1 Research Institute of Exploration and Development, PetroChina Changqing Oilfield Company, Xi'an 710018, China
2 National Engineering Laboratory for Exploration and Development of Low-Permeability Oil & Gas Fields, Xi'an 710018, China
3 College of Geosciences, China University of Petroleum(Beijing), Beijing 102249, China
4 State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum(Beijing), Beijing 102249, China
LI Chenxi,born in 1994,Ph.D.,is engaged in researches on structural analysis of petroliferous basins,seismic data interpretation and integrated studies of petroleum geology. E-mail: 13299125257@163.com.
Inversion structures are significant superimposed structures that play an important indicative role in the development of hydrocarbon traps in petroliferous basins. To clarify the characteristics of inversion structures in the Yindongzi area,southern segment of the western Ordos Basin thrust belt,and further investigate their control on the distribution of the Wulalike Formation shale,this study first identifies typical features of inversion structures using 2D framework profiles and 3D seismic data. Then,balanced cross-section techniques and discrete element numerical simulation are applied to typical seismic profiles for evolutionary analysis,determining the stages and processes of inversion structure evolution. Finally,the paleogeomorphology at the end of Wulalike Formation deposition is restored using 3D seismic data,and the shale thickness of the Wulalike Formation is predicted based on a seismic velocity-lithology analysis model,enabling an in-depth discussion of tectonic inversion control on Wulalike Formation shale development. (1)Typical positive inversion structures developed in Yindongzi area. During the early depositional stage of the Wulalike Formation,the area was characterized by normal faults and negative structural elements. After the Permian,fault properties reversed from “normal” to “reverse”,and structural elements changed from “negative” to “positive”. Compressional structures continued to superimpose after the Triassic,forming a “lower depression and upper protrusion” structural style in profiles;(2)Two stages of positive tectonic inversion occurred in Yindongzi area: the first stage was a weak compressional inversion from the Late Ordovician to Early Carboniferous,a far-field compressional response triggered by the subduction mechanism transition of the Paleo-Qinling Ocean,characterized by regional broad uplift and long-term depositional hiatus,marking the beginning of the tectonic transition from extension to compression. The second stage was a strong thrust inversion,initiated in the Late Triassic and culminating in the Late Jurassic-Cretaceous. It began with initial compression induced by the North China-Yangtze block collision along the Qinling Belt during the Indosinian and was strongly activated during the Late Jurassic-Cretaceous by the superimposed effects of Pacific Plate subduction,completing the fundamental transformation from pre-existing extensional faults to a compressional structural system;(3)The distribution of Wulalike Formation shale is controlled by these two tectonic stages: The first stage,involving syn-sedimentary normal faults in a back-arc extensional setting,controlled the original deposition of thick deep-water shale. The weak tectonic inversion potentially formed favorable hydrocarbon “sweet spots” characterized by shale enclosing brecciated limestone. The second stage of strong tectonic inversion transformed pre-existing faults into thrust faults,causing the shale distribution to migrate eastward to higher structural positions;(4)The most favorable depositional area for Wulalike Formation shale is identified as the region west of the hanging wall of the Huishan Fault and east of the footwall of the Qingping Fault,with the planar favorable depositional zone located in the deep depression in the northwestern part of southern segment of Western Ordos Basin Thrust Belt. This paper proposes an analytical approach combining balanced cross-section techniques and numerical simulation for studying inversion structure evolution,which not only guides the exploration of Paleozoic shale oil in the southern segment of the western Ordos Basin but also provides a methodology for structural analysis in similar petroliferous basins.
LI Chenxi,born in 1994,Ph.D.,is engaged in researches on structural analysis of petroliferous basins,seismic data interpretation and integrated studies of petroleum geology. E-mail: 13299125257@163.com.
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LI Chenxi,born in 1994,Ph.D.,is engaged in researches on structural analysis of petroliferous basins,seismic data interpretation and integrated studies of petroleum geology. E-mail: 13299125257@163.com.
反转构造是地质体在不同的时期受到不同类型的应力作用后形成伸展和挤压构造叠加的复合构造类型(Glennie and Boegner,1981;Bally,1983;Harding,1985;Cooper and Williams,1989;Ventisette et al., 2006;Moustafa,2014)。按照伸展和挤压构造形成的先后顺序一般可将反转构造划分为正反转构造和负反转构造,其中正反转构造为先存伸展构造叠加后期挤压构造,反之则为负反转构造(Williams et al., 1989;Mitra,1993)。随着构造地质学和油气勘探技术的发展,已在反转构造中发现了大量的油气资源,揭示出反转构造与油气的形成、运聚和分布具有密切关系,因此反转构造越来越受石油地质学家的关注(胡望水,1996;赵永强,2007;李伟和高日胜,2010;曾维特等,2012;毕珉烽等,2015;陈念楠等,2025)。
离散元数值模拟通过将地质体简化为一系列可独立运动的颗粒或块体,基于牛顿运动定律和本构关系,定量模拟构造变形过程。数值模拟在构造分析中的核心作用在于量化揭示构造形成的动力学机制,如断层带的演化、褶皱作用、裂缝网络生成及应力-应变分布,从而将宏观构造与微观机制动态关联。近年来该技术正从二维、小尺度、单一过程向三维、大尺度、多场耦合(热-流-力-化)及与连续介质方法(有限元)杂交的方向迈进,并与地震解译、物理模拟和野外露头数据深度融合,成为验证构造假说、预测深部构造和非常规储层裂缝的关键工具(Cundall and Strack,1979;Dean et al., 2013;李长圣等,2022)。
鄂尔多斯盆地西南缘在晚三叠世末(印支期)和晚侏罗世—白垩纪(燕山期)发生第2期构造正反转,分为反转孕育阶段和强烈反转阶段。孕育阶段在晚三叠世,该时期随着扬子板块与华北板块沿秦岭带发生陆—陆碰撞(印支运动主幕),产生了强烈的近南北向挤压应力。应力场向北传递至鄂尔多斯盆地西南缘,使得在早古生代弧后拉张盆地时期形成的先存正断层发生构造反转,早期控盆的张性断层转变为挤压环境下的逆冲断层(刘池洋等,2005;任战利等,2006)。此次反转主要表现为区域性宽缓隆升和晚三叠世与早侏罗世地层间的沉积间断,奠定了盆地西缘构造格架的雏形。晚侏罗世以来,太平洋板块(特别是伊邪那岐板块)向欧亚板块之下发生高速、低角度的俯冲,产生了强大的北西—南东向区域挤压应力。此应力场与持续活动的秦岭带远程效应叠加,共同作用于鄂尔多斯西南缘(Zhu et al., 2011)。强烈的挤压使得先存断层(包括印支期已经弱反转的断层)剧烈复活,形成了大规模的逆冲推覆构造与相关褶皱,构成了现今地质图上的主体构造形迹,并造成了侏罗系与白垩系之间的显著不整合,至此构造反转完成。
去压实校正是古地貌恢复和盆地模拟中最基础、最关键的步骤之一。地层被上覆岩石压实,孔隙度变低,厚度变小。去压实校正就是通过数学模型,逆向恢复地层在沉积末期未被压实时的原始厚度和孔隙度。具体过程为:在Geoscope软件中输入当前地层深度、厚度、岩性,以及不同岩性(泥岩、砂岩等)的初始孔隙度和压实系数;从顶到底逐层剥离,对于其下每一层,根据它被移除后的新埋深,利用“孔隙度-深度”关系公式(Athy,1930)计算出它在该古埋深下的原始孔隙度;根据“骨架体积不变”原则,利用新计算的孔隙度,反推出该地层在过去的原始厚度;重复此过程,直到所有目标地层都被恢复,最终得到古地貌面。整个去压实校正中最关键的参数就是地表孔隙度和压实系数,根据前人在鄂尔多斯盆地西缘的热演化与古地温研究领域的成果,笔者采纳的参数为:地表泥岩孔隙度Φ0=65%,压实系数c=0.39;地表砂岩孔隙度Φ0=49%,压实系数c=0.27(Sclater and Christie,1980;任战利等,2006)。
从岩性组合特征来看,乌拉力克组岩性构成较为复杂,非纯泥页岩沉积,而是夹杂着灰岩、云岩等,因此准确预测其厚度较难。本研究通过计算泥岩指数(泥岩在地层总厚度中所占百分比),进而以地层厚度来估算泥页岩总厚度。地层中泥质地层百分含量与地层的速度之间可以用岩石体积物理模型来表示(Li et al., 2022):
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