四川盆地及邻区中上寒武统洗象池群碳同位素特征与芙蓉统底界标志

四川盆地中上寒武统洗象池群为一套以台地相为主的白云岩,过去一直未能找到有效的方法来确定上寒武统的底界标志,导致对该层位的天然气勘探工作较为滞后。以岩屑碳氧同位素剖面的建立为依托,发现洗象池群中部存在着碳同位素正漂移事件且这一现象可全球比对,可作为识别芙蓉统(相当于上寒武统)的重要标志。进而结合电性曲线特征,确定了芙蓉统底界的划分标志,初步找到了解决芙蓉统划分难题的方案。研究结果表明:(1)洗象池群沉积时该区古地貌为西高东低的缓坡碳酸盐岩台地,沉积了一套西薄东厚的薄层—中厚层白云岩地层,纵向上可分4个层段;(2)四川盆地及邻区芙蓉统底界划分标志可以由碳同位素SPICE(Steptoean Positive Carbon Isotope Excursion)事件与电性曲线特征结合来确定;(3)芙蓉统底界的碳同位素标志是区域可对比的正漂移所对应的SPICE事件,电性标志是SPICE事件对应的洗象池群中部自然伽马值低峰,而岩性标志则是该自然伽马值低峰所对应的泥质云岩。该研究成果有助于对该盆地及其邻区洗象池群的进一步细分并推进其沉积演化与储层分布规律研究,以加快对该层系的勘探步伐。

Carbon isotope characteristics of the Middle-Upper Cambrian Xixiangchi Group and bottom boundary marks of Furong series in the Sichuan basin and its adjacent areas

The Middle–Upper Cambrian Xixiangchi Group in the Sichuan Basin is mainly a set of dolomites of platform facies. In the past, there was no effective way to determine the bottom boundary mark of the Upper Cambrian strata, so natural gas exploration of this horizon lagged behind. In this paper, it is revealed from the C and O isotopic profile of debris that in the middle of Xixiangchi Group, there is a positive drift event of carbon isotopes that can be correlated all over the world, so it can be used as an important mark to identify the Furong Series (equivalent to the Upper Cambrian strata). Then, based on the characteristics of electric curves, the Furong Series boundary mark was determined and its classification program was worked out preliminarily. It is shown that during the deposition of the Xixiangchi Group, it is a paleo-geomorphologically gentle-slope carbonate platform which is high in the east and low in the east, and a thin–moderate dolomite layer which is thin in the west and thick in the east is deposited. This layer is vertically divided into 4 intervals. Besides, the bottom boundary mark of the Furong Series in the Sichuan Basin and its adjacent areas can be determined by combining the SPICE (Steptoean Positive Carbon Isotope Excursion) event of carbon isotopes with the characteristics of electric curves. Finally, the carbon isotope mark of the Furong Series bottom boundary is the SPICE event corresponding to the positive drift which is regionally correlated, the corresponding electrical mark is the GR small peak areas in the middle of the Xixiangchi group, and the corresponding lithologic symbol is the argillaceous dolomite. The research results are conducive to the further subdivision of the Xixiangchi Group in the Sichuan Basin and its adjacent areas and promoting the study on sedimentary evolution and reservoir distribution laws so that gas exploration in this series of strata can be speeded up.