basin under the constraint of carbonate clumped isotopes

有效古温标的缺乏制约了碳酸盐岩层系热史的研究,团簇同位素作为一种新兴古温标,对碳酸盐岩层系热史重建具有重要意义。四川盆地二叠系是天然气勘探开发的重点层系,本文通过对川东地区二叠系碳酸盐岩样品进行团簇同位素测试,联合镜质组反射率重建了二叠纪以来的热史,在此基础上明确了二叠系烃源岩的热演化,并讨论了峨眉山地幔柱对川东热体制的影响。川东地区碳酸盐岩的团簇同位素温度在101.7~178.7℃,利用交换/扩散模型进行热史模拟认为:川东地区晚白垩世古温度最高,最高古温度在213~225℃,古地温梯度在23~33℃/km;川东地区二叠纪古热流最高,最高古热流约为55~70 mW/m^(2),二叠纪之后热流逐渐降低至现今大地热流。烃源岩热演化分析认为:川东地区二叠系烃源岩热演化具有阶段性,热演化受控于构造运动,受印支期运动和燕山-喜马拉雅期运动影响存在两期热演化停滞,晚白垩世达到最高古温度后停止生烃,现今处于生气阶段。川东地区二叠纪热流值和烃源岩热演化主要受到地壳拉张减薄作用的影响,受峨眉山地幔柱的影响较小且具有局限性。

The lack of effective paleo-geothermometers restricts the study of the thermal history of carbonate formations.Clumped isotopes,as an emerging paleo-geothermometer,is of great significance for the thermal history reconstruction of carbonate formations.The Permian is the key stratum for natural gas exploration and development in the Sichuan basin.In this paper,by testing the clumped isotopes of Permian carbonate samples in the eastern Sichuan basin and combined with vitrinite reflectance,the thermal history since Permian was reconstructed.On this basis,the thermal evolution of Permian source rocks was clarified,and the influence of the Emeishan mantle plume on the thermal regime of the eastern Sichuan basin was discussed.The clumped isotope temperature in eastern Sichuan basin is 101.7~178.7℃.The thermal history simulation using the exchange/diffusion model suggests that the highest paleotemperature in the study area was in the late Cretaceous.The highest paleotemperature is between 213~225℃.The paleotemperature gradient is 23~33℃/km.The highest paleo-heat flow in the eastern Sichuan basin was in the Permian,about 55~70 mW/m 2.The heat flow gradually decreased to the current terrestrial heat flow since the Permian.The thermal evolution analysis of source rocks shows that the thermal evolution of Permian source rocks is staged and controlled by tectonic movements.The Indosinian and Yanshan-Himalayan movements caused two stages of thermal evolution stagnation.After reaching the highest paleotemperature in the late Cretaceous,hydrocarbon generation stopped.It is in the gas generation stage nowadays.The Permian heat flow and thermal evolution of source rocks in the eastern Sichuan basin are mainly affected by crustal tension thinning,and the influence of the Emeishan mantle plume is small and limited.