断层活动性及其对烃源岩发育的影响——以柴达木盆地西部为例

Fault reactivation and its effect on the formation of source rock——a case study of western Qaidam basin,Tibet Plateau

断层活动性分析对于研究盆地构造演化、古环境等信息具有重要意义。本文提出运用平衡地质剖面恢复断层历史活动性,该方法可考虑地质历史时期的地层差异压实减薄、地层剥蚀厚度缺失等重要信息,同时能够直接以曲线方式测量断层的滑距大小,特别是在识别某一特定地质历史时期断层的活动方向上具有重要应用价值,是传统断层活动性分析法的进一步发展和完善。本文运用平衡剖面技术法分析了柴西地区地质历史时期的断层活动性,识别出两期构造反转,第一期由中生代晚期的挤压环境转换为新生代古近纪的伸展构造环境,第二期为上干柴沟组沉积时期(N1)由伸展构造背景转换为挤压构造背景。下干柴沟组上段沉积时期(E32)的伸展构造背景控制了优质烃源岩的发育,上干柴沟组沉积时期(N1)之后转换为挤压构造背景,控制了柴西现今主要圈闭的形成,捕获晚期形成的烃类。靠近断层的反转构造可形成早期圈闭,具有捕获早期生成的低熟烃类的潜力,是深层油气勘探的潜在领域。

Fault reactivation study is significant for reconstructing basin evolution and paleoenvironment.Balanced geological profile method was proposed to restore past fault reactivation in this paper.This method can take into account important information such as differential compaction and strata erosion thickness.It can directly measure the slippage of faults by curve,and has important application value in identifying the direction of fault reactivation in a specific geological period,which is an improvement on the traditional fault reactivation analysis method.Two episodes of structure inversion were identified by analyzing the fault reactivation characteristic in the western Qaidam basin using this method.The first episode was converted from the compression environment in the late Mesozoic to the extensional tectonic environment in the Paleogene of the Cenozoic.The second phase is the change in the deposition period of the Shangganchaigou Formation(N1)from the extensional tectonic background to compressional tectonic background.The extensional tectonic background of the sedimentary period of the upper Xiaganchaigou Formation(E32)controlled the development of high-quality source rocks.After the depositional period of the Shangganchaigou Formation,the Qaidam basin was converted into a compressional tectonic background,which controlled the traps formation that captured hydrocarbons that were formed at a later stage.Inversion structures near faults can form early traps and have the potential to capture early generated low-mature hydrocarbons,and are promising for deep petroleum exploration.