激光原位U-Pb同位素定年技术及其在碳酸盐岩成岩-孔隙演化中的应用

Laser ablation in situ U-Pb dating and its application to diagenesis-porosity evolution of carbonate reservoirs

为解决溶液法难以实现的古老海相碳酸盐岩取样和超低U、Pb含量样品测年的难题,通过激光剥蚀进样系统与多接收电感耦合等离子体质谱仪的连用和对年龄为209.8 Ma实验室工作标样的开发和标定,建立了适用于古老海相碳酸盐岩的激光原位U-Pb同位素定年技术,并应用于四川盆地震旦系灯影组成岩-孔隙演化研究。通过充填孔洞、孔隙和裂缝中不同期次白云石胶结物的测年,指出灯影组白云岩储集层的埋藏成岩过程主要是原生孔隙和表生溶蚀孔洞逐渐被充填的过程。孔洞的充填作用发生在早加里东、晚海西—印支、燕山—喜马拉雅期3个阶段,孔隙的充填作用主要发生于早加里东期,未被胶结物充填的残留孔洞、孔隙和裂缝构成了主要储集空间,明确了四川盆地灯影组白云岩储集层的成岩-孔隙演化史。这些认识与该地区的构造-埋藏史、盆地热史具有很高的吻合度,说明测年数据的可靠性和激光原位U-Pb同位素定年技术的有效性,为古老海相碳酸盐岩成岩-孔隙演化研究和油气运移前有效孔隙评价提供了新的方法。

Through the development and calibration of a standard carbonate sample with an age of 209.8 Ma using a newly-developed Laser Ablation(LA)Multi-Collector Inductively Coupled Plasma Mass Spectrometry(MC-ICP-MS)technique,we successfully overcome the difficulty in sampling and dating ultra-low U-Pb ancient marine carbonates,which was previously untenable by isotope dilution(ID)methods.We developed the LA-MC-ICP-MS in situ U-Pb dating technique for ancient marine carbonates for the study of diagenesis-porosity evolution history in Sinian Dengying Formation,Sichuan Basin.By systematically dating of dolomitic cements from vugs,matrix pores and fractures,we found that the burial and diagenetic process of dolomite reservoirs in Sinian Dengying Formation was characterized by progressive filling-up of primary pores and dissolution vugs formed by epigenic karstification.The filling of vugs happened in three stages,early Caledonian,late Hercynian-Indosinian and Yanshanian-Himalayan,while the filling of matrix pores mainly took place in early Caledonian.The unfilled residual vugs,pores and fractures constitute the main storage space in the reservoir.Based on the above knowledge,we established the diagenesis-porosity evolution history of the dolomite reservoir in Sinian Dengying Formation,Sichuan Basin.These findings are highly consistent with the tectonic-burial and basin thermal histories of the study area.Our study demonstrates the reliability of in situ U-Pb dating for providing an effective way for the investigation of diagenesis-porosity evolution history and evaluation of porosity in ancient marine carbonate reservoirs before hydrocarbon migration.