摘要： 川北元坝气田二叠系长兴组49个碳酸盐岩样品的稀土元素(REE)和钇(Y)含量的测试结果表明:稀土元素总量(ΣREE)普遍较低(平均1.50×10^-6),说明岩石样品几乎无陆源碎屑的混入,且未受到深部热液的影响。页岩标准化后的白云岩REE特征表现为轻稀土(LREE)亏损、正La异常、负Ce异常(0.49～1.08,平均0.75)、普遍的正Eu异常(0.89～46.00,平均4.68)及较高的Y/Ho值(28.82～63.47,平均43.04),总体表现出与海水的REE配分特征类似,表明白云化流体为还原性的海源流体,白云岩形成环境为埋藏环境。元坝气田长兴组构造平缓,白云岩横向上连续分布,碳酸盐岩δ¹⁸O(-7.07‰～-2.92‰,平均-4.95‰)相对较重,岩心与岩石薄片未观察到角砾状或条带状的热液白云岩典型构造,均说明元坝气田长兴组未受到深部热液流体的影响,白云岩较高的正Eu异常与深部热液流体无关;电子扫描显微镜下观察到大量的热化学硫酸盐还原反应(TSR)的产物单质硫和富硫沥青,表明元坝气田长兴组基质白云岩形成之后的TSR作用导致了地层水极度还原的环境,从而造成了白云岩较高程度的正Eu异常,反过来说明了TSR作用对白云岩储集层具有一定的改造作用。

关键词: 稀土元素, 白云岩, TSR作用, 四川盆地, 元坝气田, 二叠系

Abstract: Rare earth element(REE)and Yttrium(Y)concents were determined from 49 carbonate samples of the Permian Changxing Formation of Yuanba Gasfield in northern Sichuan Basin,and the results show that the total content of rare earth elements(REE)is generally low(average 1.50×10^-6),which indicates that rock samples were almost without any terrigenous clastics,and not be affected by the deep hydrothermal fluids. Shale-normalized REE characteristics of dolostone including light rare earth element(LREE)depletion,positive La anomalies,negative Ce anomalies(0.49 to 1.08,average 0.75),positive Eu anomalies(0.89 to 46.00,average 4.68) of different degrees and the higher Y/Ho ratios(28.82 to 63.47,average 43.04) recorded seawater-like REE and Y signatures,which show that the dolomitization fluids is reductive seawater derived,and the dolomitization occurred in a burial environment. The brecciated or zebra typical structure of hydrothermal dolostones were not observed in the cores and petrographic thin sections. Flattened structure and the horizontal continuous distribution of dolomite reservoirs and the higher δ¹⁸O value(-7.07‰ to-2.92‰,average -4.95‰)illustrate that no obvious hydrothermal activities occurred in the study area,and the higher degree of positive Eu anomalies have no relationship with the deep hydrothermal fluids. Many elemental sulfurs and sulfur-rich pyrobitumen originated from thermochemical sulfate reduction(TSR) can be observed by scanning electron microscope,which indicates that TSR created an extremely reducing environment of stratal seawater,and then caused the higher degree of positive Eu anomalies,which in turn explains that TSR had a certain dissolution on dolomite reservoirs.

Key words: REE, dolostone, TSR, Sichuan Basin, Yuanba Gasfield, Permian

中图分类号: 

• P88.24+5

• P9