湖相混积层系沉积特征和高频层序定量划分——以饶阳凹陷肃宁-大王庄构造带沙一下亚段为例

Sedimentary Characteristics and Quantitative High-frequency Sequence Division of Lacustrine Mixed Depositional Systems:A Case Study from the Lower Member of the Shahejie Formation in the Suning-Dawangzhuang Structural Unit of the Raoyang Sag

为定量划分湖相混积层系高频层序,以肃宁-大王庄构造带沙一下亚段为例,利用岩心和地震资料分析混积体系的岩石学特征和空间分布,运用频谱分析和滤波等方法识别标准井自然伽马测井曲线蕴藏的米兰科维奇旋回并计算沉积速率,据此进行高频层序划分并讨论地层超压的成因。结果表明:研究区沙一下亚段混积层系岩性以粉砂岩、钙质砂岩、白云质灰岩、油页岩和暗色泥岩为主,混积主体位于肃宁-大王庄构造带的中部,沉积微相包括分流河道、河口坝、席状砂、碳酸盐质滩坝、砂质滩坝和湖泥;研究区沙一下亚段混积层系受控于短偏心率和斜率周期,高频层序划分的最高精度可以达到约12 m,可识别出8个中期旋回和17个短期旋回;沙一下亚段烃源岩内部的异常高压源于细粒沉积物的欠压实作用和生烃增压作用,高沉积速率是欠压实作用的主要成因。可见,高频湖平面变化对混积体系地层划分具有很大影响。

To quantitatively divide high-frequency sequences in lacustrine mixed depositional series,taking the lower member of the Shahejie Formation(SF)in the Suning-Dawangzhuang structural unit as the case,firstly,the core data and seismic volumes were used to analyze petrology and spatial distribution of mixed depositional systems.Then,spectrum analysis and filtering methods were used to recognize Milankovitch cycles contained in GR series of standard wells,based on which the sediment accumulation rate was calculated.At last,high-frequency sequence division was performed and the origin of formation overpressure was discussed.The results show that mixed depositional series of the lower member of the SF in the research area are dominated by siltstone,calcareous sandstone,dolomite limestone,oil shale,and dark mudstone.The main body of the mixed depositional system is situated at the center of the research area.Dominant sedimentary micro-facies are distributary channels,mouth bars,sheet sand,carbonate-rich beach bar,sand-rich beach bar,and lacustrine mud.Mixed depositional series of the lower member of the SF in the research area are controlled by short eccentricity and oblique.The maximal accuracy of high-frequency sequence division reaches 12 m.Eight mid-term cycles and 17 short-term cycles is identified.Formation overpressure within source rocks of the lower member of the SF originates from under-compaction and hydrocarbon generation pressurization of fine-grained sediments.High sediment accumulation rate is the main cause for under-compaction.Hence,high-frequency lake level change has a significant effect on the stratigraphic division of lacustrine mixing depositional system.