Mechanics and analog modeling of the Huo-Ma-Tu thrust sheet in the southern Junggar Basin fold and thrust belt
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摘要: 逆冲岩席发育的力学机制厘定是认识冲断带变形过程的基础与难点。文章以准南冲断带为研究实例,结合力学模型与物理模拟对逆冲岩席发育的力学机制进行探讨。依据地质调查、地震资料解释和钻井资料证实霍−玛−吐背斜核部和南翼发育有逆冲断层,断层向南延伸到第一排背斜之下,并形成广泛分布的霍−玛−吐逆冲席体,席体内部基本上无构造变形。霍−玛−吐构造带的钻井资料显示准南逆冲带前缘逆冲断层及滑脱断层均发育在流体压力系数较高的异常超压层中,说明该逆冲岩席属于强干变形席体、弱滑脱断层组合。根据玛纳斯构造带的钻井实测地层压力计算出逆冲断层的上下盘压力系数明显不同,且逆冲断层上盘的流体压力系数在逆冲断层处急剧降低,说明上盘逆冲岩席底部为弱滑脱层,有效分隔了上、下盘流体压力系统。文章依据地震剖面和钻井资料标定获得的霍−玛−吐逆冲岩席变形几何学特征,建立了后缘挤压下逆冲席体变形的简化力学模型,并计算出逆冲席体后缘水平挤压应力与垂直方向的重力比值与逆冲席体几何参数及断层摩擦系数和断坡角之间的数学关系式,分别给出了无流体超压的逆冲席体和含有流体超压滑脱层逆冲席体的数学关系式。为了验证力学模型可靠性,对准南逆冲带中的霍−玛−吐逆冲席体发育特征进行了物理模拟实验研究。结果证实该类型的刚性逆冲席体变形规律前缘逆冲断层发育样式符合地震剖面的构造解释认识,也佐证了简化的力学模型符合实际地质情况。实验结果表明逆冲席体底部大位移是多地段同时发生的小范围位移累积而成大位移,为认识造山带内推覆体或造山带前缘逆冲席体大规模远距离推覆问题提供了理论依据。Abstract:
Objective Understanding the mechanical development of thrust sheets is fundamental, yet challenging, to comprehensively understand the deformation processes of thrust belts. Various models explain the mechanics behind thrust sheet development, yet significant controversies persist. Methods This study takes a comprehensive approach, focusing on the southern Junggar Thrust Belt. We combine a variety of methods including surface geological surveys, seismic reflection profiles, and drilling data analysis with mechanical and physical modeling to thoroughly investigate this issue. Results (1) Based on surface geological surveys, seismic data interpretation, and drilling data, we confirm that thrust faults have developed in the core and southern limb of the Huo-Ma-Tu anticline. These faults extend southward beneath the front of anticlines, forming the extensively distributed Huo-Ma-Tu thrust sheet, which exhibits none to weak internal structural deformation. (2) Drilling data from the Huo-Ma-Tu structural belt clearly show that the frontal thrust faults and detachment faults have developed in layers with abnormally high fluid pressure, indicating that the thrust sheet is a combination of strong deformation sheets and weak detachment faults. Analysis of in-situ formation pressure data suggests that the thrust faults within these overpressure layers can segregate fluid pressure coefficients between the hanging wall and the footwall. (3) Using the geometric deformation characteristics of the Huo-Ma-Tu thrust sheet obtained from seismic profiles and drilling data, a simplified mechanical model is established. This model calculates the mathematical relationship between the horizontal compressive stress-to-gravity ratio at the back of the thrust sheet, the geometric parameters of the thrust sheet, the fault friction coefficient, and the fault dip angle. Separate equations are provided for thrust sheets without fluid overpressure and those with fluid overpressure detachment layers. (4) A physical model of the development characteristics of the Huo-Ma-Tu thrust sheet in the Southern Junggar Thrust Belt confirms that the deformation pattern of such rigid thrust sheets aligns with the structural interpretation from seismic profiles. This supports the validity of the simplified mechanical model in reflecting the actual geological conditions. Conclusions The simplified mechanical model demonstrates that the required horizontal tectonic stress-to-gravity ratio at the back of the thrust sheet decreases significantly with increasing fluid pressure coefficient. The physical modeling results also verify that the deformation pattern of strong deformation sheets/weak detachment fault thrust sheets generally conform to the structural interpretations of the seismic profiles. The experiments reveal that the large displacements at the base of the thrust sheet result from the accumulation of small-scale displacements coinciding across multiple segments. Significance This study proposes simplified rectangular with triangular geometric models of thrust sheets, which can preliminarily explain the kinematics and dynamics of thrust sheets, especially those with fluid overpressure. The derived mathematical relationships accurately describe the geometric, kinematic, and dynamic relationships of thrust sheets and are robustly validated by physical simulation experiments, reinforcing the reliability of our findings. -
图 1 准噶尔盆地南缘逆冲褶皱带地质简图
a—准南逆冲褶皱带地质简图;b—齐古背斜−吐北背斜构造解释剖面
Figure 1. Geological map of the southern Junggar Basin thrust and fold belt
(a) Simplified geological map of the southern Junggar fold and thrust belt; (b) Structural interpretation of the seismic profile across the Qigu-Tugulu anticline based on surface geology and well data
图 2 准南逆冲褶皱带实测地层压力与深度及流体压力系数关系图。准南逆冲带钻井实测压力数据(压力数据来自新疆油田分公司,2007)
Figure 2. Graph showing the relationships between the fluid pressure and the Hubbert-Rubey pore-fluid pressure ratio (λ) as functions of depth. Fluid pressure data of wells from the southern Junggar fold and thrust belt, derived from in situ formation test. ( The data are from Xinjiang Oilfield Company, 2007)
图 3 玛纳斯背斜钻井地层压力与深度及孔隙流体压力系数关系图
a— MN002井MDT实测紫泥泉子组地层压力图; b— MDT实测安集海河组地层压力图(压力数据来自新疆油田分公司,2007)
Figure 3. Graphs showing the relationships between fluid pressure and Hubbert-Rubey pore-fluid pressure ratio (λ) as functions of depth through wells in the Manasi anticline.
(a) Pressures measured by the MDT technology in the MN002 well for the Ziniquanzi Formation (E1-2z); (b) Pressures measured by the MDT technology in the MN1, MN001, and MN002 wells for the Anjihaihe Formation (E1-2a)(Formation pressure data from Xinjiang Oilfield Company, 2007)
表 1 不同断层摩擦系数(μb)和不同流体压力系数下计算的水平挤压应力与重力比值
Table 1. The tectonic stress to gravitational stress ratio varies with different sliding friction coefficients and Hubbert-Rubey pore-fluid pressure ratio (λ)
水平压力与
重力比值摩擦系数 0.40 0.50 0.60 0.70 0.80 0.90 流体压力系数 0.0 2.60 3.31 4.06 4.86 5.72 6.66 0.4 1.68 2.15 2.65 3.19 3.77 4.43 0.5 1.45 1.86 2.30 2.77 3.29 3.87 0.6 1.22 1.57 1.94 2.35 2.80 3.31 0.7 0.99 1.28 1.59 1.93 2.32 2.76 0.8 0.76 0.99 1.24 1.51 1.83 2.20 0.9 0.53 0.70 0.88 1.10 1.34 1.64 -
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