Analogue experiments on the piggyback propagation in northwestern Sichuan and latest propagation in its deeps
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摘要: 近年来油气勘探揭示了川西前陆北段油气资源的巨大潜力，但因受控于龙门山—川西前陆系统多期构造活动和山前带深埋藏等，川西前陆山前隐伏构造带含油气构造模型解释具有明显的多解性和不确定性。基于川西北前陆原型与实验室砂箱构造物理模型间几何学—运动学—动力学相似性原理，通过两组川西前陆北段扩展变形构造物理模型对比实验（即标准模型实验和断坪—断坡模型实验），揭示川西冲断褶皱带—前陆盆地受控于中、下三叠统膏盐和下寒武统泥岩2套主滑脱层系，晚中生代—新生代呈现出分层式前陆扩展变形特征，基底断坪—断坡结构对川西北前陆深层冲断—冲起构造的控制影响作用最为明显。进一步结合川西北前陆双鱼石地区三维地震解释资料，揭示川西北前展式扩展晚期变形的古生界隐伏冲断—冲起构造构成了川西北深层主要的勘探潜力区。Abstract: Recent explorations reveal a great potential for oil and gas resources in the northwestern Sichuan foreland basin. However, constrained by multi-phase tectonic activities of the Longmenshan-West Sichuan foreland system and the deep burial of the piedmont belt, there remain many uncertainties in the interpretation of petroleum structure models in the foreland concealed tectonic belt. Based on the geometry-kinematics-dynamic similarity theory between the foreland prototype of the northwestern Sichuan and the sandbox analogue model, two groups of controlled experiments were carried out (including standard scaled experiments and analogue experiments with a ramp-flat structure). The fold-thrust belt and foreland basin of the western Sichuan are controlled by two sets of main detachments of the Middle-Lower Triassic gypsum salt and the Lower Cambrian mudstone, leading the layered-style propagation to the foreland in the Late-Middle Cenozoic. The ramp-flat structure plays a profound influence on deep thrust and pop-up structures in the northwestern Sichuan foreland basin. Analogue results combined with seismic interpretation of the northwestern Sichuan illustrates that the main exploration potential may rely on the Paleozoic blind thrust and pop-up structures caused by the latest propagation to the northwestern Sichuan foreland basin.
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图 2 川西北段砂箱物理模型边界条件示意
Figure 2. Boundary conditions for sandbox model of northwestern Sichuan Basin
图 3 川西北段地层比例标准(a)和断坪—断坡结构(b)模型实验演化及其解释
①-⑨.挤压缩短量逐渐增加导致造山带楔形体物质序次变形过程，⑨.模型挤压结果构造解释图。图中冲断层及其相关褶皱结构按照形成演化序列标注，T6a等.浅部滑脱层系逆冲断层，T6b等.深部前展式逆断层，T6b'.深部反冲断层
Figure 3. Evolution and interpretation of standard scaled (a)and the pre-existing ramp-flat structure(b)experiment of northwestern Sichuan Basin
图 4 川西北段砂箱物理模型模拟楔形体几何学特征
Figure 4. Geometric features of wedges in sandbox analogue experiments of northwestern Sichuan Basin
图 6 川西北前陆区地震测线及其深部冲起结构构造解释对比
测线位置见图 1。
Figure 6. Seismic lines of the foreland zone in northwestern Sichuan Basin and the interpretation of their thrust and pop-up structures
表 1 砂箱物理模型模拟实验比例系数特征
Table 1. Scale coefficient for sandbox physical model of simulation experiments
参数 物理模型(M) 自然实例(N) 比例系数(M/N) 长度(L) /m 8.5×10-1 1.7×105 L*=5×10-6 缩短量(D)/m 0.30 6.0×104 D*=5×10-6 内聚力 20~100 Pa 5~20 MPa 重力加速度(g)/(m·s-2) 9.81 9.81 g* =1.0 密度(石英砂)/(kg·m-3) 1 550 2 200 ρ*1=0.70 密度(玻璃珠)/(kg·m-3) 1 370 2 400 ρ*2=0.57 密度(硅胶)/(kg·m-3) 960 2 300 ρ*3=0.42 应力(σ) σ* =ρ*g*L* =6.60×10-6 动力黏度(η)/(Pa·s) 2.1×104 1.7×1019 η* =1.24×10-15 时间 1×105 s tN=tM/t*=8.44 Ma t* =η*/σ* =3.76×10-10 速率 0.003 mm/s 7.1 km/Myr 注：tN，tM分别代表自然实际和实验模型中的时间跨度，t*为实验—实例时间比例系数，η*为动力黏度比例系数。表中部分参数来源于文献[23]，计算公式参考文献[4]。 
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