北部湾盆地涠西南低凸起构造演化特征及其成因

Tectonic evolution characteristics and genesis of Weixi’nan low uplift in Beibu Gulf Basin

北部湾盆地涠西南低凸起呈NEE—SWW向延伸,北缓南陡,与以似花状或"Y"字型构造样式为主的(2)号断裂带、(3)号断层相接,是重要的油气富集区带。低凸起地层剥蚀严重,构造现象复杂,制约了对区域油气成藏的准确认识。基于最新的勘探进展,通过古地貌恢复、右行力偶产生的走滑应变椭圆应力机制分析及构造物理模拟,解析了涠西南低凸起及周缘区域剪切应力环境下伸展、走滑、挤压并存的复杂构造现象。研究认为:在区域右行剪切应力作用下,模拟有效区域内剪切主变形带及在剪切带内雁列式分布的同向剪切断层的变形特征与(2)号断裂带相似;斜交于主剪切变形带形成多个背斜、向斜交替出现的褶皱带,褶皱轴线均呈NNE向,与涠西南低凸起及周缘的褶皱走向相同;涠西南低凸起及周缘的地层剥蚀区主要受渐新世末期的褶皱控制。渐新世末期在右行剪切应力控制下形成的褶皱与走滑断裂带是下一步油气勘探的重点区带。

Weixi’nan low uplift is an important oil and gas accumulation area in Beibu Gulf Basin, which extends in NEE-SWW direction, and is gentle in the north and steep in the south, respectively connected with No.2 fault zone dominated by flower-like or “Y” structural style and No.3 fault. The serious stratum denudation and the complex structural features restricted the accurate understanding of regional oil and gas accumulation. Based on the latest exploration progress in this area, the complex structural phenomenon of extension, strike-slip, and compression coexisting in the shear stress field environment of Weixi’nan low uplift and its periphery is analyzed through paleogeomorphic restoration, elliptical stress mechanism analysis of strike-slip strain generated by right-hand couple and structural physical simulation. It is considered that:(1)Under the action of regional dextral shear stress, the deformation characteristics of the principal displacement zone(PDZ) and the en echelon shear faults in the shear zone in the simulated effective area are similar to those of the No.2 fault zone.(2)The fold axes of multiple anticline and syncline alternating fold belts forming in oblique intersection with the PDZ are in NNE direction, which is the same as the fold trend of Weixi’nan low uplift and its periphery.(3)The stratigraphic denudation area in Weixi’nan low uplift and its periphery is mainly controlled by the fold at the end of Oligocene. It is pointed out that the fold and strike-slip fault zones formed under the control of dextral shear stress at the end of Oligocene are the key areas for oil and gas exploration in the next step.