鄂西—黔南地区下寒武统页岩旋回地层学研究

Cyclostratigraphic Analysis of the Lower Cambrian Shales in Western Hubei and Southern Guizhou

【目的】鄂西—黔南地区下寒武统富有机质页岩层系是页岩气勘探的重要目标,查明不同地区富有机质页岩层段的等时地层关系及其成因已成为页岩气勘探的关键问题。【方法】通过对鄂西ND1井及黔南HY1井下寒武统测井自然伽马数据的旋回地层学分析,识别出下寒武统主要的天文周期,以文献报道的锆石年龄作为锚点建立了两口井的天文年代标尺,并基于沉积噪音模型恢复了两口井下寒武统时期的相对海平面变化。【结果】ND1井牛蹄塘组36 m和9 m的沉积旋回及HY1井16.6 m和4.0 m的沉积旋回代表了轨道周期405 kyr和100 kyr偏心率;ND1井牛蹄塘组和HY1井九门冲组持续时间分别为3.30 Myr和2.64 Myr。给出两种方案的天文年代标尺,方案一以ND1井牛蹄塘组顶作为锚点起点,对应的天文年代标尺年龄区间处于535.20±1.70 Ma~538.50±1.70 Ma;方案二以HY1井九门冲组顶作为锚点起点,对应的天文年代标尺年龄区间处于535.20±1.70 Ma~537.84±1.70 Ma。经过对比分析,两口井海平面变化对应于1.20 Myr旋回和两个三级层序(SQ1和SQ2)。将下寒武统富有机质页岩总有机碳(Total Organic Carbon,TOC)含量和405 kyr旋回滤波及海平面变化曲线对比,结果显示TOC高值段对应于405 kyr旋回最大值,而ND1井TOC低值段对应于两个405 kyr旋回之间的低值。此外,TOC高值段既对应于海平面较高时期,也对应于海平面较低时期。【结论】由于太阳系的混沌行为,无法准确预测寒武纪早期理论轨道偏心率对富有机质页岩驱动的机制。假设偏心率最大值和总有机碳含量最大值对应,则更长的时间段内强季节性变化在某种程度上触发了黑色页岩的富集。此外,相对海平面变化和总有机碳含量对比表明海平面变化和有机质富集不具有因果关系。

[Objective]The Lower Cambrian organic-rich shale is an important marine shale exploration target in western Hubei and southern Guizhou.Understanding the chronostratigraphic relationships and their underlying causes of organic-rich shale intervals in different areas is crucial for shale gas exploration.[Methods]In this study,we conducted cyclostratigraphic analyses using natural gamma-ray logging data from the Lower Cambrian well ND1 in western Hubei and the well HY1 in southern Guizhou and determined the primary astronomical periods.Using previously reported zircon ages as anchoring points,we constructed an astronomical time scale for the Lower Cambrian.Using a sedimentary noise model,relative sea-level changes in the Lower Cambrian were reconstructed.[Results]Our results reveal that wavelengths of 36 m and 9 m in well ND1 and 16.6 m and 4 m in well HY1 correspond to orbital 405 kyr and 100 kyr cycles,respectively.Furthermore,we calculated the durations of the Niutitang Formaion in well Niedi-1 and the Jiumengchong Formation in well HY1 to be 3.30 and 2.64 Myr,respectively,The astronomical age scales of two options are presented.In Option 1,the top of the Niutitang Formation in well ND1 serves as the starting point for the anchor point,with an age interval of 535.20±1.70 Ma to 538.50±1.70 Ma.Option 2 employs the top of the Jiumengchong Formation in well HY1 as the starting point of the anchor point,with the corresponding age interval spanning from 535.20±1.70 Ma to 537.84±1.70 Ma.Through correlation sea-level change curves(DYNOT andρ1)with eccentricity cycles and sedimentary cycles,our results indicate that sea-level changes correspond to 1.2 Myr obliquity modulation cycles and two sedimentary sequences(SQ1 and SQ2).Comparing the total organic carbon(TOC)content of the Lower Cambrian organic-rich shale with orbital eccentricity,sedimentation rates,and sea-level changes,our findings reveal that the intervals with high TOC value correlate with the maximum eccentricity of 405 kyr,whereas those with low TOC value correspond to the minimum value in well ND1.In addition,the high TOC value corresponds to both the high sea level period and the low sea level period[.Conclusions]Because of the chaotic behavior of the solar system,accurately predicting the mechanism that drives organic-rich shale during the Early Cambrian is impossible.Assuming that the maximum eccentricity corresponds to the maximum organic carbon content at that time,strong seasonal variations have triggered the enrichment of black shale over a more extended period.Furthermore,the comparison between relative sea-level changes and organic carbon content reveals no causal relationship between sea-level fluctuations and the enrichment of organic matter.