准噶尔盆地玛湖凹陷下二叠统风城组碱湖古沉积背景

准噶尔盆地玛湖凹陷下二叠统风城组发育全球最古老的碱湖优质烃源岩，但其古沉积背景尚未得到系统剖析，限制了对风城组发育模式与成烃演化的认识。通过岩石学与地球化学相结合的方法，研究了古沉积的地貌、气候、水深、盐度和水温等特征，结果表明，玛湖凹陷在风城组沉积期总体属于闭塞型湖泊，古地貌表现为西陡东缓的不对称箕状凹陷，凹陷中分布有大小不等的湖湾；风城组沉积期碱湖形成的气候条件为半干旱，蒸发量大于补给量，并且季节性的潮湿环境与干旱环境相交替；古水深总体较浅，发育深色细粒的烃源岩；古水体为高盐度水介质，风城组二段盐度最高，指示风城组二段沉积时期碱湖最发育；古水温较高，相当于暖相—偏暖相。

Paleo-Sedimentary Setting of The Lower Permian Fengcheng Alkali Lake in Mahu Sag,Junggar Basin

The high?quality hydrocarbon source rocks of the world’s most ancient alkali lake are found in the Lower Permian Fengcheng formation of Junggar basin. The understanding of development model and hydrocarbon?forming evolution of the Fengcheng formation have been limited due to unsystematic analysis about its paleo?sedimentary setting up to now. This paper uses petrological and geochemical methods to study the characteristics of its landform, climate, lake water depth, salinity and water temperature, and the results show that Mahu sag is a closed lake in the depositional stage of Fengcheng formation as a whole, and its ancient landform appears asymmetric halfgrabenlike sag with steep slope in the west and gentle slope in the east, and the sizes of lake bays are distributed in it; the climatic condition of the alkali lake formation in this stage is semi?arid, characterized by that the evaporation was greater than the recharge, with seasonal humid?arid?alternate environment; the lake water depth is shallow in general, developing dark fine?grained source rocks; the water body is of high?salinity medium, the highest salinity occurs in second member of Fengcheng formation, indicating the alkali lake is most developed in this member; and the paleotemperature of the water body is relatively high, equivalent to warm to partial warm phase