库车前陆盆地幕式冲断活动、层序-沉积响应特征及油气地质意义

Episodic thrusting and sequence-sedimentary responses and their petroleum geological significance in Kuqa foreland basin,NW China

综合利用野外露头剖面、钻井岩心、二维地震大剖面及重点区三维地震资料,建立库车前陆盆地三叠系构造-层序地层序列,探讨幕式冲断构造对沉积充填演化和烃源岩分布的控制作用。研究表明:①库车前陆盆地三叠纪的形成与发展经历了完整的强烈—减弱—松弛—宁静幕式冲断活动,与之对应可以划分4个二级层序(对应三叠系俄霍布拉克组、克拉玛依组、黄山街组、塔里奇克组)和11个三级层序(SQ1—SQ11),每个层序或层序内幕受次级幕式冲断构造活动影响,形成“下粗上细”的“二元”结构。②幕式冲断作用与区域层序序列、沉积充填以及烃源岩发育与分布具有明显的响应,幕式冲断作用初始强烈至宁静过程中,三叠纪自下而上的沉积演化呈扇三角洲—辫状河三角洲—湖泊—辫状河三角洲—曲流河三角洲沉积演化的特点,幕式冲断活动初始强烈—减弱期对应于厚层粗粒砂砾岩储层发育期,幕式冲断活动松弛—宁静期对应于烃源岩发育期。③受幕式冲断活动期次与强度控制,近东西走向逆冲断层下盘烃源岩不仅在黄山街组沉积时期明显沉积增厚,成为三叠系烃源岩发育中心,而且烃源岩软地层经历多期次的逆掩推覆,呈“叠垛式”堆积。(4)库车前陆盆地深层具备形成规模气藏的基础与条件,可以形成自生自储型岩性、下生上储型断块-岩性、地层不整合等多种类型的气藏,是未来天然气勘探规模增储建产的重要领域。

This study integrates field outcrop profiles,drilling cores,2D seismic profiles,and 3D seismic data of key areas to analyze the Triassic tectonic-sequence stratigraphy in the Kuqa foreland basin,and investigates the impact of episodic thrust structures on sedimentary evolution and source rock distribution.(1)The Kuqa foreland basin has experienced stages of initial strong,weakened activities,relaxation and inactivity of episodic thrusting,resulting the identification of 4 second-order sequences(Ehebulake Formation,Karamay Formation,Huangshanjie Formation,Taliqike Formation)and 11 third-order sequences(SQ1–SQ11).Each sequence or secondary sequence displays a“coarse at the bottom and fine at the top”pattern due to the influence of secondary episodic thrust activity.(2)The episodic thrusting is closely linked to regional sequence patterns,deposition and source rock formation and distribution.The sedimentary evolution in the Triassic progresses from fan delta to braided river delta,lake,braided river delta,and meandering river delta,corresponding to the initial strong to the inactivity stages of episodic thrusting.The development stage of thick,coarse-grained sandy conglomerate reservoirs aligns with the strong to weakened thrust activities,while the source rock formation period coincides with the relaxation to inactivity stages.(3)Controlled by the intensity and stages of episodic thrust activity,the nearly EW trending thrust fault significantly thickened the footwall source rock during the Huangshanjie Formation,becoming the development center of Triassic source rock,and experienced multiple overthrust nappes in the soft stratum of the source rock,showing“stacked style”distribution.(4)The deep layers of the Kuqa foreland basin have the foundation and conditions necessary for the formation of substantial gas reservoirs,capable of forming various types of reservoirs such as self-generating and self-storing lithology,lower generating and upper storing fault-lithology,and stratigraphic unconformity.This area holds significant importance for future gas exploration efforts aimed at enhancing reserves and production capabilities.