轨道周期约束下海-陆过渡相页岩层系高精度层序界面识别及其地质意义——以鄂尔多斯盆地东缘二叠系山西组2~3亚段为例

Orbital forced high-resolution sequence boundary identification of marine-continental transitional shale and its geological significance:A case in Shan 2~3 sub-member at the eastern margin of Ordos Basin

海-陆过渡相页岩气具有良好的勘探前景,但其产层多、单层厚度薄且岩相变化快等特点导致高精度层序地层格架构建难度大,而旋回地层学理论为高频旋回划分和对比提供了有效的手段。基于岩心分析测试及测井资料,结合高分辨率层序地层学及旋回地层学理论,厘定了鄂尔多斯盆地东缘二叠系山西组2^(3)亚段海-陆过渡相页岩层系高精度层序界面。运用山2^(3)亚段GR值、Th/U元素含量比值及总有机碳含量(TOC)数据序列,滤波输出记录在沉积物中的天文轨道参数,通过对山2^(3)亚段岩性、测井数据及地化元素分析数据进行划分,识别了四级层序界面,405 kyr长偏心率周期与四级层序耦合关系良好,山2^(3)亚段共划分为4个四级层序(PSQ1-4)。依据短偏心率与五级层序的对应关系,山2^(3)亚段共划分为12个五级层序(FSQ1—12)。在此基础之上,分析了轨道周期与高分辨率层序、海平面升降、沉积环境演化及岩相组合规律的耦合关系,长偏心率周期通过调节0.4 Myr尺度海平面变化控制沉积相演化,影响优势页岩储层段的发育;短偏心率周期通过调节0.1 Myr尺度海平面变化控制沉积亚相演化,影响页岩甜点层的发育。基于旋回地层学理论提出的页岩层系高精度地层划分与对比技术,能为精准识别优势页岩层段及水平井导向设计提供理论借鉴及技术支撑。

The marine-continental transitional shale is of good exploration prospect,but it is difficult to construct a highresolution sequence stratigraphic framework due to its multiple pay zones,and thin single-layer thickness with rapid lithofacies change.The theory of cyclostratigraphy provides an effective means for the division and correlation of highfrequency cycles.The study determines the high-resolution sequence boundary of marine-continental transitional shale in the 2^(3) sub-member of Shanxi Formation(Shan 2^(3) sub-member) at the eastern margin of Ordos Basin,based on core analysis and logging data,combined with the high-resolution sequence stratigraphy and cyclostratigraphy.The GR,Th/U and TOC sequences of the Shan 2^(3) sub-member are applied to obtain the astronomical parameters recorded in the sediments by filtering.A comprehensive analysis of the sub-member in terms of lithology,logging and geochemical elements serves to identify fourth-order sequence boundaries.The 405 kyr long eccentricity cycle has a good coupling relationship with the fourth-order sequences,and the sub-member can be divided into four fourth-order sequences(PSQ1—4).According to the relationship between the short eccentricity and the fifth-order sequence,the sub-member can be sub-divided into 12 fifth-order sequences(FSQ1—12).On this basis,we analyze the coupling relationship of orbital cycle with high-resolution sequence,sea level fluctuation,sedimentary environment evolution and lithofacies association.The long eccentricity cycle controls the evolution of sedimentary environment(facies) by adjusting 0.4 Myr scale sea level change,and affects the development of dominant shale reservoirs;while the short eccentricity cycle controls the evolution of sedimentary environment(sub-facies) by adjusting the sea level change of 0.1 Myr scale,and affects the development of shale reservoir sweet spots.In all,the high-resolution sequence division and correlation technology of shale strata as proposed in cyclostratigraphy,can be of theoretical reference and technical support to precisely identifying dominant shale and geo-steering design of horizontal wells.