基于自然伽马能谱测井资料的碳酸盐岩层序地层划分及其对古环境恢复的指示意义——以川东北LT1井下寒武统龙王庙组为例

Sequence stratigraphy interpretation and implications for paleoenvironment reconstruction based on gamma-ray spectrum logging data of carbonates——A case study from the Lower Cambrian Longwangmiao Formation of the Well LT1 in northeast Sichuan Basin

自然伽马能谱测井资料在碳酸盐岩层序地层研究中具有较好的应用效果,Th/U和Th/K曲线的振荡趋势可为层序地层的旋回性识别提供依据。笔者等选取东北地区LT1典型井的自然伽马能谱测井资料为对象进行层序地层分析,结合岩芯碳氧同位素资料,探讨了沉积期古环境和相对海平面升降之间的耦合关系。结果表明,通过对自然伽马能谱测井资料进行INPEFA分析和Demy小波变换分析,可将LT1井下寒武统龙王庙组划分为1个三级层序和6个四级层序。INPEFA处理得到的Th/U_1曲线和Demy小波变换得到的Th/U_3_d8曲线可指示LT1井龙王庙组三级层序的相对海平面变化,Th/U_3_d6曲线与四级层序的相对海平面变化曲线相当。LT1井龙王庙组δ^(13)C变化受三级层序相对海平面变化控制明显,最大海泛面位置对应ROECE碳同位素负漂移事件开始位置。结合龙王庙组δ^(13)C变化和基于自然伽马能谱测井的相对海平面变化分析,认为ROECE事件可能与深部缺氧海水上涌和河流的持续输入以及风暴沉积带入的陆源碎屑物质有关。整体而言,LT1井龙王庙组沉积期古海水温度和古盐度的振荡变化趋势与三级和四级层序的相对海平面变化曲线(Th/U_3_d8和Th/U_3_d6)形态具有一定相关性,说明基于自然伽马能谱测井资料开展碳酸盐岩层序地层划分具有可行性,对沉积古环境恢复具有一定的指导和借鉴意义。

Objectives:Based on the multiple analyses of natural gamma-ray spectrum logging( NGRSL) data,this work aims to conduct a comprehensive sequence stratigraphic interpretation of the Lower Cambrian Longwangmiao Formation in the Well LT1 in northeast Sichuan Basin.With the guidance of the interpreted stratigraphic framework,the sedimentary paleoenvironment of the Longwangmiao period is also investigated.Methods:The NGRSL data has been well applied in interpreting carbonate sequence stratigraphy,and the oscillation trend of Th/U and Th/K curves can provide a basis for the cycle identification of sequence stratigraphy.In this paper,the NGRSL data of the Well LT1 in northeast Sichuan Basin is selected for sequence stratigraphic analysis.Combined with the study of carbon and oxygen isotopes,the corresponding relationship between sedimentary paleoenvironment and relative sea-level oscillation is discussed.Results and Conclusions:The results show that the Lower Cambrian Longwangmiao Formation in the Well LT1 can be divided into one third-order sequence and six fourth-order sequences according to the NGRSL data processed by INPEFA analysis and Demy wavelet transform.The Th/U_1 curve obtained by INPEFA and the Th/U_3_d8 curve obtained by Demy wavelet transform can be regarded as the relative sea-level oscillation curve of the third-order sequence of the Longwangmiao Formation in the Well LT1,whereas the Th/U _ 3 _ d6 curve and the relative sea-level oscillation of the fourth-order sequence are quite similar.The change of δ^(13)C of the Longwangmiao Formation in the Well LT1 is obviously controlled by the relative sea level change of the third-order sequence,and the position of the maximum flooding surface corresponds to the starting position of the negative ROECE carbon isotope shift event.Combined with the change of δ^(13)C and relative sea-level change,it is believed that the ROECE event may be related to the upwelling of deep anoxic seawater and the continuous input of rivers and terrigenous debris brought in by storm deposits.The obvious abrupt change in paleo-seawater temperature at the maximum of the Th/U _ 3 _ d8 curve indicates that the ROECE event may be related to the organism extinction triggered by temperature change.Overall,the oscillation trends of paleo-seawater temperature and paleo-salinity,during the deposition of the Longwangmiao Formation in the Well LT1,have a certain correlation with the relative sea-level variation curves of the third-and fourth-order sequences( Th/U_3_d8 and Th/U_3_d6).This indicates that it is feasible to conduct sequence stratigraphic division of carbonate rocks based on NGRSL data.In return,this has certain guiding and reference significance for the reconstruction of sedimentary paleoenvironment.