玛湖凹陷是准噶尔盆地重要的生烃凹陷,凹陷中下二叠统风城组的湖相优质烃源岩被认为是中国至今发现的最古老的碱湖沉积,是准噶尔盆地西北缘百里油区形成的重要基础.通过区域构造、沉积学以及岩石矿物学方面的综合研究表明,风城组沉积于西准噶尔地区古生界基底上所发育的二叠纪造山后伸展断陷中,整体上是一套扇三角洲—湖相沉积体系,其中碱湖沉积集中于玛湖凹陷的中心地带.风城组发育大量的特征性碱类矿物,包括硅硼钠石、氯碳酸钠镁石、碳酸钠钙石、水硅硼钠石、碳镁钠石、碳氢钠石和苏打石等,表征了碱湖沉积特点,碱化程度高,区别于常见的盐湖沉积.碳酸钠钙石、碳氢钠石、硅硼钠石和水硅硼钠石等碱类矿物的发育,证实玛湖凹陷风城组碱湖烃源岩形成于受深部热液作用影响的高盐度闭塞性湖泊环境,而不是蒸发成因,这也和伸展断陷背景下活跃的火山活动密切相关.%Mahu sag is one of the most prolific oil-gas sags in the Junggar basin.The high-quality lacustrine source rocks in the Lower Permian Fengcheng formation are considered as the oldest alkaline lacustrine deposits in China,which provides the resource foundation for the formation of hundred-mile oil province in northwestern margin of Junggar basin.Based on the investigations of regional geology,sedimentology,mineralogy and petrology,Fengcheng formation is inferred to be developed in the post-orogenic extensional fault depression which is found on the Paleozoic basement in the western Junggar basin.The formation is overall a set of fan delta-lacustrine deposits with the depocenter in the center of Mahu sag.Abundant alkaline minerals are observed in the Fengcheng formation,including reedmergnerite,northupite,shortite,searlesite,eitelite,wegscheiderite and nahcolite etc.,which indicates a representative alkaline lacustrine facies with high salinity,distinct from other common saline-lake deposits.The presence of shortite,wegscheiderite,reedmergnerite and searlesite confirms that the alkaline lacustrine source rocks of the Fengcheng formation were deposited in a closed and high-salinity lacustrine environment highly effected by deep hydrothermal fluids,not due to evaporation,which is also strongly associated with volcanic activities under the extensional fault depression setting.
展开▼
