准噶尔盆地热史恢复及其对早—中二叠世时期盆地构造属性的约束

摘要

大地热流是盆地动力学成因及构造演化过程的客观反映,不同时代、不同动力学背景形成的盆地,大地热流差异极大,因此盆地构造—热演化研究不仅能够揭示盆地不同演化阶段的地温场特点,而且能够有效地约束盆地在特定地质演化历史时期的动力学机制和构造属性.本文针对准噶尔盆地深层多期复杂热史的特点,在盆地现今地温场研究的基础上,采用镜质体反射率和裂变径迹等古温标,结合古地温梯度法和古热流法定量恢复了准噶尔盆地二叠纪以来的热历史,进而分析了早—中二叠世期间盆地构造属性.研究表明,准噶尔盆地从早二叠世开始呈现出热流持续降低的热流演化特征,二叠纪期间,盆地热流值很高,多数钻井的古热流在75～85mW/m2之间,少数钻井揭示的古热流更高,超过了100mW/m2;中—新生代,热流持续、逐渐降低,直至现今的42.5mW/m2.早—中二叠世,盆地的最高热流地区在中央坳陷和南部坳陷.以早—中二叠世期间高古热流为切入点,结合区域地质、地球物理和地球化学等资料,论证了准噶尔盆地早—中二叠世期间的裂谷构造属性.这一认识不仅是重新认识准噶尔地区晚古生代碰撞造山和陆内盆山体制转换的基础,而且对于准噶尔盆地深层石炭系、二叠系烃源岩油气进一步勘探具有重要意义.%Heat flow is an objective reflection of the dynamic mechanisms and tectonic evolution process in sedimentary basins.Basins,which formed in different geodynamic setting at different time,show huge difference in terrestrial heat flow.Therefore,the research on the tectono-thermal evolution of the basin can not only reveal the geothermal characteristics in different evolution stages,but also effectively constrain dynamic mechanism and tectonic attributes of the basin in the specific geological history.With aiming at features of the deeply-buried,multiple-stage and complicated thermal history of the Junggar basin,combined with the present study of geothermal field of basins,this study reconstructed the thermal history of the basin since Permian,using paleothermometers such as vitrinite reflectance and fission track,paleo-geothermal gradient and paleo-heat flow methods,and further discussed the structural property of the basin during the Early-Middle Permian.The study shows that the Junggar basin presented the continuous evolution feature of heat flow since Early Permian.During the Permian,the Junggar basin experienced much higher heat flow of around 75 ～ 85 mW/m2,even higher than 100 mW/m2 in a few boreholes located in the central depression and southern depression.During the Mesozoic to Cenozoic,the heat flow in the basin maintained and gradually decreased until to the present of 42.5 mW/m2.During the Early-Middle Permian,the highest heat flow area in the basin was distributed mainly in central depression and southern depression.High heat flow resulting from the intensive lithosphere stretching and thinning suggests that the Junggar basin was a rifting basin during the Early-Middle Permian,which is supported by many geological,geophysical and geochemical data.The realization is the important basis for understanding the collisional orogenesis and intracontinental basin-mountain system transformation during the Late Paleozoic,but also of great significance in deepening the Carboniferous and Permian deep oil-gas exploration in the Junggar basin.