川中地区震旦系“葡萄花边”白云岩的形成时代与成因

Age and origin of Sinian grape-lace shaped dolostone in central Sichuan Basin

四川盆地震旦系灯影组是重要的油气勘探开发层系,复杂的成岩演化和多期流体活动导致多种类型的白云岩形成。揭示震旦系白云岩的形成时代和成因不仅可以为四川盆地震旦系下一步的油气勘探提供更加充实的科学依据,也可为理解全球大规模存在前寒武纪白云岩的原因提供有益启示。本文针对川中地区震旦系灯影组“葡萄花边”白云岩的形成时代和成因进行了综合研究。岩相学研究表明,“葡萄花边”白云岩充填于藻白云岩的孔隙内,向内对称生长,结晶程度明显好于藻白云岩。在岩相学的基础上,利用激光剥蚀-电感耦合等离子体质谱(LA-ICP-MS)微区原位U-Pb定年确定其形成时代,研究结果表明:川中地区震旦系灯影组“葡萄花边”白云岩形成时代为530±12Ma~502±12Ma,略新于灯影组地层年代。结合微量元素分析以及碳-氧-锶同位素数据,对川中地区震旦系灯影组“葡萄花边”白云岩进行成因分析。结果表明,其稀土元素总量(∑REE)相对较低(0.680×10^(-6)~8.963×10^(-6)),δ13 C VPDB(-0.1‰~1.7‰)略低于与同时期海水沉积白云岩的碳同位素值,δ18 O VPDB(-2.3‰~1.4‰)略高于与同时期海水达到氧同位素平衡的白云岩氧同位素值,87 Sr/86 Sr值(0.70876~0.70909)基本落入同时期海水范围。稀土元素配分总体为水平的模式,所有样品表现出Ce负异常。表明其形成环境既非海水沉积环境,也不完全是表生大气水岩溶条件。综合上述特征,推断川中地区震旦系灯影组“葡萄花边”白云岩是在震旦纪末-寒武纪早期海平面高频低幅度波动这一背景下,灯影组地层多次暴露与淹没,形成岩溶裂缝和孔洞的同时在溶蚀空间内结晶形成。残留孔洞为其后流体活动、油气充注提供了储集空间。本研究为认识川中地区震旦系灯影组“葡萄花边”白云岩成因、震旦纪末-寒武纪早期水化学条件、及超深层油气勘探开发提供了有益参考。

Sinian Dengying Formation is one of the most important oil and gas exploration and development targets in Sichuan Basin.Complex diagenetic evolution and multi-stage fluid activities lead to the formation of various types of dolostones in this formation.Revealing the age and origin of the Sinian dolostones can not only provide a more substantial scientific basis for the next Sinian oil and gas exploration in the Sichuan Basin,but also provide useful enlightenment for understanding the cause of the large-scale existence of Precambrian dolostones in the world.This study focuses on the grape-lace shaped dolostone of Sinian Dengying Formation in central Sichuan Basin to disclose its formation time and genetic environment.The petrographic study shows that the grape-lace dolostones fill in the pores of algal dolostone and grow symmetrically,whose degree of crystallization is obviously better than that of algal dolostone.On the basis of petrography,the age of the grape-lace dolostones were determined by in situ laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)U-Pb dates.The results show that the age of grape-lace dolostone is 530±12Ma~502±12Ma,which is slightly younger than that of Dengying Formation.Trace element analyses and carbon-oxygen-strontium isotope data were used to place constraints on its origins.The total amount of rare earth elements(∑REE)of grape-lace shaped dolostone is relatively low(0.680×10^(-6)~8.963×10^(-6)).Itsδ13 C VPDB value(-0.1‰~1.7‰)is slightly lower than that of the dolostones depositing in Sinian seawater,but theδ18 O VPDB value(-2.3‰~1.4‰)is slightly higher than that of the contemporaneous dolostones.Its 87 Sr/86 Sr values(0.70876~0.70909)basically fall into the range of seawater in the same period.The distribution diagram of rare earth elements shows an overall horizontal pattern,with all samples showing negative Ce anomaly.It suggests that the formation environment of the grape-lace dolostone is neither seawater sedimentary environment nor supergene atmospheric water karst condition.Based on the above characteristics,it is concluded that the grape-lace shaped dolostone of Sinian Dengying Formation in central Sichuan was exposed and submerged many times under the background of high frequency and low amplitude fluctuation of sea level from the end of Sinian to the Early Cambrian,which formed karst fractures and pores accompanied by crystallization in the dissolution space.The residual pores provide reservoir space for subsequent fluid movement and oil and gas charging.This research provides useful information for understanding the origin of"grape-lace"dolostones in Dengying Formation of Sinian in central Sichuan,the chemical conditions of the seawater from the end of Sinian to the Early Cambrian and the petroleum exploration and development in ultra-deep formation.