川西北地区断裂带内部结构及其流体运移路径的判识

Fault is one of important pathways of the hydrocarbon vertical efficient migration and has great significance to hydrocarbon accumulation. Study on the favorable pathways of vertical fluid migration in fault zones from the structures of fault zones and the evidence of fluid flow by fault perspective, still does not go far enough at present. There are a lot of various outcrop faults and the imprinting information of fluid flow by faults in the northwest of Sichuan, where is the ideal place to study the internal structures of fault zones and analyze the characteristics of fluid flow by faults. Seeing that the fault structures isn't identified effectively by seismic data, and the information on drilling and logging can only provide the feature of fault breakpoint instead of overall perspective. Hence, this project will probe into the internal structures of fault zones and their controlling factors by means of contrastive analysis of various kinds of outcrop faults and taking samples. And then, the C, O isotopes of calcite and H, O isotopes、trapped temperature、salinity of fluid inclusions in calcite veins are tested. By mutual restraint and checking of these testing results, the fluid source of forming calcite veins in fault zones and the fluid flowing characteristic are analyzed. Combining the features of oil charging and the bitumen filling, the migration characteristics of the formation fluid from the bottom to up are studied. Finally, the vertical fluid migration pathways in fault zones under different controlling factors are revealed, which provide the theory basis for the favorable pathways forecasting of hydrocarbon vertical migration along the deep buried faults in basin.

断裂是油气发生垂向高效运移的重要路径之一，对油气成藏意义重大。从断裂带结构及其内部流体活动证据方面，研究断裂带内流体的垂向运移路径，尚待深入。川西北地区，野外露头断裂数量、类型多，断裂带内流体活动的印记发育，是进行断裂带结构及其内部流体活动特征分析的理想场所。鉴于目前地震资料不能有效识别断裂带结构，而探井资料仅能反映断点处的特征，难及全貌。本项目拟通过野外露头考察、取样及对各种类型断裂的对比分析，研究断裂带内部结构及其控制因素，借助断裂带内方解石的碳、氧同位素测试，方解石脉中流体包裹体的氢、氧同位素、捕获温度、盐度测试，经过相互约束和验证，分析形成方解石脉的流体来源及活动特征，结合断裂带内古油藏中石油的充注与沥青的充填特征，研究地层流体在断裂带内自下而上的运移特征，最终揭示不同控制因素下断裂带内流体垂向运移的有利路径，为盆地内油气沿断裂垂向运移的有利路径的预测提供理论依据。

断裂带作为一个三维地质体，流体在其内部结构中运移的过程尚存争议，特别是对逆冲断裂在这方面的研究更少。鉴于目前探井、地震资料不能有效的识别断裂带的内部结构，且在探井中对断裂带进行连续取样受限。鉴于此，本次研究以中国四川盆地西北部为例，通过大量的野外观察、取样，实证了流体在以碳酸盐岩和砂、泥岩为原岩的逆冲断裂带内进行流动的过程。研究中对于碳酸盐岩断裂，以其内部大量分布的方解石脉为线索，通过13C、18O同位素测试、卤水包裹体测温，实证了卤水、大气水在逆冲断裂带内的流动过程；对砂、泥岩断裂，以断裂带内大量发育的油砂为线索，通过与石油共生的卤水包裹体测温及沥青在断裂带内分布位置的分析，实证了古油藏在被断裂破坏的过程中，石油如何沿断裂带进行泄漏及如何与大气水间发生作用。研究结论表明，中国四川盆地西北部碳酸盐岩逆冲断裂中流体的活动过程为：深部地层中的热流体首先进入断层核，而后沿断层核向上运移，当接近地表时与大气水混合，同时热流体的温度降低，之后进入断裂破裂带；古油藏中的石油在砂、泥岩逆冲断裂带中的泄漏过程为：发育在断层破裂带位置的古油藏，在构造抬升后，石油首先沿裂缝向断层核运移，大量的石油在断层核发生泄漏，而后遭遇大气水的降温、水洗和氧化，在断裂带内形成了低温流体包裹体，并主要在断层核内形成了沥青。断层核内主滑动面发育的断层泥薄膜封闭了一部分石油向对盘的运移。以上的研究表明，断层核与内破裂带是流体沿逆断层发生垂向运移的主要通道。该认识解决了逆断层内流体垂向运移路径的科学问题，为盆地内流体沿逆断层运移路径的预测提供了理论依据。