基于沉积学原理的被动陆缘陆坡古地貌恢复方法

深入了解被动陆缘陆坡古地貌如何控制深水重力流储层的形成与分布对深水油气勘探意义重大。在少井探区, 基于三维地震的地层厚度印模法在古地貌研究中应用广泛, 但是, 陆坡区容易因沉积物供给不足而处于“半饥饿欠补偿”状态和陆坡地形坡度的存在会影响该方法的适用性和准确性, 对沉积体系和油气储层的分布特征分析产生不利影响。为此, 根据沉积型陆坡的特点, 对地层厚度印模法进行了两方面的补充与改进: 其一, 通过三维地震资料的分析明确目的层陆坡补偿状态, 讨论了地层厚度印模法的适用性; 其二, 根据研究区陆坡演化过程, 以沉积物体积等分方式求取目的层段各层序对应的潜在坡度趋势, 基于此实现古陆坡地形坡度校正。利用改进后的方法对非洲西部尼日尔三角洲盆地下陆坡古地貌进行恢复的结果与研究区现有构造演化认识高度一致, 符合“构造活动主导地貌特征”这一普遍认识。另外, 地震属性刻画出的深水沉积体系分布特征与古地貌恢复结果间存在良好的耦合关系。应用实例证明了改进方法的科学性和研究区古地貌恢复结果的客观性。

Ancient land form of the passive continental marginal slope is important to deep-water petroleum exploration owing to its influence on gravity-flow reservoirs.In an area with a small number of wells, a routine practice for palaeogeomorphologic restoration is using formation thickness impression method based on 3D seismic data, which suffers from two problems: insufficient sediment supply and consequent semi-starved undercompensation caused by long distance from the slope zone to the sedimentary source and slope gradient leading to morphological distortion of geomorphic units.Inaccurate palaeogeomorphologic restoration has a negative impact on the study of sedimentary system and reservoir distribution.To address these issues, formation thickness impression method is ameliorated from two perspectives according to the features of sedimentary continental slope.One is the feasibility of the method in light of the state of slope compensation obtained using 3D seismic data, and the other is ancient slope correction based on the equilibrium surface of each sequence derived from equal division of sediment volume in accordance with the process of slope evolution in the area of interest.In the case study of the lower continental slope in the Niger Delta Basin, western Africa, there is a good consistency between palaeogeomorphologic restoration and structural evolution in accordance with the idea of tectonic activity domination on geomorphic features.Vigorous thrust-fault activities resulted in faulted anticlines and intra-slope basins, which prevented gravity-flow transportation.Sediments were confined to local areas to form huge lobes.Controlled by mud diapirs, narrow intra-slope basins formed on both sides of the diapiric anticline.Channel-lobe complexes were localized within intra-slope basins and ran subparallel to the inclination of the slope.In addition, there is a good correlation between deep-water sedimentary system derived from seismic attributes and the results of palaeogeomorphologic restoration, which indicates the scientific nature of the modified method and the objectivity of the results.