基于米氏旋回的页岩高频层序划分与页岩有机质富集模式—以松辽盆地梨树断陷北斜坡营城组为例

Division of high-frequency shale sequences and organic matter enrichment patterns in shales based on Milankovitch cycles:A case study of the Yingcheng Formation in the Northern Slope of the Lishu Fault Depression,Songliao Basin

松辽盆地梨树断陷营城组页岩气具有巨大的勘探开发前景。与此同时,针对页岩层段的高频层序划分逐渐成为研究热点。但传统层序地层学难以针对页岩层系划分层序,页岩层系的有机质富集模式认识不清,对页岩油气的勘探开发造成阻碍。利用旋回地层学方法可在页岩层系中划分高频层序,还可基于米氏旋回探究页岩有机质的富集模式,是预测甜点层的有利手段。通过对研究区营城组的自然伽马数据进行天文旋回分析,得出以下结论:(1)SN65井营城组较好保存了米氏旋回信号,并以长、短偏心率作为四级、五级层序的参考曲线,划分了14个四级层序与51个五级层序,建立了研究区高精度等时地层格架。(2)梨树断陷北斜坡营城组页岩有机质富集受控于天文旋回,且二者互为反相位,即长偏心率高值时期,气候暖湿,对应总有机碳(TOC)低值;长偏心率低值,气候干冷,对应TOC高值。(3)建立暖湿与干冷两种页岩沉积模式。通过引入温跃层溶解氧最小值段(MOM)的概念,证明干冷环境有利于页岩有机质富集。(4)以长偏心率旋回曲线的半幅点为界,划分4个富有机质页岩层段和3个含有机质页岩层段。优质页岩层段发育于各个层序的顶部和底部,而不是最大湖泛面。浅水环境对富有机质页岩发育有利。研究成果不仅为松辽盆地梨树断陷页岩气甜点预测提供理论支撑,也为其他湖相盆地的有机质富集模式提供参考依据。

The Early Cretaceous Yingcheng Formation shale gas in the Lishu Fault Depression of the Songliao Basin presents immense prospects for exploration and development.Concurrently,the division of high-frequency sequences within shale intervals has gradually become a research focus.However,traditional sequence stratigraphy faces challenges in the precise delineation of sequences within the shale series,while the understanding of organic matter enrichment patterns within the shale series remains unclear,hindering the exploration and development of shale oil and gas.The utilization of cyclostratigraphy meth-ods allows for the subdivision of high-frequency sequences within the shale series,and further,it provides a beneficial means to explore the enrichment patterns of organic matter in shale based on Milankovitch cycles,aiding in the prediction of"sweet spots"within the layers.Through astronomical cycle analysis of natural gamma-ray data from the research area in the Yingcheng Formation,the following conclusions have been drawn:(1)Well SN65 effectively preserves the signals of Milankovitch cycles within the Yingcheng Formation.Long and short eccentricities serve as reference curves for the fourth and fifth-order sequences,respectively,delineating 14 fourth-order sequences and 51 fifth-order sequences,thus establishing a high-precision isochronous stratigraphic framework for the study area.(2)The organic matter enrichment of the shale within the Yingcheng Formation on the northern slope of the Lishu Fault Depression is controlled by astronomical cycles,exhibiting an antiphase relationship.During periods of high long eccentricity,the climate was warm and humid,corresponding to low Total Organic Carbon(TOC)values;conversely,during periods of low long eccentricity,the climate was cold and dry,corresponding to high TOC values.(3)Two shale sedimentation models have been established:warm-humid and cold-dry.Through the introduction of the concept of the metalimnetic oxygen minimum(MOM),it has been demonstrated that cold-dry environments are conducive to the enrichment of organic matter in shale.(4)The data were divided into 4 organic-rich shale intervals and 3 organic matter-bearing shale intervals based on the half-amplitude points of the long eccentricity cycle curve.High-quality shale intervals develop at the tops and bottoms of various sequences rather than at the maximum flooding surface.Shallow water environments are conducive to the development of organic-rich shale.These research findings not only provide theoretical support for predicting"sweet spots"of shale gas within the Yingcheng Formation in the Lishu Fault Depression of the Songliao Basin but also serve as a reference for the organic matter enrichment patterns in other lacustrine basins,holding significant implications for the evaluation of shale oil and gas exploration selection.