玛湖凹陷二叠系-三叠系沉积模式与分支河流体系演变

Permian-Triassic sedimentary model and distributive fluvial system evolution of Mahu Sag,Junggar Basin

玛湖凹陷呈现常规-非常规油气有序共生的全油气系统格局,研究储层沉积体系展布模式和优势储层分布规律是完善全油气系统理论的重要内容。以分支河流体系(DFS)沉积理论为指导,综合利用岩心、测井、地震等资料,分层段绘制玛湖凹陷二叠系-三叠系各地层组的沉积相平面分布图,总结不同地质时期的沉积模式,依据沉积相平面图定量测算DFS的数量规模,研究沉积体系的演变规律。结果表明,玛湖凹陷各地层组普遍发育DFS,部分DFS入湖形成扇三角洲和河流三角洲。佳木河组具有火山岩、火山碎屑岩、碎屑岩共生的特征,主要发育冲积扇等小型DFS及扇三角洲沉积体系;风城组、夏子街组和乌尔禾组火山活动减弱,发育冲积扇和河流扇等类型的DFS,受湖平面变化影响,沉积体系类型和规模变化很大;上乌尔禾组和百口泉组主要发育粗粒砾质河流扇沉积体系;克拉玛依组和白碱滩组发育砂质巨型扇和河流扇沉积体系。DFS类型和规模与盆地构造发育阶段和气候变化相关,随着盆地构造由压陷向挠曲和坳陷阶段演化,DFS规模逐渐增大,由短流程冲积扇为主演变为以长流程河流扇和巨型扇为主,三角洲类型由以扇三角洲向以辫状河三角洲和曲流河三角洲演化。

Mahu Sag presents an orderly symbiosis of conventional and unconventional oil and gas systems.Studying the distribution patterns of sedimentary systems and of advantageous reservoirs is helpful to understand the orderly coexistence and accumulation theory of conventional and unconventional hydrocarbons.Guided by the theory of distributive fluvial system(DFS),by using cores,logging and seismic data,the mapping of spatial distribution pattern of sedimentary facies of Permian-Triassic in Mahu Sag is drawn by layers and sedimentary models of different geological periods are summarized.According to the spatial distribution map,the quantity and scale of DFS are measured and the evolution law of sedimentary system is studied.The results show that DFS is well developed in Permian-Triassic strata,with some flowed into lacustrine to form fan delta and river delta.Jiamuhe Formation is characterized by the coexistence of volcanic rocks,pyroclastic rocks and clastic rocks,mainly developing small DFS as alluvial fan and fan delta depositional system.During the deposition of Fengcheng,Xiazijie and Wuerhe Formations,volcanic activities were weakened and the fluvial fan and alluvial fan developed.Due to the change of lacustrine level changes,the type and scale of sedimentary systems vary greatly.In upper Wuerhe and Baikouquan Formations,the coarse-gravel fluvial fan sedimentary system was mainly developed,while in Karamay and Baijiantan Formations,sandy giant fan and fluvial fan sedimentary systems were developed.The type and scale of DFS are related to tectonic development stage and climate change.With the evolution of basin structure from pressure-flexure to flexure-depression stage,DFS scale increased gradually from smaller alluvial fan to larger fluvial fan and mega-fan,the deltas changed from fan delta to braided river delta and meandering river delta.