四川盆地龙门山前复杂构造带北段精细构造建模及对油气勘探的启示

四川盆地西北部地区受到多期次逆冲推覆变形叠加作用,构造变形非常复杂,致使龙门山前复杂前锋带构造模式不清、构造圈闭不落实。为了降低油气勘探的风险,利用最新线束三维地震、测井、地表地质等资料,基于断层褶皱理论,在分析构造运动与地表地质特征的基础上,开展了精细构造建模研究,结合计算机运动学演化模拟对龙门山前复杂前锋带北段进行构造几何学分析,解析该区的构造变形模式、落实断层变形特征和构造圈闭,进而提出了下一步油气勘探的有利目标。研究结果表明:(1)以马角坝断裂和(1)号断裂为分界断层,可将龙门山冲断带北段划分为前山推覆带、前锋带、潜伏构造带3套构造单元;(2)前锋带(1)号断裂前端发育产状陡立倒转的断层传播褶皱,其下盘可识别出逆掩隐伏构造;(3)前锋带发育4套垂向叠置的二叠系—三叠系岩片,该构造带在印支期发生构造变形、喜马拉雅期活化;(4)隐伏前缘带受寒武系泥页岩和下三叠统嘉陵江组膏盐层两套滑脱层夹持形成3套构造层,构造变形主要发生于喜马拉雅期。结论认为:(1)号断裂下盘隐伏的上古生界构造(中构造层)和上盘的红星构造背斜形态完整,是下一步油气勘探的有利目标区;计算机运动学演化技术及精细构造建模方法,给复杂构造带的油气勘探提供了重要的技术支撑。

Accurate structural modeling on the northern section of complex structural belt at Longmenshan piedmont in the Sichuan Basin and its implications for oil and gas exploration

The northwestern area of the Sichuan Basin has experienced the superimposition of multi-stage thrust nappe deformation and its structural deformation is quite complicated. As a result, the structural mode of the complicated surging zone of Longmenshan piedmont is not figured out and the structural traps in the area are not ascertained. For reducing the risk of oil and gas exploration, accurate structural modeling was carried out based on the fault-related fold theory, combined with latest swath, log, surface and geological data after tectonic movement and surface and geological characteristics were analyzed. Then, the kinematic evolution process of the northern section of the complicated surging zone of Longmenshan piedmont was simulated by using the computer to study the structural geometry, analyze the structural deformation modes and confirm the faulting deformation characteristics and structural traps in this area. Finally, the favorable targets for future oil and gas exploration were presented. And the following research results were obtained. First, separated by Majiaoba fault and No. 1 fault, the northern section of the Longmenshan thrust zone can be sub-divided into three tectonic units, i.e., front nappe belt, surging zone and buried structural belt. Second, fault propagation fold with the occurrence of pitch and inversion is developed at the front tip of No.1 fault in the surging zone and a buried overthrust structure is identified at its lower wall. Third, four sets of vertically stacked Permian–Triassic microlithon are developed in the surging zone, which is structurally deformed in the Indosinian period and activated in the Himalayan period. Fourth, three sets of structural layers are formed in the buried frontal zone by the support of two detachment layers (i.e., the Cambrian shale and the gypsum salt layer of the Jialingjiang Fm, Lower Triassic), and its structural deformation mainly happens in the Himalayan period. In conclusion, the buried Upper Paleozoic structure (middle structural layer) at the lower wall of No.1 fault and Hongxing anticline at the upper wall are morphologically complete, so they are the favorable target areas for the future oil and gas exploration. In addition, the computer based kinematic evolution technology and the accurate structure modeling provide an important technological support for the oil and gas exploration in complex structural belts.