曲流河三角洲—辫状河三角洲控制因素及演化过程探讨

鄂尔多斯盆地是我国最大含油气之一,随勘探不断深入,盆地东南部山2段-盒8段展现出巨大的潜力。早期研究主要针对单一层段储集砂体发育特征,而多层段整体沉积演化未进行深入探讨。在综合岩芯特征、测井相标志以及分析化验资料识别三角洲相标志的基础上,通过基准面变化、物源供给条件、古坡度恢复及古气候分析,明确了曲流河三角洲-辫状河三角洲演化的控制因素,再现了其沉积转换过程。研究表明,鄂尔多斯盆地东南部山2段-盒8段沉积过程中,基准面逐步下降导致河流沉积作用逐渐增强;SN向与EW向物源供给逐渐增加;利用"压实模拟计算地层古厚度"的方法恢复的古坡度逐渐变陡;通过岩芯相与地球化学标志恢复的古气温呈现逐渐降低的趋势。在这些因素控制下,研究区由山2段曲流河三角洲逐步向盒8段辫状河三角洲过渡。重建的沉积演化过程对区域内油气勘探及不同类型三角洲时空转换均具有一定的意义。

Control Factors and Evolution Progress of Depositional System Transition from Meandering River Delta to Braided River Delta: Case study of Shan2 member to He8 member,Ordos Basin

The Ordos Basin is one of the largest petroliferous basins in China. Exploration has shown that the Shan2 to He8 members in the SE part of the basin has considerable exploration potential. Previous studies have focused mainly on the sand body distribution within a single member,but have not discussed the overall sedimentary evolu-tion for a number of members. To infer the transition from meandering river delta to braided river delta and analyze the main controlling factors of the evolutionary process,and to reconstruct the true evolution,in this study facies log-ging from core samples was integrated with analytical data to recognize the delta sedimentary facies. Changes of the base-level elevation enabled the sediment supply to be recognized,and to reconstruct the paleo-geomorphology and paleoclimate in the study area,and the evolution of a meandering river delta to a braided river delta. The elevation of the base-level surface sloped downwards between Shan2 and He8,enhancing channel sedimentation and increasing the supply of sediment. It was then inferred that the gradation of paleo-thickness due to compaction gradually in-creased the ancient slope. Analysis of lithofacies together with geochemical characterization indicated that the ancient air temperature gradually decreased. A comprehensive analysis of these four factors has provided a theoretical basis for the study of the evolution of meandering river deltas to braided river deltas. Such reconstruction of the sedimentary evolution process is significant for hydrocarbon exploration. In addition,the study provides a theoretical basis for the spatiotemporal evolution of various types of delta.