基金项目：中国石油化工股份有限公司海相前瞻性研究项目(YPH08101)

摘要：四川盆地位于扬子板块西缘,是中国重要的含油气盆地之一，在震旦纪—早奥陶世处于裂陷特征的被动大陆边缘阶段。利用地球动力学理论在岩石圈尺度探讨盆地构造热演化特征，恢复早古生代热历史，为盆地生烃状态、生烃期次等研究提供重要热参数。首先，利用回剥技术得到盆地构造沉降史，并作为构造热演化模拟的目标函数；然后，基于二维多期拉张模型，分别与晚震旦世、寒武纪和奥陶纪等3个时期构造沉降量拟合得到3期拉张系数，揭示岩石圈底界和温度场的演化以及基底热流随时间的演化特征。模拟结果显示：在岩石圈拉张作用下，软流圈上涌形成热扰动，震旦纪时期基底热流值略有升高，其中高值主要分布在盆地西北与西南，而川东北受到的影响最小；热扰动在寒武纪有所减弱，至奥陶纪时期基本消失，盆地热流值呈缓慢下降趋势；盆地基底热流在早古生代始终在52~59 mW·m-2范围内。

关键词：构造热演化；多期拉张模型；早古生代；基底热流；岩石圈尺度；数值模拟；四川盆地

中图分类号：P314.2文献标志码：A

Tectonothermal Evolution of Sichuan Basin in Early Paleozoic

HE Lijuan, HUANG Fang, LIU Qiongying, LI Chunrong, WANG Jiyang

(Key Laboratory of Petroleum Resources Research of Chinese Academy of Sciences, Institute of Geology

and Geophysics, Chinese Academy of Sciences, Beijing 100029, China)

Abstract: Sichuan Basin, located in the western margin of Yangtze Plate, is one of the important oilgasbearing basins in China. The basin is in the passive continental margin stage with rift in SinianOrdovician. The tectonothermal evolution of basin was simulated by a geodynamical method in the lithospheric scale, and the thermal history of Early Paleozoic was obtained in order to provide important thermal parameters for assessment of hydrocarbon generation of basin. The tectonic subsidence of basin was firstly calculated by backstripping technique to provide objective function for the tectonothermal modeling; secondly, based on twodimensional multiphase extensional model, three episode extensional coefficients were estimated through fitting with the tectonic subsidences of Late Sinian, Cambrian and Ordovician in order to reveal the evolution of lithospheric base and temperature, as well as the basal heat flow. The modeling results show that the thermal disturbance, which is induced due to lithospheric extension and upwelling of asthenosphere, increases the basal heat flow in Late Sinian, and the effects are high in the northwest and southwest of basin and little in the northeast; the thermal disturbance weakens in Cambrian and disappears gradually in Ordovician, and consequently the basal heat flow decreases slowly; the basal heat flow of basin is still 5259 mW·m-2 in Early Paleozoic.

Key words: tectonothermal evolution; multiphase extensional model; Early Paleozoic; basal heat flow; lithospheric scale; numerical modeling; Sichuan Basin

0引言

盆地热历史控制着油气的生成、运移、聚集以及保存等过程，是含油气盆地分析的重要环节。热历史的恢复可以利用岩石圈尺度的构造热演化方法或盆地尺度的古温标方法［15］进行。四川盆地现今大地热流值平均为53 mW·m-2,具有典型克拉通型盆地中低热流特征［6］。近年来，许多学者对四川盆地的热历史恢复作了很多工作［716］，研究结果表明早二叠世—中三叠世是盆地热演化的重要时期，其间盆地古热流达到了其历史最高值。传统古温标方法由于无法恢复盆地达到最高古地温以前的热历史，因而在四川盆地古生代热历史研究中受到局限。构造热演化模拟可以有效补充古地温方法在该盆地应用的局限性，填补该盆地早期热历史研究的空白。构造热演化模拟作为研究沉积盆地的重要手段之一，与古温标方法不同的是，构造热演化模型依赖于盆地成因类型。它必须建立在盆地成因机制的地质地球物理模型基础之上。

位于扬子克拉通的四川盆地是一个大型古生代—中新生代海相陆相叠合盆地,蕴含着丰富的油气资源［1722］。它经历了中—晚元古代扬子地台基底形成阶段、震旦纪—中三叠世被动大陆边缘阶段、晚三叠世盆山转换与前陆盆地形成演化阶段、侏罗纪—第四纪前陆盆地沉积构造演化阶段。其基本性质是多旋回叠合构造改造型盆地［23］。震旦纪开始的伸展作用在中国南方是区域性的［24］。扬子克拉通在震旦纪—早奥陶世处于裂陷特征的被动大陆边缘阶段。盆地内大型构造圈闭主要发育古生代—中生代烃源岩，形成多套生储盖组合，是重要的油气生、储场所，因此，对古生代热历史的研究迫在眉睫。笔者拟将多期拉张模型应用到四川盆地早古生代岩石圈拉张期(或称为伸展期)的热历史研究中，利用地球动力学方法在岩石圈尺度探讨盆地晚震旦世—奥陶纪构造热演化特征，为盆地生烃状态、生烃期次等研究提供热参数。