鄂尔多斯盆地中西部深部流体活动及其对奥陶系天然气形成的热作用

鄂尔多斯是一个非常稳定的克拉通盆地,深部流体一直被认为是禁区。通过构造学、火成岩岩石学、矿物学、同位素以及热史分析等方法,首次系统证实了鄂尔多斯盆地中西部发生过强烈的深部流体活动,并分析了其形成机制和对奥陶系天然气形成的热演化作用。在中生代晚期,祁连构造域的挤压作用导致盆地西缘发生强烈逆冲推覆、断裂活动及三次大规模的深部流体上涌。这些流体在构造应力作用下主要沿J/T、O/C不整合面运移至盆地中部。流体携带了大量深部热能,使奥陶系上部古地温升高约50℃,并将其中的碳酸盐岩生烃速度提高了至少32倍。最终,深部流体一方面导致奥陶系生烃量显著增加,另一方面也降低了天然气的散失,有助于其聚集成藏。深部流体使鄂尔多斯盆地中西部奥陶系天然气成因复杂化,另外它对深入探索铀和其它金属矿床的形成及盆地演化也具有重要意义。

Deep fluid activity in middle and western Ordos Basin, Northwest China and its heating effect on hydrocarbon generation in Ordovician

Ordos is a very stable intracratonic basin that is considered as a prohibited place for deep fluids.By means of tectonics,igneous petrology,mineralogy,isotope and temperature history analysis,the authors have systematically confirmed for the first time that there had really happened violent hot fluid activities in Ordos Basin,and have studied their mechanism and their heating effect on Ordovician gas formation.In the Late Mesozoic,the strong deformation of Qilian structural domain caused obduction,faulting,and upwelling of hot fluids from lower earth crust and mantle for three times in western Ordos.The fluids then migrated into the middle basin along J/T and O/C unconformities or cracks by strong stress.The fluid was so hot that it made upper Ordovician palaeotemperature go up 50 ℃,and increased the hydrocarbon-generating rate of carbonate rock in Ordovician 32 times at least.As a result,the hydrocarbon quantity increased,and the hydrocarbon-dissipating rate was greatly lowered.This was beneficial for the hydrocarbon accumulation.The effect of deep fluid resulted in a more complex hydrocarbon-generating mechanism.This is also of great significance for investigating the formation of deposits of uranium and other metallic minerals,as well as the evolution of Ordos Basin.