天文旋回约束下东营凹陷中始新统含碳酸盐细粒沉积岩成因分析

Astronomical Forcing of Origins of Eocene Carbonate-bearing Finegrained Sedimentary Rock in Dongying Sag

【目的】通过对中国东部渤海湾盆地东营凹陷发育的大套具有韵律特征的富含碳酸盐物质和有机质的湖相细粒沉积岩进行深入研究,以期探究其成因机制以及古气候、古环境的变化特征。【方法】基于岩心和薄片观察,依据沉积构造特征和矿物组分含量进行岩相划分。结合沉积环境和沉积速率的差异,将目标层段划分为四个单元,采取Multi-Taper Method(MTM)方法对樊页1井(FY1)沙四上(Esu4)到沙三下(Esl3)亚段的自然伽马(GR)测井曲线进行分段频谱分析。【结果】通过宏观和微观沉积观察,将东营凹陷地区发育的细粒岩划分为纹层状泥质灰岩、纹层状灰质泥岩、弱纹层状灰质泥岩、透镜状泥质灰岩和块状泥岩五种岩相。频谱分析结果显示,四个单元均记录了米兰科维奇旋回信号,包括125 kyr、38.7 kyr和18.7 kyr的周期。基于火山灰测年数据,我们建立了精度为38.7 kyr的“浮动”天文年代标尺,并确定东营凹陷沙四上到沙三下细粒岩沉积的总持续时间为5.3 Myr。进一步分析发现,18.7 kyr岁差旋回与碳酸盐岩含量变化之间存在良好的对应关系,表明地球轨道参数对含碳酸盐细粒沉积岩的沉积作用具有显著影响。【结论】本研究利用旋回地层学理论,从天文周期控制沉积作用的角度,揭示了湖泊沉积的古气候、古环境变化特征。通过分析,我们发现地球轨道参数(特别是18.7 kyr岁差旋回和125 kyr偏心率旋回)共同驱动了气候的变化,进而控制了含碳酸盐细粒沉积岩的沉积作用。具体表现为,在岁差极大值点或偏心率极小值处,日照量增多,湖盆水体温度升高,有利于方解石结晶和灰岩沉积;而在岁差极小值或偏心率极大值处,日照量减少,湖盆水体温度降低,细粒物质主要来自陆源输入,以泥岩沉积为主。这一研究成果对于非常规油气勘探开发具有重要的科学和指导意义。

[Objective]This study aims to conduct an in-depth investigation into the large set of rhythmicallycharacteristic lacustrine fine-grained sedimentary rocks rich in carbonate materials and organic matter developed in the Dongying Sag area of the Bohai Bay Basin in eastern China,exploring their genetic mechanisms as well as characteristics of paleoclimate and paleoenvironment changes.[Methods]Based on core and thin-section observations,we classified the lithofacies according to sedimentary structural features and mineral component content.Considering the differences in sedimentary environment and deposition rate,the target interval was divided into four units.The Multi-Taper Method(MTM)was applied to perform segmented spectral analysis on the natural gamma(GR)logging curve from the upper Fourth member of the Shahejie Formation(Es_(4)^(u))to the lower Third member(Es_(3)^(l))in well Fanye 1(FY1).[Results]Through macroscopic and microscopic sedimentary observations,we identified five types of fine-grained rocks in the Dongying Sag area:laminar argillaceous limestone,laminar calcareous mudstone,weakly laminar calcareous mudstone,lenticular argillaceous limestone and massive mudstone.Spectral analysis results revealed that all four units recorded Milankovitch cycles,including periods of 125 kyr,38.7 kyr,and 18.7 kyr.Based on volcanic ash dating data,we established a"floating"astronomical timescale with a precision of 38.7 kyr and determined that the total duration of fine-grained sedimentary deposition from the upper Es4 to the lower Es3 in the Dongying Sag was 5.3 Myr.Further analysis indicated a good correspondence between the 18.7 kyr precession cycle and changes in carbonate content,suggesting a significant influence of Earth's orbital parameters on the deposition of carbonate-bearing fine-grained sedimentary rocks.[Conclusions]Utilizing the theory of cyclostratigraphy,this study revealed characteristics of paleoclimate and paleoenvironment changes in lacustrine sediments from the perspective of astronomical cycles controlling sedimentation.Our analysis showed that Earth's orbital parameters,particularly the 18.7 kyr precession cycle and the 125 kyr eccentricity cycle,jointly drove climate changes,which in turn controlled the deposition of carbonate-bearing fine-grained sedimentary rocks.Specifically,at maxima of precession or minima of eccentricity,increased sunlight and warmer lake water temperatures favored the crystallization of calcite and the deposition of limestone.Conversely,at minima of precession or maxima of eccentricity,decreased sunlight and cooler lake water temperatures resulted in fine-grained material being predominantly derived from terrestrial sources,with mudstone deposition dominating.This research has important scientific and guiding implications for unconventional oil and gas exploration and development.