大陆架科学钻探CSDP-2井的大地热流和岩石圈热结构研究

The heat flow is a direct parameter that characterizes the outward heat transfer from the Earth, while the lithosphere thermal structure refers to the heat flow distribution and structural relationship between the mantle and different strata layers within the crust, both of which reflect the current geothermal state and have a key influence on the geophysical fields such as magnetic, gravity, conductivity and seismic wave. The study of heat flow is the basis for further studying the tectonic thermal evolution from basin scale and lithospheric scale. The South Yellow Sea (SYS) has experiences complex tectonic evolution processes, and the study of heat flow, which is still lack of research, is of great significance to solve problems of regional tectonic and the Mesozoic-Paleozoic marine oil and gas resources. The CSDP-2 well, supported by continental scientific drilling project (CSDP), is the exclusive deep exploratory welll. It reveals the integrated stratigraphic sequence from the Quaternary to the Upper Ordovician, with a total depth of 2843m. As a whole core drilling project, the core recovery of CSDP-2 can reach up to 97.7%, providing excellent opportunity for this study. Based on measurement of thermal conductivity, rock heat production and other geothermal parameters, together with five temperature logging curves, the vertical heat flow profile and the crustal production model can be established. The study of lithospheric thermal structure, heat flow distribution, thermal lithospheric thickness and other parameters will lay an important foundation for further study of tectonic evolution and petroleum resources evaluation.

大地热流是表征地球向外热传导的直接参量，岩石圈热结构是地壳、地幔以及地壳内部各岩层之间的热流配分和组构关系，两者反映着现今地热状态，对磁力、重力、导电率和地震波等多种地球物理场有关键影响，也是进一步研究盆地尺度和板块尺度构造热演化的基础。南黄海历经复杂的构造演化过程，既面临下扬子在海区边界的构造问题，也面临中-古生界海相油气需要突破的资源问题，急需基本地热状态作为重要参考，然而至今此项研究几近空白。大陆架科学钻探CSDP-2井，是南黄海中部隆起唯一深井，完井深度2843m，自第四系钻至上奥陶统，全取心钻井的综合取心率高达97.7%，为该项研究提供了绝佳机会。本项目拟通过系统测定岩石热导率、生热率等热物性参数，结合5次系统测温结果，建立垂向热流剖面和地壳生热率模型，深入剖析岩石圈热结构， 获得各岩层大地热流配分、热岩石圈厚度等地热参量，为南黄海进一步的构造演化和油气资源研究奠定重要基础。

大地热流与岩石圈热结构是构造热演化及地球动力学过程研究的基础。南黄海处于华北、扬子块体和西太平洋板块中­新生代相互作用的关键区，大陆架科学钻探CSDP-2井获取的近2800m的连续岩心为研究南黄海大地热流、生热率及东亚陆缘岩石圈热结构提供了一个绝佳的机会。通过对井中295块沉积物或沉积岩进行了热导率测量，利用5次系统测温数据，并考虑地层温度、压力和裂缝孔隙影响，大地热流确定为66.7mW /m2。通过对井中302块岩心样品的U、Th和K等放射性元素含量测试及分析，结合下扬子块体87块露头样品，得到震旦纪至奥陶纪生热率为0.22-0.29μW/m3，上奥陶统至志留系、泥盆系与下石炭统、上石炭统、二叠系、下三叠统、新近系与第四系的生热率分别为2.01、1.90、0.62、2.98、1.95和1.21μW/m3。基于实测数据和岩性模型，建立了南黄海的地壳生热率模型。南黄海热流在东亚大陆边缘处于偏低的水平，反映了南黄海在东亚陆缘相对偏冷的地质背景。地壳放射性生热对大地热流的贡献自边缘向内部变大，与新生代区域构造活动强度呈负相关。1-D热模型计算的南黄海Moho温度为652℃，热岩石圈厚度为96km，2-D热模型计算的Moho温度为620-638℃，热岩石圈厚度为94-104km。壳幔边界温度在整个东亚大陆边缘较低，热岩石圈厚度则相对较厚，与新生代伸展前先存的岩石圈相对较厚，而未受郯庐断裂等深大断裂深部物质上涌的影响有关。