四川盆地热演化异常成因及热场演化特征分析

Causes of abnormal thermal evolution and characteristics of thermal evolution in Sichuan Basin

为探讨四川盆地热演化异常成因,分析不同地史时期热场分布及其主控因素,通过四川盆地不同构造单元典型井、剖面露头镜质体反射率与深度热演化剖面的建立,分析了四川盆地纵向上热演化异常成因;在热演化剖面建立的基础上,通过镜质体反射率梯度法求取了四川盆地晚古生代—中三叠世、晚三叠世—侏罗纪及现今的热场分布,并进行了热史演化特征分析。研究认为:(1)烃源岩生烃作用产生超压并对镜质体反射率和烃类裂解产生差异性抑制,是四川盆地纵向有机质热演化"负"异常的主要原因。(2)盆地不同时期构造运动及盆地性质控制着四川盆地热场纵向的演化:中二叠世晚期区域性拉张作用改变了四川盆地早期统一的低热场,导致断陷内热流值不断升高,平均地温梯度在3.5℃/hm以上;晚燕山—早喜马拉雅期,盆地东部开始隆升剥蚀,盆地逐渐向西萎缩,周缘冲断作用趋于平静,高热场开始降温;至喜马拉雅晚期的快速隆升及降温作用,形成现今低地温场分布面貌,地温梯度总体小于2.5℃/hm。

The causes of thermal evolution anomalies in the Sichuan Basin,and the distribution of thermal fields and their main controlling factors during different geohistory periods were analyzed.The Ro-H thermal evolution profiles of typical wells and outcrops in various tectonic units in the Sichuan Basin were established to discuss the causes of vertical thermal evolution anomalies.The thermal field distributions in the Sichuan Basin from the Late Paleozoic to the Middle Triassic,the Late Triassic to the Jurassic,and at present,respectively,were obtained by a Ro gradient method,and a thermal history analysis was carried out.Some conclusions were made as follows.(1)Hydrogen generation from source rocks produced overpressure,leading to the differential inhibition of Ro and hydrocarbon cracking,which was the main cause of the“negative”anomaly of longitudinal organic matter thermal evolution in the Sichuan Basin.(2)The tectonic movements and basin properties in different periods controlled the vertical thermal evolution in the basin.At the end of middle Permian,regional extension changed the former unified low?heat field,resulting in increasing heat flow value in the fault depression,with an average geothermal gradient above 3.5°C/hm.From the Late Yanshanian to the Early Himalayan periods,the east part of the basin began to uplift,and the basin gradually shrank to the west.The peripheral thrusting action tended to decrease,and the high thermal field began to cool.The rapid uplifting and cooling effects at the end of Himalayan period formed the current low?heat field,with a geothermal gradient less than 2.5°C/hm.