基于重磁震资料的南海新生代盆地分布综合研究

作为西太平洋最大的边缘海，南海分布有30多个新生代沉积盆地，其蕴含着丰富的油气资源.但由于资料的限制，南海存在不同区域盆地研究程度不同，不同区域盆地面积差别较大，部分盆地只是坳陷而没有达到盆地的级别以及盆地外围可能存在凹陷等问题.南海新生代盆地分布问题制约了其油气分布规律、储量等基础地质问题的研究.本文以地震剖面数据为约束，以重力资料为主、辅以磁力资料，研究了南海新生代盆地分布及构造区划.通过提取新生代盆地及其构造单元引起的重力异常，结合地震剖面等资料反演了新生界底界面深度及新生界厚度.在充分调研已有盆地和构造单元划分方案的基础上，根据南海的地质及地球物理特征，确定了盆地及构造单元划分标准.以新生界厚度为基础并结合重、磁、震、地质等资料，进行地质-重磁震联合解释，将南海原有的36个盆地重新划分为24个盆地，盆地总面积扩大了约15万km2.研究表明，南海新生代盆地沉积层厚度在1.5~16 km之间，有6个北东东/北东向沉积坳陷带、2个近南北向沉积坳陷带以及1个三角沉积坳陷区；盆地展布方向主要为北东和北东东向，其次为北西和近南北向，呈现"南三北三"的分布特征.

An integrated study on distribution of Cenozoic basins in the South China Sea based on gravity,magnetic and seismic data

The South China Sea (SCS), as the largest marginal sea in the western Pacific Ocean, hosts more than 30 Cenozoic sedimentary basins that contain abundant oil and gas. Due to the limit of survey data available, some of these basins lack exhaustive studies or have controversial issues, especially in the central and southern regions. For example, different boundaries of basins in the central and southern SCS have been delineated. The relationship between the basins remain unclear and some basins (e.g. Nanweidong basin and Jiuzhang basin) may not attain the level of a true basin, instead merely a depression. In addition, some sags may exist on the periphery of some basins. To address these problems, we have studied the distribution of the Cenozoic basins in the SCS based on gravity, magnetic seismic data in an integrated way.
We collected the latest satellite altimetry gravity and magnetic ΔT anomalies and analyzed the physical properties of the strata and rocks in the SCS. Then we obtained the Bouguer gravity anomalies by removing the effect caused by seawater and computed the magnetic anomalies reduced to the pole using a varying inclination method. Based on the analysis of the characteristics of the gravity field, we extracted the gravity anomalies of the Cenozoic by using the minimum curvature technique for potential field data separation and inverted the thickness of Cenozoic in the study area under the constraint of seismic profiles.
We established the criteria to divide basins and other tectonic units on the basis of the thickness of Cenozoic and the relative relief of the basement assisted with the characteristics of the gravity and magnetic fields and trend lines of the faults combining other geologic data. Then we re-divided the original 36 Cenozoic basins into 24 sedimentary ones and defined their internal tectonic units. The total area of the newly divided basins has increased by about 150 thousand square kilometers. The thickness of the sedimentary layer of Cenozoic basins in the SCS is between 1.5 km and 16 km, which is rather changeable. These basins are divided into six NE or NEE trending sedimentary depression zones, two nearly NS trending sedimentary depression zones and one triangular sedimentary area. The directions of the basin distribution are mainly NE and NEE, followed by NW and nearly NS. Under the control of the major faults, these structures show a ‘south three and north three’ pattern.