Tectonic Evolution of the Middle Frontal Area of the Longmen Mountain Thrust Belt,Western Sichuan Basin,China

By analyzing the balanced cross sections and subsidence history of the Longmen Mountain thrust belt, China, we concluded that it had experienced five tectonic stages： （1） the formation stage （T3x） of the miniature of Longmen Mountain, early Indosinian movement, and Anxian tectonic movement created the Longmen Mountain; （2） the stable tectonic stage （J1） where weaker tectonic movement resulted in the Longmen Mountain thrust belt being slightly uplifted and slightly subsiding the foreland basin; （3） the intense tectonic stage （J2-3）, namely the early Yanshan movement; （4） continuous tectonic movement （K-E）, namely the late Yanshan movement and early Himalayan movement; and （5） the formation of Longmen Mountain （N-Q）, namely the late Himalayan movement. During those tectonic deformation stages, the Anxian movement and Himalayan movement played important roles in the Longmen Mountain＇s formation. The Himalayan movement affected Longmen Mountain the most; the strata thrust intensively and were eroded severely. There are some klippes in the middle part of the Longmen Mountain thrust belt because a few nappes were pushed southeastward in later tectonic deformation.

Differential Tectonic Deformation of the Longmen Mountain Thrust Belt,Western Sichuan Basin,China

Field investigation and seismic section explanation showed that the Longmen Mountain Thrust Belt has obvious differential deformation： zonation, segmentation and stratification. Zonation means that, from NW to NE, the Longmen Mountain Thrust Belt can be divided into the Songpan- Garz~ Tectonic Belt, ductile deformation belt, base involved thrust belt, frontal fold-thrust belt, and foreland depression. Segmentation means that it can be divided into five segments from north to south： the northern segment, the Anxian Transfer Zone, the center segment, the Guanxian Transfer Zone and the southern segment. Stratification means that the detachment layers partition the structural styles in profile. The detachment layers in the Longmen Mountain Thrust Belt can be classified into three categories： the deep-level detachment layers, including the crust-mantle system detachment layer, intracrustal detachment layer, and Presinian system basal detachment layer; the middle-level detachment layers, including Cambrian-Ordovician detachment layer, Silurian detachment layer, etc.; and shallow-level detachment layers, including Upper Triassic Xujiahe Formation detachment layer and the Jurassic detachment layers. The multi-level detachment layers have a very important effect on the shaping and evolution of Longmen Mountain Thrust Belt.

Structural Styles of Longmen Mountain Thrust Belt,SW China

Through field geological investigation and seismic interpretation of the Longmen （龙门） Mountain thrust belt, we summarized the following structural styles： thrust belt, fault-related fold （fault bend fold, fault propagation fold, and fault decollement fold）, pop-up, triangle zone, duplex, superimposed fold, ductile deformation structures, reverse thrust fault, klippe, decollement structure,etc.. These structural styles have evident distribution characteristics; they had zonation and segmentation in plane. The zonation presents as the thrust nappe tectonic zone to the west of Tongjichang （通济场） fault, fault-related folds between Tongjichang fault and Guankou （关口） fault, and low and mild folds to the east of Guankou fault. The segmentation is evidenced as the scale of reverse thrust faults was minor between Tongjichang No. 1 fault and Tongjichang No. 2 fault. The distance between these two faults became long in the Daynanbao （大园包） structure, and there developed typical fault propagation fold and pop-up between these two faults. Furthermore, the structures had stratifi-cation in profile. The salt layer of T21 provided good conditions for the formation and development of large listric thrust faults; the thrust fault slipped in the salt layer and formed decollement structures and fault-related fold. At the same time, there formed duplex and reverse thrust faults between the two decollement layers.

Fault Characteristics in Longmen Mountain Thrust Belt, Western Sichuan Foreland Basin, China

Through field geological survey, the authors found that abundant thrust faults developed in the Longmen （龙门） Mountain thrust belt. These faults can be divided into thrust faults and strike-slip faults according to their formation mechanisms and characteristics. Furthermore, these faults can be graded into primary fault, secondary fault, third-level fault, and fourth-level fault according to their scale and role in the tectonic evolution of Longmen Mountain thrust belt. Each thrustfault is such as composed of several secondary faults, Qingchuan （青川）-Maowen （茂汶） fault zone is composed of Qiaozhuang （乔庄） fault, Qingxi （青溪） fault, Maowen fault, Ganyanggou （赶羊沟） fault, etc.. The Longmen Mountain thrust belt experienced early Indosinian movement, Anxian （安县） movement, Yanshan （燕山） movement, and Himalayan movement, and the faults formed gradually from north to south.

Characteristics of Nappes and Segmentation of the Longmen Mountains Thrust Belt, Western Sichuan Basin, China

In order to reveal the nature of the segmentation of Longmen Mountains Thrust Belt caused by the three nappes (Jiaoziding, Jiudingshan, and Baoxing Nappe), several methods are applied in this paper, including field investigation, seismic explanation and balanced crossed section, etc. Results of research reveal that nappes in Longmen Mountains vary in geometry, kinematics, and dynamics. Jiaoziding Nappe has generally behaved in a ductile manner, whereas Jiudingshan Nappe has been rigid, and the rheology of Baoxing Nappe has been intermediate between that of the other two nappes. The development of nappes has resulted in tectonic segmentation of Longmen Mountains: the main structural style of the northern segment is thrust faulting, with Jiaoziding Nappe representing a giant syncline. Given its ductility, it absorbed lots of stress, with the least amount of tectonic shortening in the SE part of the nappe. In the middle segment, the deformation is controlled by the rigid Jiudingshan Nappe, whose frontal area records lots of tectonic shortening. Deformation in the southern segment is intermediate in character between that of the other two segments, characterized by horizontal zonation, as demonstrated by fault development, and vertical stratification, which indicates that fault development was controlled by lithology.

龙门山冲断带构造特征研究主要进展及存在问题探讨

在前人研究成果基础上,对龙门山冲断带差异构造变形特征,构造几何样式,构造演化等几个方面进行了综述。研究表明,龙门山冲断带具有明显差异构造变形特征,即东西分带、南北分段、上下分层,同时发育大量构造变形样式,如断层相关褶皱、三角带、飞来峰、双重构造等等,在经历了多期复杂构造演化之后,构造变形样式具有多期叠加特征,最后提出详细地表构造地质建模、地震剖面精细解释及合理模型的建立,同时结合多种量化测试分析手段能够有效推进龙门山冲断带复杂构造样式与多期构造演化阶段分析研究工作,以及推动该地区的油气勘探。

龙门山构造转换带对油气成藏的控制作用

龙门山冲断带前缘地区发育大量油气田或含油气构造,主要分布在安县构造转换带和灌县构造转换带,通过野外地质调查和典型地震资料解析,结合沉积相分布特征,研究其构造带对油气成藏的控制作用。结果表明,安县构造转换带和灌县构造转换带具有较好的陆相碎屑岩储集层展布特征,并且具有较为有利的构造圈闭,即中浅部发育冲起构造型圈闭,深部发育与滑脱层相关的双重构造圈闭,因此安县构造转换带和灌县构造转换带具有较好的油气勘探前景。

龙门山冲断带滑脱构造变形样式

龙门山冲断带发育多套滑脱带,如寒武系、志留系、下三叠统嘉陵江组、中三叠统雷口坡组等。通过野外地质调查和地震剖面精细解释,认为在具有滑脱层的构造变形中,滑脱层本身发生强烈的挤压揉皱,使得上下地层发生显著不同的构造变形,深部的构造变形在纵向上形成两个构造体系,其中上部构造变形体系具有强烈的构造变形,形成大量的逆冲断裂带、背冲断块等构造变形样式,而下部构造变形体系则经历较弱的构造变形,地层保存完好,形成分层差异构造变形,地表可以见到特征明显的坡坪式断层、节理发育的断开与连续、逆冲断层的尖灭消失、构造虚脱和弯流褶皱、强烈的褶皱或者揉皱等。

川西龙门山冲断带构造油气藏类型及分布预测

野外地质考察和地震剖面解释表明川西龙门山冲断带具有复杂的构造地质特征,认为研究区发育类型众多的构造变形样式,主要有背冲断块、三角带构造、逆冲断层带和断块构造、断层相关褶皱构造以及双重构造等,不同类型的构造变形样式形成不同类型的构造油气藏。不同类型构造油气藏具有一定的分布规律,在平面上具有分带性,并与龙门山冲断带的主干断裂密切相关,如北川—映秀断裂与马角坝—通济场—双石断裂之间主要发育与断层相关的褶皱型油气藏;马角坝—通济场—双石断裂与广元—关口—大邑断裂之间主要发育三角带构造型油气藏以及一些断滑褶皱型油气藏,等等。在纵向上具有分层性,区域性滑脱层之上发育各种类型的构造油气藏,主要有背冲断块型油气藏、逆冲断裂带内的构造油气藏等,滑脱层之下发育双重构造型油气藏、断层相关及与背斜有关的构造油气藏。

龙门山冲断带前缘地区侏罗系出露与油气分布关系

龙门山冲断带前缘地区目前已发现的油气田或含油气构造主要分布在马角坝—通济场—双石断裂以东,统计分析结果表明52%分布在马角坝—通济场—双石断裂与广元—关口—大邑断裂之间(即前缘褶皱冲断带),其地表出露为侏罗系,其余则主要分布在广元—关口—大邑断裂以东的前陆坳陷带,地表为白垩系或第四系覆盖,认为侏罗系出露地区具有较好的油气勘探前景。通过研究区典型地震剖面解释,从构造变形角度解释这种油气分布特征的成因。

川西龙门山褶皱冲断带分带性变形特征

通过野外地质考察和地震资料解释,将龙门山褶皱冲断带划分为5个构造带,即青川-茂汶断裂以西为松潘-甘孜构造带,青川-茂汶断裂与北川-映秀断裂之间为韧性变形带,北川-映秀断裂与马角坝-通济场-双石断裂之间为基底卷入冲断带,马角坝-通济场-双石断裂与广元-关口-大邑断裂之间为前缘-褶皱冲断带,广元-关口-大邑断裂以东为前陆坳陷带,在构造变形特征上,各条断裂在演化上具有前展式特征,在松潘-甘孜构造带和韧性变形带构造变形强烈,形成推覆构造带等构造变形样式,在前缘-褶皱冲断带和前陆坳陷带,变形强度较弱,形成背冲断块或断层相关褶皱等构造,西北部区域的变形表现为塑性变形特征,向南东方向渐变为塑-脆性变形和脆性变形,在剖面上各条断裂所形成的深度向盆地方向逐渐递减。龙门山褶皱冲断带的分带性变形特征是由多种因素共同影响的结果,这些因素主要有板块构造背景的决定作用、多套滑脱层的控制作用和岩性因素的制约作用。

川西龙门山油气保存条件探讨

在对国内外推覆带油气研究现状调研的基础上,从构造形变、盖层及油水与油气保存关系等方面,对龙门山逆冲推覆带油气保存进行深入研究,总结油气成藏主控因素。在区带评价的基础上,指出江油—都江堰断裂以东保存条件好,有利于油气成藏。建立了龙门山逆冲推覆带构造形变保存模式,以及适应本区特征的评价技术方法和指标。据此预测石板滩背斜、鸭子河背斜为龙门山前有利勘探目标。

平衡剖面技术在龙门山中段构造解释中的应用

龙门山推覆冲断带地形条件恶劣,构造断裂复杂,阻碍了该区油气资源的勘探进程。为搞清龙门山推覆冲断带下的构造样式,验证其构造解释方案,应用平衡剖面技术,对龙门山中段具典型构造特征的地质、地震综合解释剖面进行平衡恢复。实例分析证明,平衡剖面技术可正确指导龙门山中段构造剧烈变形区的构造解释方案,并可定性、定量分析构造变形期次和变形程度,为下一步油气勘探提供可靠依据。

龙门山中段彭灌断裂带汶川M_S8.0地震同震破裂断层识别及其浅、表构造特征

2008年汶川M_S8.0地震在龙门山中段的彭灌断裂带产生的地表破裂,是该地震产生的第二大地表破裂带。综合应用地质、钻井以及二维、三维地震数据,利用横贯前山带的多条人工地震反射剖面,对彭灌断裂带产生同震破裂的断层进行准确识别和解释。研究表明,龙门山中段的彭灌断裂带是一套由3条主要断层和次级广泛发育的断裂组合构成,浅层表现为叠瓦构造样式,断裂之间发育小型双重构造。汶川M_S8.0地震在彭灌断裂带产生的地表破裂是由该断裂带前缘③号主断层活动产生的。彭灌断裂整体看上陡下缓,为轴面多次转折形成断坪一断坡式组合样式。分析认为彭灌断裂初步形成于晚三叠世末期,晚侏罗世之后大规模逆冲推覆,新生代继续活动,是多期次继承性断层。根据浅表地质主要断裂构造以及同震地表破裂特征,龙门山中段彭灌断裂带沿走向可分为白鹿和汉旺两个次级构造段。对龙门山中段彭灌断裂带的浅、表构造特征进行研究,可为进一步认识前山带主要断裂构造带提供一定的依据和约束。

龙门山冲断带分段机理研究

前陆冲断带具有巨大的油气勘探潜力,构造分段性控制油气分布。龙门山冲断带构造分段特征明显,但对分段性的形成机理缺乏深入研究。总结前人砂箱模拟实验成果认为:盆地基底差异和推覆带附近刚性体分布的差异,是造成冲断带构造分段的主要原因。龙门山前川西古拗拉谷的发现,为盆地基底存在差异提供了有利证据,古拗拉谷两侧发育大邑古隆起、江油——老关庙古隆起,在后期逆冲推覆过程中形成阻挡。通过对龙门山推覆带基底特征和刚性地体进行分析,结合前人模拟实验结果,明确提出龙门山冲断带构造分段机理:龙门山北段构造的形成是以碧口地块为动力,龙门山初始裂谷边缘古隆起形成阻挡,在古隆起上方形成冲断带;中段以彭灌杂岩体[彭县——灌县(都江堰)杂岩体]传递动力,在川西古拗拉谷坳陷部位,刚性体挤入盆地内部,印支期——喜马拉雅期,古坳陷部位继承性地发展为川西前陆盆地;南段以宝兴杂岩体传递动力,在大邑古隆起上方形成冲断带。

利用Walkaway VSP技术预测川西北深层碳酸盐岩储层

四川盆地川西北龙门山冲断带下二叠统海相碳酸盐岩勘探潜力巨大,SYS地区主要目的层栖霞组发育一套孔隙型白云岩储层,该套储层埋藏深、厚度小、非均质性强,导致钻前预测误差大,储层预测难度大。为此,结合钻井、测井、井中地震和三维地震资料,提出了井地联合提高钻前预测精度的思路。通过初至拾取、三分量旋转和共炮集的中值滤波、保幅SVD滤波得到高信噪比的反射波记录,由VSP直接测得的下行波提取子波进行已知子波反褶积压制多次波,进一步提高分辨率;然后基于零井源距VSP资料约束进行Walkaway VSP速度建模;最后采用基于射线追踪共接收点归位和相干叠加成像方法得到Walkaway VSP成像。通过约束反演和属性分析开展储层横向预测,取得了良好的效果。