准噶尔盆地玛湖凹陷深层构造—热演化研究

Tectono-thermal evolution is an important aspect of the basin dynamics and petroleum geology. With the increasing petroleum shortage year by year, the deep zones of superimposed basins in western China are the most realistic zones to succeed the strategic reserves area of oil and gas. However, as an example, the deep zone in Mahu sag has experienced multi-phase complex tectono-thermal evolution history since the Carboniferous period, then the early thermal recording has superimposed and reformed by the late hot events. There are many issues on the tectono-thermal history reconstruction in the deep zone of Mahu sag, such as lack of sufficient and effective thermal indicator data, too simple method of thermal history reconstruction, which make it difficult to recover the thermal history of the study area. It leads to different understanding on periods of hydrocarbon generation, thereby affecting the correct understanding of the law of deep oil and gas accumulation, which has restricted the deep petroleum exploration. Therefore, the project intends to establish effective paleothermometers profile on the base of systemic testing of different types and different closure temperatures thermal indicator; then reconstruct the multi-phase thermal history since the Late Paleozoic of the Mahu sag by joint inversion of low-temperature thermochronology and traditional geothermometer, and tectono-thermal evolution numerical simulation of the Carboniferous large-scale magmatism and the lithospheric thinning/thickening during Early-Middle Permian. The results can provide important scientific basis for a correct understanding of tectono-thermal evolution relationship since the Late Paleozoic, and hydrocarbon accumulation in deep zone of Mahu sag.

构造—热演化是盆地动力学和石油地质学领域研究的重要内容。随着我国油气短缺逐年加大，以玛湖凹陷为代表的西部叠合盆地深层成为了我国最现实的油气后备储量接替区。然而，玛湖凹陷自石炭纪以来经历了多期复杂构造—热历史，早期热记录被后期热事件叠加改造，深层热史研究中缺乏系统的有效古温标、热史恢复方法单一，难于有效恢复研究区深层热历史，导致对深层烃源岩生烃期次认识不一，进而影响对深层油气成藏规律的正确认识，这已经制约着研究区深层油气勘探。因此，该项目拟系统测试不同类型、不同封闭温度的古温标，建立有效古温标剖面；通过低温热年代学和传统古温标的耦合反演，并对石炭纪岩浆作用、早—中二叠世岩石圈拉张减薄/挤压增厚等重要热事件辅以构造—热演化数值模拟，正反演相结合系统厘定研究区晚古生代以来的多期复杂热史。研究成果可以为正确认识玛湖凹陷晚古生代以来构造—热演化关系以及深层油气成藏演化规律提供重要科学依据。

针对准噶尔盆地深层多期复杂热史的特点，综合盆地演化阶段、构造与岩浆作用过程，在盆地现今地温场与岩石圈热结构研究的基础上，采用镜质体反射率、磷灰石裂变径迹等古温标，定量、系统、有效地恢复了准噶尔盆地晚石炭世以来的热历史，进而分析其演化特征和主控因素，建立了构造-热演化关系模型。研究表明，准噶尔盆地现今地温梯度介于 11.6~27.6℃/km，平均21.3±3.7℃/km，大地热流介于23.4~56.1mW/m2，平均42.5±7.4 mW/m2，表现为低地温梯度、低大地热流的“冷”盆特征。准噶尔盆地大地热流与地温梯度分布规律基本一致，主要受控于基底的构造形态，东部隆起最高，陆梁隆起次之，乌伦古坳陷、中央坳陷和西部隆起较低，北天山山前坳陷最低。准噶尔盆地地壳热流介于18.8~26.0mW/m2，地幔热流介于16.5~23.7mW/m2，壳幔热流比值介于0.79~1.58，属于典型的“冷壳冷幔”型热结构。准噶尔盆地地幔热流值与莫霍面起伏一致，隆起区地幔热流高，坳陷区地幔热流低。准噶尔盆地古热流较高，晚石炭世—早中二叠世一般可达80~100mW/m2；中、新生代，热流持续、逐渐降低，直至现今的42.5mW/m2。盆地的热流演化与构造演化息息相关。综合盆地及其周缘晚古生代火山机构的平面分布、火山活动期次和上石炭统的沉积序列，认为晚石炭世的超高古热流受控于同时期的强烈的岩浆活动。结合盆地地质、地球物理和地球化学资料，论证了早—中二叠世准噶尔盆地的裂谷属性，即岩石圈的减薄与强烈拉张造成了早—中二叠世的高热流。中生代为陆内坳陷阶段，盆地的构造稳定性增强以及缺乏幔源热活动的动力学机制导致盆地热流降低。新生代，印度板块与欧亚板块碰撞产生的远程效应，使盆地转为前陆盆地演化阶段，盆地南缘岩石圈的挠曲沉降及快速、巨厚的磨拉石建造是现今低地温梯度、低大地热流的主要原因。