The central-eastern Ordos Basin develops evaporate and carbonate cyclic strata with a thickness of 500 ～900m.The study on lithology and sedimentary facies of the fifth member (Ma5) of Lower Ordovician Majiagou Formation,which has a thickness of 200 ～ 350m,shows that it has secondary depositional cycle.The discussion on sedimentology and lithofacies paleogeography of its subdivision intervals not only makes the understanding of the variation in lithologic facies and the sedimentary evolution much clear,but also plays an important role in the Ma5 gas exploration.The key results are got as follows:(1) the prediction for favorable sedimentary facies zone (especially of the sedimentary facies for gas reservoir) is much precise and the lithofacies paleogeography mapping for submembers of the main play is more purpose-direction;(2) the mechanism for dolomitization is further explored,and dolomitization in the short-period transgressive sequence is influenced by the inherited paleogeography pattern and sedimentary sequence evolution;(3)the establishment of a large-scale lithologic play model for the Ordovocian middle assemblage (consisting of the Ma5_5 to MaS_10 submembers) in the eastern slope of the central paleo-uplift and the discovery of the gas province with a reserve of trillions cubic meters;and (4) the proving of the lateral hydrocarbon charging model for Ordovician sub-salt reservoirs and its important breakthrough in the recently exploration.The lithofacies paleogeography mapping for sub-members thus holds an important advance in paleogeography and it plays an important role not only in the study on facies zone distribution,sequence evolution and its dolomitization,but also in the prediction of favorable reservoirs and understanding of trap and pool-formation.%鄂尔多斯盆地中东部奥陶系发育巨厚的蒸发岩-碳酸盐岩旋回性沉积,地层厚度达500～ 900m.对其上部厚约200～350m的马家沟组第五段岩性、沉积相的进一步分析表明,其内部仍发育次一级的沉积旋回.而通过细分小层的沉积学研究和岩相古地理编图,对马五段的岩性相变规律及沉积演化特征的认识则更为明晰,对该区奥陶系的沉积学研究与天然气勘探部署也发挥了重要作用,主要体现在以下几方面:一是对于有利沉积相带(尤其是相控储层)的预测更为精准,细分小层的岩相古地理编图对主力目的层的研究分析更具针对性;二是对白云岩化机理的认识进一步深入,基本明确了短期海侵层序中的白云岩化受继承性古地理格局与层序演化的共同控制;三是推动古隆起东侧奥陶系中组合(马五5-马五10)岩性圈闭大区带成藏认识的形成,勘探发现了千亿方规模储量接替区;四是催生奥陶系盐下“侧向供烃成藏”模式的建立,指导近期盆地中部奥陶系盐下勘探取得重大突破.因此可以初步认为,细分小层的岩相古地理编图可能代表了古地理学发展的一个重要方向,无论是对相带展布、层序演化及白云岩化等沉积学方面的研究,还是对油气勘探工作中的有利储集体预测及圈闭成藏规律的认识,都具有一定的实际意义.
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