塔里木盆地玉北地区中下奥陶统鹰山组溶洞充填物稀土元素特征及其意义*

中下奥陶统碳酸盐岩古岩溶型储集层的特征、演化及纵横向分布是塔里木盆地玉北地区勘探关注的重要问题,而YB7井在鹰山组钻遇的溶洞及其充填物为研究该地区中下奥陶统古岩溶作用提供了契机。研究区YB7井中下奥陶统鹰山组溶洞充填物以长石石英砂岩为主,文中将其稀土元素组成特征与该地区志留系依木干他乌组和克孜尔塔格组、泥盆系东河塘组、石炭系巴楚组及塔中地区志留系柯坪塔格组和塔塔埃尔塔格组进行对比,以分析其物源及溶洞的演化时限。结果显示: 4个溶洞充填物样品稀土元素配分模式表现为轻稀土轻微富集,重稀土稍显亏损,具有明显的Ce负异常和轻微的Eu正异常,与东河塘组和柯坪塔格组的特征较为接近。溶洞充填物的ΣREE平均为36.48 μg/g,与东河塘组最为接近,而与柯坪塔格组相距甚远。玉北地区缺失柯坪塔格组,至巴楚组沉积前一直处于构造高部位,故柯坪塔格组不具备为YB7井溶洞提供物源的基础。玉北地区西部局部钻遇东河塘组,而YB7井周边缺失该组,故综合稀土元素组成特征、沉积特征及构造背景认为,YB7井溶洞充填物来源于东河塘组,其形成于暗河系统,且东河塘组沉积后至巴楚组沉积前是玉北地区重要的岩溶储集层发育时期。

REE characteristics and their significance of cave fillings of the Lower-Middle Ordovician Yingshan Formation in Yubei area,Tarim Basin

The characteristics,evolution and distribution of palaeokarst reservoir of the Lower-Middle Ordovician are the key factors in carbonate exploration in Yubei area,Tarim Basin. Fortunately,cave fillings of the Lower-Middle Ordovician Yingshan Formation in the karst cave of Well YB7 provide the opportunity to research the palaeokarst of this period. The cave fillings were composed mostly of feldspar-quartz sandstone. Compared the characteristics of their REE element with the counterpart of the Formations of Yimugantawu,Keziertage,Donghetang,Bachu,Kepingtage and Tataaiertage,the provenance of cave fillings and the time limit of palaeocave evolution were analyzed. The results show that four cave filling samples are characterized by LREE slight enrichment,HREE slight deficit,negative Ce anomaly and positive Eu anomaly,which are similar to those of the Donghetang and Kepingtage Formations. The average ΣREE is 36.48,which is most similar to that of the Donghetang Formation,while different greatly from that of the Kepingtage Formation. Because Yubei area is located in structurally high position before the depositional period of Bachu Formation and the Kepingtage Formation in this area was missing,the Kepingtage Formation was not the provenance of the cave fillings of Well YB7. The Donghetang Formation distributes in part of western Yubei area and is missing in the vicinity of Well YB7. Based on the REE characteristics, sedimentary characteristics and structural background, it is concluded that the Donghetang Formation is the provenance of cave fillings of Well YB7, and the palaeocave was formed in the underground river system during the depositional period of Donghetang Formation to Bachu Formation.