Forward and Inverse Modeling of Anomalous Post-Rifting Subsidence in Extensional Sedimentary Basins
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摘要: 世界上许多张性沉积盆地存在远大于McKenzie模型理论值的裂后异常沉降, 南海北部陆缘的沉积盆地也是如此; 确定裂后异常沉降的特征和分布是认识其成因机制和对油气成藏影响的前提.介绍了估算裂后异常沉降的3种方法: 古水深比较法、应变速率反演法和沉降过程二维正反演法, 并指出了各方法的应用前提和优缺点.对于由作者提出的后一种方法还结合在珠江口盆地的应用实例进行了较详细的讨论, 表明这种方法能在考虑岩石圈挠曲强度的基础上正演模拟出单幕或多幕盆地沉降及相应的岩石圈伸展系数, 从而计算出盆地理论热沉降, 与通过回剥反演得到的实测构造沉降进行对比; 还指出了该方法存在的问题和需进一步研究之处.Abstract: Anomalous post-rifting subsidence, i.e. the post-rifting subsidence larger than the thermal subsidence predicted by the McKenzie model, observed in many extensional sedimentary basins including those in the northern South China Sea. The characterization of anomalous post-rifting subsidence is the first step in understanding its origin and mechanism and its impact on the formation of oil/gas reservoirs. This paper introduces three methods for characterizing anomalous post-rifting subsidence, namely, the paleo-water depth comparison, the strain-rate inversion, and the 2D forward-backward modeling. The conditions, advantages and disadvantages of these methods are discussed. The application of the third method, which is proposed by the authors, in the Pearl River Mouth basin of the northern South China Sea is presented. Taking into account the lithospheric flexural rigidity, the method calculates the lithosphere stretch factor and theoretical thermal subsidence through 2D forward modeling, and then compares the theoretic subsidence with the observed one given by 2D backstripping. Necessary improvements of the method are also discussed.
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图 1 判识裂后异常沉降的二维正反演流程
Fig. 1. Flow chart of 2D forward-inversed modeling for anomalous post-rifting thermal subsidence
图 2 珠江口盆地简图(示1530剖面位置)
Fig. 2. Simplified map of the Pearl River Mouth basin showing the location of Line 1530
图 7 1530剖面北段裂后理论热沉降后的盆地理论形态与现今实测盆地形态的对比
Fig. 7. Comparison of the modeled basin shape after 30-0 Ma post-rift thermal subsidence and the observed basin shape for northern Line 1530
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