准噶尔盆地玛湖凹陷风城组烃源岩沉积古环境

Sedimentary paleoenvironment of source rocks of Fengcheng Formation in Mahu Sag,Junggar Basin

准噶尔盆地下二叠统风城组发育一套重要的烃源岩。基于岩心和薄片资料,利用电感耦合等离子质谱计(ICP-MS)和X射线衍射荧光光谱仪(XRF)测试了玛湖凹陷玛页1井风城组25件烃源岩样品的主、微量元素含量。结合前人研究成果,综合研究玛湖凹陷风城组烃源岩沉积古环境。结果表明,玛湖凹陷风城组主要以碎屑岩为主,包括泥岩、灰质泥岩、粉砂质泥岩、泥质粉砂岩以及泥质细砂岩等,风城组的沉积演化可分为5个阶段:第一和第二阶段(风一段沉积期)以蒸发咸化为主;第三和第四阶段(风二段沉积期)在热液作用影响下以钙镁碳酸盐沉积、钠碳酸盐沉积的咸化、碱化为主;第五阶段(风三段沉积期)在火山作用、湿冷气候条件下沉积凝灰质泥岩、陆源碎屑岩,以淡化为主。相应地,玛湖凹陷所发育的湖盆水深呈周期性变化,总体为逐渐变浅,对应的湖水古盐度演化特征为淡—半咸—咸—半咸—淡。古气候演化特征为半干旱—潮湿—半干旱—干旱—半干旱,整体以半干旱—干旱气候为主。风城组碎屑岩的粒度及沉积构造变化可以作为很好的环境指标,沉积作用受火山活动和古气候的控制,热液作用提供了大量营养物质,促进碱湖的形成。

An important set of source rocks is developed in Fengcheng Formation of Lower Permian in Junggar Basin. This research took full advantages of ICP-MS,XRF in the measurements around major elements and trace elements,along with biomarkers of saturated hydrocarbons in 25 source rock samples from Fengcheng Formation in Well Maye 1,Mahu Sag. Based on previous research results,the geochemical characteristics and sedimentary paleoenvironment of the source rocks of Fengcheng Formation in Mahu Sag were comprehensively studied. The research demonstrated that the sedimentary evolution of Fengcheng Formation can be divided into five stages. The first and second stages(sedimentary period of Feng1 section)were mainly defined as evaporation and salinization. In the third and fourth stages(sedimentary period of Feng2 section),calcium and magnesium carbonate deposits,as well as sodium carbonate deposits were salinized and alkalized under the influence of hydrothermal action. In the fifth stage(sedimentary period of Feng3 section),tuffaceous mudstone and terrigenous clastic rock were deposited under the condition of volcanic activities and damp climate,while the desalination took the role of primary reaction. Accordingly,the water depth of the lake basin developed in Mahu Sag changed periodically and became gradually shallower in general. The corresponding paleo-salinity evolution characteristics of lake water were light-semi-saline-saline-semi-saline-light,while the paleoclimate evolution features are semi-arid-humid-semi-arid-arid-semi-arid,but the semi-arid-arid climate performed generally. The grain size and sedimentary tectonic changes of clastic rocks in Fengcheng Formation can be used as an appropriate environmental index,while the deposition of Fengcheng Formation was controlled by volcanic activities and paleoclimate,as the preservation of organic matter was closely related to paleo-bathymetric and redox preservation conditions of water body. The arid climate results in a large amount of evaporation of water,promoting salinization,while hydrothermal action provides a general series of nutrients,and promotion of the alkaline lake.