Structure characteristics and deep hydrocarbon significance of Nanhua rift in central area of Sichuan Basin

  • Shunqi CHAI , 1 ,
  • Chuanxin LI , 1 ,
  • Congsheng BIAN 2 ,
  • Tian GAO 1 ,
  • Zheng SUN 2
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  • 1. School of Energy Resources,China University of Geosciences (Beijing),Beijing 100083,China
  • 2. Research Institute of Petroleum Exploration and Development,PetroChina,Beijing 100083,China

Received date: 2023-01-02

  Revised date: 2023-02-23

  Online published: 2023-07-13

Supported by

The National Natural Science Foundation Enterprise Innovation and Development Joint Fund Project(U19B6003-01)

Highlights

Affected by the breakup of Rodinia supercontinent, rift basins were widely developed in the Yangtze block, North China block and Tarim block during the Nanhua Period. The existence of deep rift basins played an important role in controlling the late tectonic evolution of the basin and the distribution of deep oil and gas. The Sichuan Basin is located in the western margin of the Yangtze block. Because there are few wells in the basin that reveal the stratigraphic characteristics of the Nanhua System, the research on the structural characteristics and genetic mechanism of the Nanhua rift in the Basin is relatively low. This paper takes the Nanhua rift in central area of Sichuan Basin as the research object, uses the method of drilling and seismic combination to analyze the geometric characteristics of the rift basin, recover the kinematic process, and preliminarily explore the genetic mechanism of the rift basin under the constraints of the regional tectonic background. The research reveals that: (1)NE trending narrow and long rifts dominated by extensionism developed in Nanhua Period in the central area of Sichuan Basin, the boundary of which is mostly “fault overthrust” or “double fault”, The internal horst graben type and ladder type faults are developed. (2)The tectonic evolution of the rift valley is divided into three main stages: the opening stage of the rift basin, the synsedimentary stage and the sedimentary overlap stage. The formation and evolution of the Nanhua rift valley has an obvious control effect on the structural deformation of the overlying strata, and then affects multiple sets of source-reservoir associations and oil and gas distribution in the basin. (3)Based on the results of previous petrogeochemical studies, it is comprehensively believed that the Nanhua rift in Sichuan Basin developed in the environment of continental crust thinning after plate subduction and collision, showing the nature of intracontinental rift. The fault activity under the extensional tectonic background is the key factor affecting the development of Cambrian carbonate hydrothermal solution reservoir and the formation of natural gas reservoir.

Cite this article

Shunqi CHAI , Chuanxin LI , Congsheng BIAN , Tian GAO , Zheng SUN . Structure characteristics and deep hydrocarbon significance of Nanhua rift in central area of Sichuan Basin[J]. Natural Gas Geoscience, 2023 , 34(7) : 1203 -1217 . DOI: 10.11764/j.issn.1672-1926.2023.03.001

0 引言

新元古代早期,Rodinia超大陆解体背景下发生了全球性裂谷事件,中国华南古陆在该阶段发生的拉张裂陷普遍伴有强烈的火山喷发活动1-4,在此拉张背景下,发育四川盆地南华纪裂谷、鄂湘桂裂谷、康滇裂谷和秦岭裂谷5-9
前人对四川盆地南华纪裂谷结构进行了许多探索性研究,谷志东等10-11综合应用地震与航磁资料,首次发现盆内川中地区新元古代发育八支伸展构造。杨志如等12通过重、磁、电、震资料解译初步勾画了川中高石梯—磨溪地区与南充—遂宁地区前震旦纪裂谷的分布特征,揭示川中地区深部发育地垒—地堑结构。在此基础上,管树巍等13利用三维地震解析在高石梯—磨深区域识别出南华系底界有一套NE向断续延伸的强振幅波组,证实川中裂谷的存在,而重新处理的地震影像识别出川中裂谷由通江—达州延伸至资阳—威远地区。盆内其他区域如“广安—达州—万源”与“剑阁—绵阳—资阳—雅安”一带存在的裂陷及沉积加厚区也逐步得以发现13-15
现今对于四川盆地内部存在南华纪裂谷已达成共识,但受限于盆内未有南华系出露,钻井资料少,对于裂谷的结构、分布仍有待进一步研究。本文以地震资料为基础,结合钻井数据和前人重磁电研究资料,明确川中地区南华纪盆地地质结构,开展区域剖面平衡恢复和构造运动学研究,恢复其演化过程,结合前人岩石学分析探讨裂谷成因,以期指导四川盆地南华系油气勘探部署。

1 区域背景与地层特征

四川盆地隶属扬子地块,西抵龙门山,东至齐耀山,北至米仓山—大巴山,南至大凉山。四川盆地在前震旦系结晶基底上发育了震旦纪—中三叠世海相沉积和晚三叠世—第四纪的陆相沉积,是一个大型叠合盆地(图1)。盆地外围有勉略—城口断裂分隔盆地与米仓山、大巴山构造;向北进入南秦岭造山带腹地,以商丹缝合带与北秦岭造山带分隔。现有研究成果表明,前震旦纪四川盆地处于克拉通背景。其北部、西北部为被动大陆边缘盆地,周缘是原特提斯洋体系,西部为川西—滇中裂谷盆地,南部为滇黔桂陆缘海盆地16
图1 扬子地区构造单元

Fig.1 Tectonic unit of the Yangtze region

1.1 扬子地块区域构造背景

扬子地区构造活动复杂,主要经历了晋宁—澄江期(前震旦纪)复杂基底的形成阶段、桐湾期(震旦纪—早寒武世)隆坳构造形成与裂陷填平补齐阶段、加里东期大面积隆升与剥蚀阶段、海西期持续隆升与剥蚀阶段,并在印支期初步呈现盆地雏形,后经喜马拉雅运动盆地全面褶皱形成了现今的构造面貌17-22。新元古代早期(900~700 Ma,相当于青白口纪—南华纪),全球Rodinia超大陆发生解体,中国华南古陆也相应发生裂解,形成了众多裂谷带,且该阶段发生的拉张裂陷普遍伴有强烈的火山喷发活动,裂陷内主要充填了一套火山岩—沉积岩23,其中形成于陆内的一些裂陷持续发育到南华纪—震旦纪或转化为沉降坳陷。南华纪—震旦纪扬子地块处于拉张背景,上扬子地区发育大规模的南华纪裂谷,四川盆地周缘发育东南缘鄂湘桂裂谷、西缘康滇裂谷、北缘秦岭裂谷3个大型南华纪裂谷2224。震旦纪区域性大陆裂谷作用结束,进入克拉通盆地演化阶段25。震旦纪灯影组沉积期—早寒武世,上扬子地区长期处于拉张背景,发育桐湾3幕运动2126-29,构造运动以垂直升降为主。灯影组沉积中晚期,台地普遍抬升遭受剥蚀。早寒武世中期,上扬子克拉通构造格局发生重大转变,垂直升降运动幅度明显减小,地貌上的隆坳格局差异逐渐消失。
前人对四川盆地的基底特征进行过很多探索性研究,认为四川盆地的基底具有双重性,即:①太古代—早元古代发育的康定群经中条运动形成距今1 700~1 900 Ma 的深部结晶基底;②中元古代—新元古代发育的峨边群经晋宁运动形成距今780~1 700 Ma 的褶皱基底30,之后沉积较厚的南华系盖层。

1.2 扬子区域南华系地层特征

裂谷内部南华系以火山碎屑岩、砂岩、泥岩、冰碛岩等为主要充填特征。扬子板块基底主要由新元古界组成,青白口系顶部花岗岩发育,与南华系的碎屑岩沉积差异明显,最新的地层划分方案把新元古界划分成震旦系、南华系及青白口系31-32。国际上最新划定南华系年龄为720~635 Ma,并将原南华系底部莲沱组划出南华系,本文研究依然采用南华系四分方案,即地层从老到新依次为莲沱组、古城组、大塘坡组及南沱组。
扬子地区是新元古界冰川沉积较发育的地区之一,一般认为扬子地区新元古界经历了古城期和南沱期2次冰期,对应全球的Sturtian和Marinoan冰期33-34。上扬子区南华系在桂北和黔东南地区发育最为完整,自东南往西北方向,地层缺失明显。鄂西宜昌地区、黔中瓮安、遵义等地仅发育南沱组。而在四川中部、北部地区,如乐山、峨嵋、南江洋坝,以及旺苍干河坝等地,南华系完全缺失,震旦系直接覆盖在冰期前地层之上35。扬子新元古界裂谷由一系列地堑—地垒式次级盆地组成,裂谷地层位于震旦系与四堡造山不整合面之间,区域上侧向延伸不连续,呈“楔状地层”展布。盆内南华系未出露,前人主要对盆外展开野外勘测,将盆缘裂谷分为3个区,分别是扬子北缘的秦岭—大别山分区、西缘康滇分区、东南缘湘渝黔分区(图2)。
图2 扬子地区南华系地层划分与对比

Fig.2 Stratigraphic division and correlation of the Nanhua System in the Yangtze region

其中扬子北缘的秦岭—大别山分区南华系出露较全,主要岩性为冰碛砾岩、砂泥岩和少量粉砂质炭质页岩。湘渝黔分区地层命名不一,如贵州东北缘主要为南沱组、大塘坡组、铁丝坳组、两界河组,主要岩性为冰碛砾岩、炭质页岩及碎屑岩。西缘康滇分区以沉积巨厚的苏雄组双峰式火山岩为特征36

2 裂谷结构特征

川中地区仅有5口井钻穿沉积盖层进入基底,分别为老龙场构造的老龙1井,威远构造的威117井、威28井与威15井以及龙女寺构造的女基井15。本文研究主要以威117井、老龙1井、五探1井为约束,对过威远地区地震剖面进行精细解析,识别裂谷的剖面结构特征与平面展布。

2.1 裂谷剖面结构特征

对钻井、地震资料展开分析,威117井钻遇盆地前震旦系基底处测井曲线显示电阻率值极高且幅度差大,深侧向电阻率(RLLd)与浅侧向电阻率(RLLs)曲线明显分离代表有岩浆侵入。自然伽马值高,自然电位曲线平直,反映基底为花岗岩。威117井过地震剖面A—A′北段(测线位置见图1)表现为强振幅、低频反射,同向轴断续分布,具有低角度斜交、杂乱反射特征(图3)。通过四川盆地航磁解释资料以及在威117井、威28井和女基井取得的基岩同位素资料表明盆地基底皆钻遇花岗岩,威28井基岩年龄为740.99 Ma36-38,女基井基底岩石年龄为701.54 Ma39,属于裂谷外沉积。
图3 威117井花岗岩反射特征与测井曲线(剖面位置见图1中A—A′)

Fig.3 Granite reflection characteristics and logging curve of Well Wei117 (the profile position is shown in Fig.1 A-A′)

老龙1井位于川西南老龙场构造,过测线A—A′南段,震旦系灯影组直接与前震旦系基底花岗岩接触,缺失震旦系陡山沱组和南华系(图4)。老龙1井钻遇基底在地震剖面上表现为低频强振幅反射特征,同向轴断续分布,具杂乱反射特征(图4)。基底测井曲线表现为高自然电位、中高伽马值,其中自然电位为50~60 μs/m,自然伽马值为100~240 API,放射性元素钍(Th)、铀(U)含量高。指示裂谷外沉积环境。
图4 老龙1井花岗岩反射特征与测井曲线(剖面位置见图1中A—A′)

Fig.4 Granite reflection characteristics and logging curve of Well Laolong 1(the profile position is shown in Fig.1 A-A′)

五探1井位于四川开江檀木场,构造位置为川北低陡褶皱带与川东高陡褶皱带的过渡部位,过测线B—B′(图1)钻遇震旦系陡山沱组,沉积岩性主要为粉砂质泥岩、泥质粉砂岩,南华系南沱组为一套泥岩,其下还未分层,但结合地震剖面来看,五探1井陡山沱组厚度大于400 m,在地震上反射连续,成层性好,与其下伏南华系反射特征一致,指示同为南华纪裂谷内部沉积(图5)。五探1井陡山沱组沉积序列与川东北城口陡山沱组相吻合,故认为城口地区与盆内裂谷同源40-42
图5 五探1井南华系地震反射特征与测井曲线(剖面位置见图1中B—B′)

Fig.5 Seismic reflection characteristics and logging curve of Nanhua System in Well Wutan1 (the profile position is shown in Fig.1 B-B′)

通过井—震结合和利用断层波阻识别等方法精细解析3条过威远隆起的NW—SE向地震大剖面(剖面位置见图1),揭示四川盆地威远隆起两侧各发育一个裂谷,西支为资阳—遂宁裂谷,东支为自贡—内江裂谷。
C—C′剖面上威远隆起两翼近乎对称,呈穹隆状。威远两侧裂谷在靠近隆起一翼地层抬升减薄,地震上可识别出超覆现象,边界断层较缓;而在远离隆起一侧,裂谷边界断层陡,地层沉积加厚,为典型的“断超式”裂谷结构。威117井位于威远隆起构造高部位,未钻遇南华系,基底为花岗岩。资阳—遂宁裂谷此处宽105 km,深1~2 km,内部识别出断面波,断层未断穿上覆灯影组底界面,呈“地堑式”发育。自贡—内江裂谷宽80 km,深度为1.5~2.5 km。在裂谷内地震反射明显且较连续,外部反射杂乱(图6)。
图6 C—C′剖面裂谷结构解释(剖面位置见图1)

Fig.6 Interpretation of rift structure of profile C-C′ (the profile position is shown in Fig.1)

D—D′剖面位于C—C′南侧,同样受控于威远隆起影响,裂谷靠近隆起一端断层较缓,远离隆起端断层较陡。资阳—遂宁裂谷在此位置宽75.7 km,深度达1~1.5 km,内部铲式正断层切割震旦系,偏移量相对较小,沉积厚度变化不大。表明裂谷内部断层是震旦系下的同向断层,边界呈“西断东超”断裂模式。自贡—内江裂谷宽65.2 km,深度达1.5~3 km,比资阳—遂宁裂谷沉积更深,内部铲式正断层发育,靠近威远隆起处地层超覆减薄,地震上识别断层同相轴错断明显,波阻弱呈空白反射。凹陷中心地层成层性好,反射强烈。自贡—内江裂谷东西边界断裂较陡,为“双断式”(图7)。
图7 D—D′剖面裂谷结构解释(剖面位置见图1)

Fig.7 Interpretation of rift structure of profile D-D′ (the profile position is shown in Fig.1)

E—E′剖面位于上述2条剖面南部,威远隆起两侧裂谷的形态与D—D′相似。资阳—遂宁裂谷在该处宽65.3 km,深度达1~1.5 km。东边界缓,地层抬升减薄、超覆现象明显,内部断层的断面波向上传导,影响上覆地层的褶皱样式。边界断层容易识别,裂谷外空白反射明显,表现为“断超式”边界断裂模式。自贡—内江裂谷宽44.3 km,深度为1.5~2 km,裂谷内地层较裂谷外部地震反射轴强,成层性好,裂谷内地震反射同相轴错断明显,断裂处相位轻微突变,坡度及厚度变化不大,“铲式”正断层发育,边界断裂呈“双断式”发育(图8)。
图8 E—E′剖面裂谷结构解释图(剖面位置见图1)

Fig.8 Interpretation of rift structure of profile E-E′ (the profile position is shown in Fig.1)

整体来看,裂谷内地震反射特征表现为强振幅、中高频率、地震同相轴连续性好,纵向上反射界面仍然较清晰,反映了这些地层的成层性好,为沉积岩的反射特征。而裂谷外沉积区,包括古隆起位置,基底特征与川西地区相似,地震反射特征表现为连续性较差、振幅能量弱,可能为板溪群以及早期的火山岩体。川中地区南华纪裂谷具有以伸展构造体系为主、垒—堑结构发育、向古隆起侧超覆的剖面结构特征。

2.2 裂谷平面展布特征

通过前期对地震剖面上裂谷的边界断裂及内部结构展开精细刻画,以钻井钻遇南华系情况对裂谷边界范围进行约束,认为川中地区南华纪主要发育北东走向的裂谷(图9)。
图9 川中地区南华纪裂谷平面

Fig.9 Map of Nanhua rift in central area of Sichuan Basin

本研究重点解剖川中裂谷,该裂谷在威远处分为2支,西支为资阳—遂宁裂谷,东支为自贡—内江裂谷。裂谷呈北东向展布,深入大巴山腹部城口地区,沿断裂走向来看,长度几乎贯穿盆地纵轴约为400 km,宽度约为50~110 km,主体为双断式,断陷中央为地堑结构,两侧地层发育不对称,呈西断东超式,最大沉积厚度约为3 km。简要统计成都—广元裂谷宽80~130 km,深2 km,南华系厚300~500 m,裂谷整体为双断式,南部受威远古隆起影响伴有超覆现象,内部发育一系列垒—堑结构。
川东裂谷深部地震影像较为模糊,隐约能识别其裂谷西边界与华蓥山断裂相近,深部断裂对晚期隆凹格局有控制作用,裂谷中央为大型地堑,两侧发育阶梯式垒—堑结构。

3 裂谷构造沉积演化

3.1 盆内外地层对比特征

分析盆内川中地区老龙1井、威117井、蓬探1井、五探1井钻井资料,结合前人3235对盆缘露头地层的研究,进行川西—盆内—川东北(图10)地层对比。整体表现出冰期在研究区的沉积响应为古城组、南沱组沉积。其中在南秦岭一带,古城组沉积以大套砾岩夹灰岩透镜体为特征,砾石粒径大、分选差、磨圆较好,具定向排列。南沱组则以灰绿色泥质砂岩、含砾砂岩为主。向湖北—重庆—贵州过渡,南沱组顶部发育一层盖帽白云岩,冰期砾石变小,间冰期大塘坡组的含锰页岩发育。盆地东北缘南华系沉积厚度大,尤其是南沱组冰碛砾岩发育,大塘坡组薄。盆地西缘南华系被喷发相的苏雄组火山岩及上覆三角洲相的列古六组火山碎屑岩充填,厚度达2 km。整体来看盆缘南华系沉积厚度明显大于盆内,且盆内五探1井处南华系沉积厚度最大。
图10 扬子地区露头与盆内钻井地层对比

Fig.10 Stratigraphic correlation between outcrop and well drilling in the basin in the Yangtze region

3.2 构造演化特征

四川盆地位于中上扬子地块,裂谷基底演化从广泛残存的早元古代结晶基底到中、新元古代褶皱基底,再到从南华纪至现今的相对稳定盖层沉积,表明它历经太古宙—古元古宙多块体离散拼合的克拉通化到新元古代晋宁Ⅱ期与华夏地块的碰撞拼合,而后进入相对稳定沉积阶段。新元古界裂陷沉降及裂谷体的充填是裂谷盆地演化的主体阶段,它以裂陷沉积、块体差异沉降及火山作用为特征43-44
由于川中裂谷演化序列较为完整,故以此为例阐述其构造演化特征。
川中资阳—遂宁裂谷为“断超式”,现今深度大于1 km,整体发育于板溪群之上,依次沉积莲沱组、古城组、大塘坡组、南沱组、陡山沱组。其中基底板溪群是一套变质岩;莲沱组沉积厚度最大,是一套厚层砂岩夹砾岩;古城组和南沱组沉积期与全球2次大的冰期事件相对应;大塘坡组沉积期为两者之间的间冰期。裂谷整体表现为西低东高的斜坡式沉积(图11)。根据不同时期沉积特征可将“资阳—遂宁裂谷”划分为3个主要演化阶段:前沉积期(裂谷开启时期)、同沉积期、后沉积期(沉积超覆期)。整体具有“断超”的构造样式,裂谷西边界断裂活动强于东边界。
图11 川中地区南华纪裂谷构造演化模式

Fig.11 Structural evolution model of Nanhua rift in central area of Sichuan Basin

南华纪之前(即板溪群沉积期),地震剖面揭示裂谷边界断裂已经开始活动。板溪群沉积末期,裂谷内部表现为两侧高、中间低的构造地貌特征,沉积中心近裂谷裂陷中心,前沉积期(裂谷开启时间)应不晚于板溪群沉积末期。
南华纪沉积阶段,裂谷处于同沉积期。莲沱组沉积期裂谷持续发育,边界断裂持续活动,同时内部伴生次级断裂,沉积凹陷中心略微向西迁移,整体表现为“向东超覆”的沉积特征;古城组到南沱组沉积期裂谷继承性发育,裂谷持续拉张加深,继承莲沱组沉积期的构造格局和沉积特点,内部次级断裂控制沉积格局,向东断裂活动逐渐减弱。
进入震旦纪,陡山沱组沉积初期继承南沱组沉积晚期构造格局,该时期断裂活动较弱,整体表现为填平补齐,沉积中心逐渐消失;灯影组沉积时期,沉积范围扩大到裂谷外,并逐渐向裂谷两侧超覆,裂谷内部次级断裂发生继承性活动并引起上覆地层褶曲变形。裂谷的发育经历青白口纪的拉伸作用,南华纪仍处于未拉开状态接受沉积,边界持续扩张,属于同沉积裂谷。早期受构造活动影响发育的断裂,在南华纪再活化生长,影响上覆地层的沉积。
裂谷整体表现为莲沱组沉积期基底接受沉积,古城组沉积时期全球进入冰期,沉积物受冰川荷载沉降于莲沱组之上,后来冰期结束,过渡到大塘坡组沉积期,气候回归温暖,冰川消融,海平面上升,裂谷内部接受细粒物质沉积,沉积厚度小,又迎来南沱期大冰期,冰碛岩覆盖于大塘坡之上。南华系各组之间未见不整合面。南沱期大冰期结束,进入陡山沱期,海平面再次下降,裂谷接受威远隆起、五探1井两处构造高部位物源供给。

4 讨论

多期幕式裂陷导致四川盆地成为一个复杂、具有多套源储组合的含油气系统。经晋宁—四堡运动形成的前震旦系裂谷基底对晚期桐湾运动、加里东运动形成的构造形态具有控制作用34。南华纪裂谷边界断裂控制上覆地层沉积特征和晚期变形。下文将基于此开展相关讨论。

4.1 裂谷成因探讨

目前对于川中地区南华纪裂谷的成因简要概括为地幔柱型和被动成因型。地幔柱型裂谷盆地主要是指由于深部热地幔上升44,热柱、热地幔羽在岩石圈的底部上拱、快速的加热减小岩石圈的密度,导致区域性的均衡上隆,上隆作用派生的拉张应力造成裂谷作用。而被动裂谷盆地是岩石圈首先出现拉张、破裂和变薄45-46,地壳的隆升和火山作用是随后出现的,裂谷作用是拉伸作用的响应。
裂谷形成机制与其大地构造背景息息相关。华南中元古代多块体在Rodinia大陆聚合过程中,通过新元古代早期和中期不同板块拼合形成新元古代中期统一的古华南大陆板块。在825 Ma之后的新元古代中晚期,华南古陆转入陆内裂解阶段47。在Rodinia超大陆裂解的动力学构造背景下,形成了以华南浙赣湘桂为中心的南华裂谷盆地,伴有相应的裂谷型岩浆活动。
对Rodinia地壳块体地质历史的分析表明,外围增生造山带的发育和板块俯冲是南华纪裂谷作用的主控因素,俯冲驱动了其内部的岩石圈伸展,大多数岩浆活动可能是由于伸展作用导致的减压熔融20。结合前人对威117井石英二长岩主微量元素含量、全岩Sr—Nd同位素和原位锆石Lu—Hf同位素的分析也得到与之相似的结论。测得威117井石英二长岩样品与扬子地区周围的同期镁铁质岩浆无线性相关性,其具有低Mg值和高Rb/Sr值,很可能来源于陆壳36-38。因此认为川中地区南华纪裂谷形成于板块俯冲碰撞的伸展环境,属于陆内裂谷。

4.2 裂谷对上覆地层的影响

南华纪裂谷形成于拉张背景。标定层位后的地震剖面(图12)揭示裂谷边界断裂顶端附近的地层表现为低缓背斜的变形特征,该套低缓背斜发育于灯影组到中寒武统;与之对应的浅部地层则发育典型的高陡褶皱带;深部双断裂控制的地垒结构在深部发育宽缓低幅背斜,浅部则发育宽缓高陡背斜。约中寒武世前,在裂谷边界断裂持续活动和地层压实作用的双重作用下,灯影组发生差异沉积,在断裂发育处,上覆地层褶皱形成低缓背斜构造;晚寒武世—晚侏罗世地层稳定沉积构造变形较弱;白垩纪以来,受陆内造山作用影响,寒武系膏盐岩以上地层发生滑脱变形形成侏罗山式褶皱,与先存断裂构造具有垂向耦合性。
图12 过沪州—綦江裂谷地震剖面及裂谷解释方案(剖面位置见图1中C—C′东段)

Fig.12 Seismic profile and rift interpretation scheme across Luzhou-Qijiang rift (the profile location is shown in Fig.1 eastern C-C′)

南华纪裂谷边界断裂控制上覆地层沉积古地理格局,部分边界断裂晚期依然具有较强的活动性。地震剖面解释结果揭示,断裂活动引起灯影组差异沉积,在断层两盘形成差异沉降,形成向裂谷内部倾斜的小型坡折带,灯影组晚期逐渐超覆于坡折带之上(图13)。
图13 过成都—中江裂谷地震剖面及裂谷解释方案(剖面位置见图1中F—F′)

Fig.13 Seismic profile and rift interpretation scheme across Chengdu-Zhongjiang rift (the profile location is shown in Fig.1 F-F′)

南华纪裂谷边界断裂控制晚期地层沉积中心位置分布。资阳—遂宁裂谷灯一段拉平剖面揭示裂谷的边界断裂和规模控制上覆地层沉积范围和沉降中心[图14(b)],裂谷西边界处引发灯三段超覆形成小型坡折带,东2断裂引起上覆地层微隆,控制震旦系灯影组—寒武系沧浪铺组沉积中心。受东1边界控制,在早三叠世沉积中心进一步扩大。
图14 过资阳—遂宁裂谷地震剖面及裂谷解释方案(a)与灯一组拉平后裂谷结构对上覆地层影响(b)(剖面位置见图1中G—G′)

Fig.14 Seismic profile and rift interpretation scheme across Ziyang-Suining rift(a) and influence of rift structure on overlying strata after leveling of Z2 d 1 Formation(b),(the profile location in shown in Fig.1 G-G′)

4.3 深层油气意义

在新元古界Rodinia超大陆裂解背景下,四川盆地演化序列最为完整,先后发育裂谷盆地、被动陆缘盆地和前陆盆地3种盆地充填序列48-50,形成了二叠系须家河组致密油系统,寒武系、志留系海相页岩气系统,震旦系碳酸盐岩含气系统。研究区南华系大塘坡组形成于气候温暖湿润的间冰期,含锰页岩发育,南华纪裂谷的形成和发育对上覆地层沉积和构造活动存在控制作用。因此南华纪不仅本身具有油气勘探潜力,对晚期含油气系统的构造圈闭识别也具有重要意义,有待进一步深化研究。

5 结论与认识

以川中地区南华纪裂谷为研究对象,采用钻井—地震结合的方法,展开裂谷盆地几何学特征解析、运动学过程恢复、并在区域构造背景约束下初步探讨裂谷盆地成因机制。取得主要认识如下:
(1)基于二维地震解析和钻井分析,识别出川中地区发育NE向的南华纪裂谷,以伸展体系为主,边界多呈“断超式”或“双断式”,内部垒—堑结构、铲式断层发育。
(2)裂谷构造演化分为裂谷盆地开启时期、同沉积期和沉积超覆期3个阶段。南华纪以前裂谷已具雏形;南华纪裂谷持续扩张并接受沉积;震旦纪沉积初期继承南沱期构造格局,断裂活动减弱,该阶段整体表现为填平补齐。南华纪裂谷的形成演化对上覆沉积地层的构造变形具有明显控制作用,影响盆内多套源储组合与油气分布。
(3)结合前人岩石地球化学研究以及裂谷盆地所处的区域构造背景,认为裂谷发育在板块俯冲碰撞后的陆壳减薄环境,表现为陆内裂谷性质。
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