Age and origin of Sinian grape-lace shaped dolostone in central Sichuan Basin
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(Σ REE)of grape-lace shaped dolostone is relatively low(0.680 × 10-6～8.963 × 10-6).Its δ13 CVPDB value(-0.1‰～1.7‰)is slightly lower than that of the dolostones depositing in Sinian seawater,but the δ18 OVPDB value(-2.3‰～1.4‰)is slightly higher than that of the contemporaneous dolostones.Its 87Sr/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.
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