准噶尔盆地玛湖凹陷P-T转换期层序结构、坡折发育及油气地质意义

Sequence architecture,slope-break development and hydrocarbon implications of the Mahu Sag during the P-T transition,Junggar Basin

对应晚二叠世—早三叠世地球系统转换期(简称P—T转换期),准噶尔盆地玛湖凹陷P_(3)-T_(1)经历了从前陆冲断向整体拗陷的转变,发育T_(1)/P、P_(3)w/P_(2)w_(2)期大型不整合及上乌尔禾组(P_(3)w)、百口泉组(T_(1)b)_(2)套退覆型低位沉积,形成了一批大型—特大型砾岩油田。针对T_(1)/P和P_(3)w/P_(2)w_(2)个三级层序界面,识别出与构造挠曲和侵蚀作用相关的2类坡折带:构造挠曲坡折带受控于持续性古隆起,控制层序界面之上地层向斜坡高部位逐层“单向超覆”,侵蚀坡折带受控于古残丘,控制层序界面之上地层向古残丘“双向超覆”或沿下切沟谷低位充填。2类坡折带均控制地层沉积变化,有利于形成上倾超覆型、低位充填型和地层削截型岩性地层圈闭。研究成果对于指导古地貌背景坡折控制下的大型岩性地层油气藏勘探具有重要意义。

Corresponding to the transitional period of the earth system from Late Permian to Early Triassic times(referred to as the P-T transition period),the Mahu Sag of the Junggar Basin experienced a transition from a foreland basin to a large depression lake basin,with the development of two sets of unconformities(T_(1)/P and P_(3)w/P_(2)w)and two sets of lowstand deposits,the Upper Wuerhe Formation(P_(3)w)and Baikouquan Forma⁃tion(T_(1)b)that host a series of large or giant conglomerate oil fields.For the two third-order sequence boundar⁃ies T_(1)/P and P_(3)/P_(2),two types of slope-break zones which are associated with flexure or erosion have been iden⁃tified:flexural slope-break zone is controlled by a persistent paleo-uplift and controls the layer-by-layer“unidi⁃rectional onlapping”of the strata above the sequence boundary towards the higher part of the slope;while ero⁃sional slope-break zone is controlled by the erosional remnant of plaeo-mounds.It is typically characterized by“two-way onlapping”towards the residual mounds or filling along the lower part of the valleys.The two types of slope-break zones both control changes in stratigraphy and sedimentation,and are conducive to the formation of lithostratigraphic traps of up-dip onlapping,lowstand filling and truncated types.The research results are of great significance to guide the exploration of large-scale lithostratigraphic plays under the control of paleotopog⁃raphy.