准噶尔盆地玛湖凹陷下二叠统风城组碱湖岩石特征与成因*
Lithological characteristics and origin of alkaline lacustrine of the Lower Permian Fengcheng Formation in Mahu Sag, Junggar Basin

风城组作为准噶尔盆地下二叠统优质烃源岩,一直是盆地油气勘探的重点。但因其埋藏深、岩性复杂、全球可对比研究实例少,也一直是研究的难点。文中以精细的岩性研究为基础,利用岩心描述、薄片观察、SEM、X衍射、TOC含量测定、包裹体测温及地化分析等手段,深入分析玛湖凹陷风城组碱性矿物特征和地质背景,探讨了碱湖成因与形成过程,得出: (1)风城组岩石可细分为5种主要类型,其中碱性岩为碱湖标志性岩类,富硅硼纳石岩类与火山喷发或热液作用相关;(2)风城组存在5种主要的岩石组合类型,从近源区至湖区依次为: 河流—冲积平原发育的组合类型Ⅴ、滨浅湖环境发育的组合类型Ⅲ与类型Ⅳ、深湖—半深湖环境发育的组合类型Ⅰ与类型Ⅱ,其中类型Ⅰ与类型Ⅲ为碱湖特有岩石组合,含优质烃源岩;(3)碱湖形成主要受火山活动和气候控制,前者为碱湖形成提供了Na⁺溶质,后者促进了淡水湖咸化,最终演化成碱湖。

The Lower Permian Fengcheng Formation,as favourable source rock in the Junggar Basin,has been the target for petroleum exploration. Due to its deep burial depth,complicated lithology and few global analogs,the evaluation of the petroleum potential of Fengcheng Formation is still at its rudimentary stage. This study discusses the characteristics of alkaline minerals,geological background,origin of alkaline lakes and its forming processes through methods including core description,thin section observation,SEM,X-ray diffraction analysis,TOC content measurements,inclusion temperature test,geochemical analysis,etc. the results suggest: (1)the Fengcheng Formation can be categorized into five rock types,among which,alkaline rock is the most distinguished lithofacies type of alkaline lakes and reedmergnerite is generally related to volcanic eruption or hydrothermal activities;(2)five rock associations are recognized in the Fengcheng Formation. From near-source to lake center,they are type Ⅴ of alluvial-fluvial facies,type Ⅲ and Ⅳ of shallow shoreface to lacustrine facies,respectively;type Ⅰ and Ⅱ of deep-semideep lacustrine facies,respectively. Type Ⅰ and Ⅲ are typical alkaline rock associations with favourable source rock potential;(3)the formation of alkaline lake is primarily controlled by volcanic activities and climate,where the former supply the Na-rich fluids,and the latter salinized the freshwater lake and facilitate the final formation of alkaline lake.