幕式流体活动历史的热年代学研究——以松辽盆地西部斜坡带为例

Conventional geochronology based on the authigenic minerals can't give quantitative results to the time and the intensity of the episodic fluid flow driven by the tectonic transition and abnormal high presssure systems. Fluid flow has resulted in mineral solution, transportation, and precipitation, but also the convective heat transportation. Thus, episodic fluid flow can also be dated according to its tansient thermal effect (TTE). This project tries to date the episodic fluid flow in the West Slope Zone of the Songliao Basin based on the new thermochronologic progress from the Apaptite Fission Track (AFT) and Apatite (U-Th)/He（AHe）dating. Systematic sampling and dating will be carried out along the main pathway systems, which have been well defined by extensive study with the evidences both from the physical properties and special compositions of the hydrocarbon and from the formation water geochemistry. The main stages of fluid flow and its intensity will be differentiated based on the overlapping effect from multi-stage of fluid flow. The effectiveness of method will first be judged by its space and dynamic match with the basin's tectonic activity because it is believed that tectonic inversion will result in massive fluid flow. It will also be judged by the abundant quantitative hydrocarbon reservoir forming ages from authigenic illite dating and qualitative results from fluid inclusion because the Cenozoic large-scale fluid flow will result in the hydrocarbon migratioin and accumulation. The research proposal switches the research focus from the framework to the geofluid, from authigenic mineral to the TTE of the fluid flow. Thus, the new thermochronologic method of AFT and AHe dating can be used to reconstruct the episodic fluid flow with the temperatrue ranging from 30-110 ℃. The research result will make it possible to date the episodic fluid flow and thus devote to the quantitative study on the hydrocarbon charging process and also the mineral forming processes.

传统的自生矿物的定年方法无法定量分析构造和超压驱动的幕式流体活动的年龄和强度。本项目拟借助低温热年代学（磷灰石裂变径迹和（U-Th）/He）的研究方法，探索利用幕式流体活动所产生的瞬时 (持续时间<0.01Ma)热效应进行流体定年。项目选择松辽盆地西部斜坡带的中部主输导通道为研究对象，通过系统的热年代学分析，阐明输导体系中多幕流体活动产生的热史叠加过程，识别主要的流体活动期次和年龄。与盆地构造变革期和已有的油气成藏年龄数据对比，评价利用瞬时热效应进行流体定年的可靠性。在思路上使年代学研究完成了从固体格架到孔隙流体，从自生矿物到流体热效应的两大转变，从而在研究手段上可以借鉴低温热年代学的最新研究进展，重建温度在30-110℃的流体活动历史，拓展了流体定年研究的方法技术手段。研究成果将使幕式流体的精确定年成为可能，为重建流体运移及其成藏和成矿过程提供精确的年代学证据。

本项目在对松辽盆地地质特征详细分析的基础上，选择西部斜坡带的中部油气运移主输导通道为研究对象，通过系统的热年代学和成藏年龄对比分析，认为西部斜坡带超压-构造驱动的油气/流体活动在输到体系中留下了明显的热史叠加证据，表现在储层中裂变径迹年龄的完全退火，而盖层中没有退火的特征。通过对热史叠加过程的解析，认为研究区存在2幕主要的流体活动，分别发生在晚白垩末期-古近纪初期（65±4Ma-51±3Ma）、始新世中晚期（41±3Ma），流体活动的强度依次减弱，影响范围逐渐缩小。盆地西部斜坡带的油气成藏年龄也表现出幕式成藏的特征，特别是白垩末期-古近纪初期（65）和始新世中晚期（45Ma）的两期成藏，表现出时间上的高度集中性和空间分布上的广泛性，说明这两期流体活动导致了西部斜坡带两次普遍的油气成藏作用，验证了利用低温热年代学开展流体活动定年研究的可靠性。研究结果表明，利用低温热年代学开展流体定年研究可以从将研究对象由固体格架转到孔隙流体，从自生矿物转到流体热效应，从而为开展流体活动及其定年研究拓展了思路。研究中试图建立不同温度范围的流体定年方法，申请发明专利三项，获得专利证书两项，建立并拓展了流体定年研究的方法技术手段。研究成果将使幕式流体的精确定年成为可能，为重建流体运移及其成藏和成矿过程提供精确的年代学证据。