准噶尔盆地玛湖凹陷下二叠统古老碱湖成因探究:来自全球碱湖沉积的启示

Genesis of alkaline lacustrine deposits in the Lower Permian Fengcheng Formation of the Mahu Sag,northwestern Junggar Basin:insights from a comparison with the worldwide alkaline lacustrine deposits

碱湖沉积在全球范围内均有分布,在特定的构造背景、火山活动、热液作用及气候条件下形成。我国准噶尔盆地玛湖凹陷风城组发育的碱湖沉积形成年代老且埋藏较深,在完整的沉积-成岩演化研究中具有独特优势。此外,风城组碱湖沉积的分布与页岩油藏的分布具有密切关系,其形成演化对碱湖页岩油藏研究提供重要支持。通过全球尺度下碱湖及碱湖沉积主要特征分析,总结碱湖沉积特征和成因,为准噶尔盆地玛湖凹陷风城组碱湖沉积成因分析提供借鉴。综合分析表明,同期火山活动和特定气候条件均可主控碱湖沉积的形成。此外,碱湖沉积形成需湖盆水文封闭、充足的溶质供给、低硫酸盐占比及强蒸发环境条件。汇总分析全球碱湖沉积矿物种类与埋深关系,发现成岩作用下矿物转变规律:单斜钠钙石随成岩作用增强依次转变为钙水碱、碳钠钙石,苏打石和天然碱转变为碳酸氢钠石。通过与全球碱湖沉积实例对比分析可知,玛湖凹陷风城组碱湖沉积中碳酸氢钠石和碳钠钙石经成岩作用形成,钠硅酸盐矿物和特征矿物与火山活动及热液作用有关。碳酸氢钠石生长形态及磁黄铁矿指示玛湖凹陷风城组碱湖沉积中心为高盐度分层且底部缺氧的常年生湖泊。下二叠统风城组碱湖沉积形成与周缘火山岩及同沉积火山活动密切相关,受气候、构造背景、火山活动和后期成岩作用的共同控制。

The alkaline lacustrine deposits are distributed all over the world and formed under special tectonic and climate conditions with volcanic and hydrothermal activities. The old, deeply buried alkaline lacustrine deposit in the Lower Permian Fengcheng Formation of the Mahu Sag provides a unique opportunity to study its sedimentary and diagenetic evolution. In addition, the distribution of alkaline lake deposits in the Fengcheng Formation is closely related to the distribution of shale reservoirs, thus a study of their formation and evolution provides important support towards the study of the shale reservoirs.A comprehensive study of sedimentary characteristics and genesis of worldwide alkaline lacustrine deposits was carried out to provide new insights into the genesis of the alkaline lacustrine deposits in the Lower Permian Fengcheng Formation of the Mahu Sag, northwestern Junggar Basin. An overview of the literature indicates that both contemporaneous volcanic activity and specific climatic could control the formation of the alkaline lacustrine deposits. The formation of alkaline lacustrine deposits required hydrologically closed basins, adequate solute supply of sodium and carbon, low concentration of sulfate, and a strong evaporative environment. The relationship between the mineral types and burial depth of the global alkaline lacustrine deposits indicated that the minerals were transformed by diagenesis. Diagenesis resulted in the conversion of gaylussite to pirssonite and finally into shortite. Meanwhile, wegscheiderite was formed diagenetically from nahcolite and trona. Comparative analysis showed that shortite and wegscheiderite in the alkaline lacustrine deposits from the Fengcheng Formation were formed by diagenetic mineral transformation. Searlesite was formed from pyroclastic deposits under burial alteration and diagenesis, and reedmergnerite was related to hydrothermal activity. The characteristic minerals were associated with the sediments formed by weathering of surrounding alkaline volcanic rocks. The depocenter of the alkaline lacustrine deposits in the Fengcheng Formation was covered by high alkaline brines and the depositional environment was anoxic as a result of perennial salinity-based lake water stratification. The alkaline lacustrine deposits in the study area were controlled by specific climate, tectonism, volcanism and diagenesis.