Dividing carbonate sequence with natural gamma data is the popular method, while it is not suitable for shallow platform because high gamma of mud flat represent shallow-water environment. Taking Majiagou Formation of Ordovician in Bozhong 21–22 structure of Bohai Sea as an example, a new sequence division method for shallow platform was presented based on natural gamma spectrum data, and the relationship between the sequences and reservoirs was discussed as well in this paper. The high natural gammas in Majiagou Formation of the study area are response for the weathering, uranium element enrichment in reservoirs and argillaceous sediment. The comparison result between well logging parameters and geological data shows that the ratio of Th/K is an effective parameter for sequence division, which can distinguish the origin of different high gammas effectively, even the mudstone from oxidized environment and reductive environment. Based on the ratio of Th/K, five third-order sequences were divided in Majiagou Formation of the study area; among them, all the sequences are developed in well BZ21-B-1, while SQ5 is missing in well BZ22-A-2 for denudation. The corresponding relationship between sequences and reservoirs indicates that the reservoirs of SQ4–SQ5 are dominated by structural fractures, which are mainly controlled by tectonic movement and weathering; while those of SQ1–SQ3 are dominated by dissolved pores, which may be related to the internal dissolution.

中文翻译：

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浅层台地层序划分新方法对储层分布的启示——以渤海湾盆地渤中21-22构造马家沟组为例

用天然伽马数据划分碳酸盐岩层序是流行的方法，但不适合浅台地，因为泥滩的伽马高代表浅水环境。以渤海渤海渤中21-22构造奥陶系马家沟组为例，提出了一种基于天然伽马能谱资料的浅台层序划分新方法，并探讨了层序与储层的关系. 研究区马家沟组的高天然伽马是对风化作用、储层铀元素富集和泥质沉积物的响应。测井参数与地质资料对比结果表明，Th/K比值是层序划分的有效参数，可以有效区分不同高伽马的来源，甚至是氧化环境和还原环境中的泥岩。根据Th/K比，将研究区马家沟组划分为5个三级层序；其中，BZ21-B-1井全部层序发育，BZ22-A-2井SQ5缺损剥蚀。层序与储层的对应关系表明，SQ4～SQ5储层以构造裂缝为主，主要受构造运动和风化作用控制；而SQ1-SQ3以溶孔为主，可能与内部溶蚀有关。研究区马家沟组划分出5个三级层序；其中，BZ21-B-1井全部层序发育，BZ22-A-2井SQ5缺损剥蚀。层序与储层的对应关系表明，SQ4～SQ5储层以构造裂缝为主，主要受构造运动和风化作用控制；而SQ1-SQ3以溶孔为主，可能与内部溶蚀有关。研究区马家沟组划分出5个三级层序；其中，BZ21-B-1井全部层序发育，BZ22-A-2井SQ5缺损剥蚀。层序与储层的对应关系表明，SQ4～SQ5储层以构造裂缝为主，主要受构造运动和风化作用控制；而SQ1-SQ3以溶孔为主，可能与内部溶蚀有关。