锶同位素在古岩溶研究中的应用——以塔河油田奥陶系为例

Application of Strontium Isotope to the Study of Paleokarst——An Case from Ordovician in the Tahe Oilfield,Tarim Basin

为探讨锶同位素在古岩溶研究中的应用,系统分析了塔里木盆地塔河油田奥陶系岩溶缝洞方解石的87Sr/86Sr值。研究结果表明,加里东中期岩溶形成的方解石以低87Sr/86Sr值为特征,其锶同位素组成主要由围岩的重溶锶控制。而海西早期岩溶形成的方解石则以高87Sr/86Sr值为特征,锶同位素组成主要由壳源锶和重溶锶控制;同时,不同产状的方解石锶同位素比值也不相同,渗流岩溶带的方解石比潜流岩溶带富集87Sr,主要受控于岩溶系统的渗流机制和水岩反应。在平面上,桑塔木组覆盖区比北部地区富集87Sr,其主要原因是来源于围岩的重溶锶比重加大。87Sr/86Sr值在区域上的变化,结合区域地质背景,表明海西早期岩溶对北部地区和桑塔木组覆盖区均有重要的影响,是奥陶系岩溶缝洞型储层的主要形成时期。另外,极低的锶同位素比值可能与幔源锶有关,早二叠纪末的火山活动是提供幔源锶的主要时期。通过塔河油田奥陶系古岩溶作用的实例研究,锶同位素具有良好的流体示踪能力,在划分古岩溶期次方面具有明显的优势。

For application of strontium isotope to study of paleokarst, the paper has systematically analyzed the calcites filled in fissure-cave of the Ordovician carbonate rocks in the Tahe oilfield, Tarim Basin. The research results indicated that lower 87^Sr/86^Sr ratios are a character of calcites formed in the karst during the Middle Caledonian, and the 87^Sr/86^Sr ratios of calcites are controlled by those of host rocks. However, the calcites of Early Herlynian paleokarst have higher 87^Sr/86^Sr ratios, which are controlled by the terrestrial strontium and the dissolved strontium from the wall rocks. The 87^Sr/86^Sr ratios of different attitude calcites differ; the products of vadose zone are enriched in 87^Sr than phreatic zone, which mainly result from the seepage mechanism of the karstic system covered with Oss Formation has higher 87^Sr/86^Sr ratios and water-rock interaction. Furthermore, the area than the north area, and mainly contributes to an increased proportion of strontium isotope of the host rocks. Regional variations of strontium isotope and geologic backgrounds may indicate that the Early Hercynian paleokarst has important influences on the Ordovician carbonate rocks in the north area and the areas comprising the O3s Formation, and it is the main formation episode of karstic fissure-cave reservoir in the Tahe oilfield. In addition, the extremely low 87^Sr/86^Sr ratios may be an indicator of mantle source, and the volcanism of Early Permian is a main period of providing mantle-derived strontium. The study on the Ordovician paleokarst in the Tahe oilfield suggests that strontium isotope is of a better ability in indicating fluid＇s provenance, and therefore, it has distinct advantages in the division of paleokarst stages.