东特提斯库车盆地始新世成盐期卤水演化及其成钾效应

The Kuqa Basin is one of the important potash-forming basins in China. It was affected by transgression-regression of the Tethys Sea in the Eocene, and deposited great thick salt-bearing stratum, which offered good conditions for studying the formation of potash deposit in the basin. At present, researches on the potash-forming process in the Kuqa Basin mostly focus on traditional mineralogy, whole-rock geochemistry and evolution characteristics of evaporites. However, the research on physical and chemical properties evolution process of brine during salt deposition period is still weak, which directly restricts the further research on potash mineralization. The project focuses on this scientific problem, taking the favorable potash-forming intervals of boreholes KL4 and DZK01 predicted by our research group as research objects. The analysis of rock salt chlorine isotope, hydrogen and oxygen isotope, homogenization temperature and chemical composition of primary fluid inclusions is carried out. Quantitatively restore the paleotemperature and the evolutionary information of chemical composition of brine during the salt-forming period, combine with the information of evaporation concentration degree, and then accurately inverses evolution law of potash-forming proxies of brine in target borehole intervals during salt-forming period. Therefore, it can be judged whether the ancient salt lake in the Kuqa Basin has evolved into the stage of potash deposition. Research results of the project provide climate and seawater composition record quantitatively for paleoenvironment reconstruction in Eocene, and provide data support for the next potash exploration in the Kuqa Basin.

库车盆地是我国重要的成钾远景盆地之一，始新世受特提斯多期次海侵—海退的影响，沉积了巨厚的含盐系地层，为盆地成钾研究提供了得天独厚的条件。目前关于库车盆地成钾过程的研究多集中于传统的盐类矿物学、全岩地球化学和蒸发岩沉积演化特征等。但对于石盐岩沉积期卤水的物理化学性质及其演化过程的研究仍十分薄弱，这直接制约了进一步对钾盐成矿方面的研究。本项目围绕这一科学问题，以KL4井和DZK01井前期预测的有利成钾井段为研究对象，开展石盐氯同位素、原生流体包裹体氢氧同位素和均一温度以及化学组分分析，定量恢复成盐期卤水的古温度和化学组成演化信息，并结合蒸发浓缩程度信息，准确反演成盐期古卤水各成钾指标在目标井段中的演化规律，从而判定库车盆地古盐湖是否演化到钾盐沉积阶段。项目研究成果对东特提斯始新世古环境重建提供定量的气候以及海水成分记录，同时对盆地下一步钾盐勘探提供数据支撑。

库车盆地是我国重要的成钾远景盆地之一，目前关于盆地成盐期卤水演化过程的研究仍十分薄弱，这直接制约了进一步对钾盐成矿方面的研究。本项目以钻孔含盐系层段和地表剖面为研究对象，开展精细的盐类矿物学、元素化学、原生流体包裹体均一温度和化学组分、多同位素联用等分析，取得了如下重要成果：（1）KL4井含盐系剖面划分了4个成盐韵律，建立了盐类矿物演化序列，并在Ⅲ和Ⅳ成盐阶段发现了3次钾盐析出事件；（2）识别了早始新世和中新世高温成盐阶段，中新世成盐卤水温度或海平面温度具有明显降低的特征，与全球以及特提斯域降温趋势一致，但库车盆地的降温幅度更大；（3）首次通过单个流体包裹体化学成分的测定，KCl含量最高达0.59%，高于工业富钾最低品位盐水（0.5%），再结合钾盐矿物和含钾矿物的发现以及δD和δ18O值特征，说明了卤水在成盐过程中经历了高度的蒸发和浓缩期，已达到钾盐沉积阶段；（4）对盆地钻孔含盐系开展氯同位素的研究，发现δ37Cl均为负值，介于-1.20‰~-0.51‰之间，显示了较高的蒸发浓缩程度，与前人分析的地表石盐δ37Cl呈正值的特征相区别，为塔里木盆地以及库车盆地找钾工作提供了新视角；（5）上古新统蒸发岩层序是由海侵作用形成的，下始新统蒸发岩序列的沉积环境先是由大陆沉积转变为海洋沉积，然后逐渐过渡到大陆沉积环境，恢复晚古新世—早始新世海侵历史演变，建立海侵沉积模式图。以上成果对实现库车盆地找钾工作具有重要的理论意义和应用价值，亦对海相、海陆交互相盐类沉积盆地的钾盐勘探具有重要的指导作用。