引用本文

Cui Haifeng,Tian Lei,Zhang Nianchun,et al.Distribution characteristics of the source rocks from Cambrian Yuertusi Formation in the Southwest Depression of Tarim Basin[J].Natural Gas Geoscience,2016,27(4):577-583.[崔海峰,田雷,张年春,等.塔西南坳陷寒武系玉尔吐斯组烃源岩分布特征[J].天然气地球科学,2016,27(4):577-583.]
doi:10.11764/j.issn.1672-1926.2016.04.0577

塔西南坳陷寒武系玉尔吐斯组烃源岩分布特征

崔海峰 ,田雷,张年春,刘军,张继娟 

摘要  
塔西南坳陷寒武系玉尔吐斯组烃源岩分布及发育模式认识是油气成藏的关键因素。利用盆地大测线恢复震旦纪末盆地古地貌,结合盆地内钻穿寒武系的11口钻井揭示信息,对比塔西南坳陷与北部坳陷震旦系—寒武系地震相特征,认为震旦纪末巴楚—麦盖提斜坡地区抬升,沉积水体较浅,在基底之上沉积了一套潮坪沉积,以其作为碳酸盐岩枢纽带南北两侧的塔西南坳陷和北部坳陷均发育震旦系—寒武系;震旦系沉积之上往往有寒武系玉尔吐斯组沉积,玉尔吐斯组的沉积与基底古地貌有关,基底隆起高部位无沉积。研究认为塔西南坳陷原地有玉尔吐斯组烃源岩发育,在麦盖提斜坡分布局限,受控于早加里东期正断层形成的洼槽,坳陷中段裂陷槽内烃源岩最为发育;昆仑山前区发育盆地相寒武系烃源岩,分布于震旦系楔状体之上,可与满加尔凹陷烃源岩发育特征对比。

关键词 塔西南坳陷       震旦系—寒武系       玉尔吐斯组       烃源岩分布       正断层      

中图分类号:TE122.3      文献标志码:A      文章编号:1672-1926(2016)04-0577-07

Distribution characteristics of the source rocks from CambrianYuertusi Formation in the Southwest Depression of Tarim Basin

Cui Hai-feng ,Tian Lei,Zhang Nian-chun,Liu Jun,Zhang Ji-juan 

Abstract  
The distribution characteristics of the source rock from Cambrian Yuertusi Formation and its development pattern are key factors to reveal the process of oil and gas accumulation in the southwest depression of Tarim Basin.Based on the palaeogeomorphic form at the end of the Sinian resumed by 2D seismic data and the geological information from the eleven wells which drilled throughout Cambrian and the features of Sinian-Cambrian seismic facies of the Southwest Depression of Tarim Basin and the North Depression,it proposed that the lifting of Bachu Uplift and Maigaiti Slope at the end of Sinian resulted in a relatively shallow water deposition where there was a set of tidal flat sedimentary deposits on the basement of Tarim Basin,and Sinian-Cambrian deposited at its both sides named as Southwest Depression of Tarim Basin and North Depression.Generally,the Cambrian Yuertusi Formation deposited on the top of Sinian,and the high position was nondepositional area since the deposition of Yuertusi Formation was in connection with the basal sedimentary paleogeomorphology.The results obtained suggest that hydrocarbon source rocks of Yuertusi Formation developed in the southwest depression of the Tarim Basin.In the Maigaiti Slope,the distribution range of source rocks from Yuertusi Formation was limited and controlled by the trough formed by early Caledonian tensional fault,and the rifted trough in the middle of the Southwest Depression developed with great scale of hydrocarbon source rocks.Moreover,in the Kunlun Mountains,basin facies hydrocarbon source rocks,which distributed in the wedge shape of Sinian,also had more development,and its sedimentary environment could be well correlated with the Manjiaer Depression of Tarim Basin.

Key words Southwest Depression of the Tarim Basin;       Sinian-Cambrian;       Yuertusi Formation;       Hydrocarbon source rock distribution;       Normal fault;      

引言

塔西南坳陷是在塔里木板块前寒武纪结晶基底之上发育的新生代前陆坳陷[1]。早加里东期开始坳陷南缘靠近昆仑山发育寒武系—奥陶系碳酸盐岩古隆起,历经多期构造叠加改造,喜马拉雅晚期古隆起转化为南倾的大型斜坡。2000年以前,在巴楚隆起南缘的玛扎塔格构造带和巴什托普构造带发现了和田河气田和巴什托普油田,直至2011年中国石化矿区玉北区块获得油气发现,但后续勘探效果不好。2009年开始在麦盖提斜坡推动群古1井、玛南1井和玛东2井3口风险探井上钻[2],目的层钻探均失利,表明了该区油气成藏的复杂性和特殊性[3]。结合奥陶系已钻井的成功失利因素分析,认为奥陶系油气成藏受控于油气供给、构造调整和碳酸盐岩储层发育,其中寒武系烃源岩的分布是该区碳酸盐岩能否大规模成藏的关键,但因没有钻井直接揭示和露头出露目前还存在不同的认识。本文从塔里木盆地寒武系钻穿井分析入手,结合盆地二维大测线地震相分析,按照震旦纪末古地貌控制寒武系玉尔吐斯组沉积的思路,提出塔西南坳陷寒武系玉尔吐斯组烃源岩发育模式,旨在为该区油气勘探部署提供建议。

1 寒武系烃源岩研究进展

前人研究认为,塔里木盆地寒武系烃源岩发育在中下寒武统,主要分布在满加尔凹陷的欠补偿盆地相和中、西部的局限台地—蒸发潟湖相中,发育富氢的有机相和较高的有机质丰度[4-7]。满加尔凹陷烃源岩最为发育,在其周缘发现塔北轮南—塔河和塔中地区大油气田,岩性为灰质硅质泥岩与泥灰岩夹灰黑色放射虫页岩,是上涌洋流的沉积产物,有机质丰度高,且分布稳定。从有机质分析数据来看,库南1井、尉犁1井、塔东1井、塔东2井等中下寒武统欠补偿盆地相主要是一套含碳的硅磷黑色泥页岩建造,库南1井为泥质灰岩夹暗色灰质泥岩、页岩,尉犁1井、塔东1井为硅质泥岩、灰质泥岩、页岩,总有机碳含量(TOC)平均值分别为1.56%、2.28%、1.24%和1.93%;柯坪露头和星火1井的玉尔吐斯组岩性主要是一套磷质岩、硅质岩和黑色页岩,也是盆地边缘—欠补偿盆地相沉积,总有机碳含量(TOC)为7%~14%,平均为5.5%。其次是中、西部蒸发潟湖相的高有机质、高丰度烃源岩,主要分布在巴楚隆起西段和阿瓦提凹陷区域,夹于膏盐岩中,生烃母质主要为藻—球状甲藻生物。从勘探效果来看,目前在盆地中西部没有发现大的油气藏,说明中寒武统烃源岩生烃潜力比下寒武统差很多。下寒武统烃源岩主要分布在玉尔吐斯组,该套烃源岩特征上可与四川盆地寒武系的筇竹寺组烃源岩对比[8],是塔里木盆地台盆区油气成藏的主要来源,其分布关系到油气勘探区带的优选和部署。 塔西南坳陷寒武系烃源岩的分布问题,受研究资料的限制(没有钻井揭示和没有露头信息),目前认识不统一,存在有、无分布2种截然不同的观点。但从沉积相带展布来看,主要发育中下寒武统的斜坡—盆地相烃源岩,对应于满加尔凹陷烃源岩的发育特征,碳酸盐岩台地内(麦盖提斜坡区)可能有玉尔吐斯组分布,昆仑山前为盆地相烃源岩分布,因而玉尔吐斯组的分布对塔西南坳陷油气勘探前景至关重要。

2 寒武系玉尔吐斯组岩相古地理

寒武系玉尔吐斯组是塔里木盆地内一套重要的烃源岩,发育在寒武系内部,主要岩性分3段:底部为灰黑色含磷结核硅质岩、磷块岩夹薄层或透镜状白云质灰岩;中部为黑色炭质页岩及黄绿色、紫红色页岩夹砂岩、白云质灰岩;上部为灰白色薄层微晶白云岩、瘤状白云岩夹页岩[9]。厚度一般为8~35m。目前盆地内钻穿寒武系的钻井有和4井、方1井、库南1井、塔东1井、塔参1井、星火1井、中深1井、中深5井、巴探5井、玛北1井和同1井,其中库南1井、塔东1井、中石化星火1井及同1钻遇玉尔吐斯组,不过同1井岩性有相变(岩性为紫红色砂泥岩,下伏地层缺上震旦统),可见盆地内寒武系玉尔吐斯组的沉积不是广泛分布的。 塔里木盆地寒武系自下而上划分为下统玉尔吐斯组、肖尔布拉克组、吴松格尔组;中统沙依里克组、阿瓦塔格组;上统下秋里塔格组。玉尔吐斯组与下伏震旦系为不整合接触。塔中东部潜山上的塔参2井揭示中寒武统吴松格尔组之下为前震旦纪花岗岩,下寒武统和震旦系无沉积。中深1井和中深5井下寒武统肖尔布拉克组之下直接钻遇前震旦纪花岗岩,玉尔吐斯组无沉积;巴楚隆起南缘的巴探5井和玛北1井揭示下寒武统肖尔布拉克组之下直接钻遇前震旦纪变质岩,玉尔吐斯组无沉积;巴楚隆起北缘的和4井、方1井寒武系最新地层划分结果,下寒武统肖尔布拉克组之下为前震旦系,无玉尔吐斯组。钻井结果表明,从塔中东部潜山至巴楚隆起,玉尔吐斯组沉积前古地貌位置相对较高,这与巴楚隆起西北部的柯坪露头和塔北西部的星火1井揭示的玉尔吐斯组沉积环境明显不同(图1)。 早寒武世早期,在区域构造伸展背景之下,塔里木南北缘为被动大陆边缘发展时期,北部中天山板块与塔里木板块分离,致使海平面极速上升,形成了中部隆起地区为浅水台地,东部、西北部和南部三面环水盆地的沉积格局。玉尔吐斯组是寒武纪的第一次海进期,与全球性海平面上升、缺氧事件相对应。恢复震旦纪末期盆地古地貌,此时塔中东部—巴楚地区为基底古隆起,海侵沉积水体较浅,为潮坪沉积[10],局部高部位海水并未覆盖,与钻井揭示结果一致。在中部隆起的南北两侧为陆棚沉积,玉尔吐斯组沉积厚度相比较厚一些,如满加尔凹陷区和塔西南坳陷区。南部从巴楚隆起至麦盖提斜坡玉尔吐斯组的沉积界限现在不好界定,这也是认识分歧的原因。早寒武世中期,海水在塔中—巴楚地区的缓慢地上升,陆源碎屑物质减少,碳酸盐岩建造逐渐形成[11,12]。 此外,对比塔西南坳陷和满加尔凹陷区的震旦系分布,也可为早寒武世早期沉积格局的认识提供依据。在塔西南凹陷(昆仑山前)的地震剖面上,可以发现有一楔状沉积体,向北沉积减薄,一直以来对楔状体的地层归属有争议,通过塔里木盆地二维大测线的精细解释[图2(a),图2(b)],对比满加尔凹陷地震相特征,结合山前露头上震旦系分布特征,笔者认为该楔状体为震旦系。楔状体向北的尖灭部位上覆寒武系发育白云岩台缘相带,台缘带北部为碳酸盐岩台地[图2(c)],这与前述寒武纪早期沉积格局相一致,也很好地解释了震旦纪末到寒武纪的沉积格局演化,与前人认为寒武纪早期塔西南坳陷为隆起区的观点不同。

图1     塔里木盆地寒武系玉尔吐斯组岩相古地理
Fig.1     Cambrian Yuertusi Formation lithofacies paleogeographic map of Tarim Basin

图2     中下寒武统沉积模式示意(a、b图位置见图1)
Fig.2     The sketch map showing the Lower Cambrian sedimentary model(profile position of Picture a、b in Fig.1 )

3 玉尔吐斯组烃源岩分布特征

利用塔西南坳陷覆盖的二维地震和盆地大测线,对震旦系和寒武系进行精细构造解释,玉尔吐斯组在塔西南坳陷的分布主要有2个部分(图3):其一是昆仑山前部位的盆地相沉积,在震旦系楔状体沉积之上;其二是麦盖提斜坡部位的台地内分布,受早加里东期活动的张性正断层形成的洼槽或者震旦纪裂谷活动控制,整体上呈北东向分布,与塔西南坳陷基底断层的分布一致。

图3     寒武系玉尔吐斯组烃源岩发育模式
Fig.3     The map showing the Cambrian Yuertusi Formation hydrocarbon source rock development pattern

早加里东期正断层形成于震旦纪—寒武纪的区域伸展背景,在塔北、塔中地区也发育该期正断层[13,14],主要有以下特征:一是断层活动没有断穿上寒武统,有断层后期反转成为逆断层持续活动;二是断层活动的断距普遍较小,形成一系列北东向展布为主、成带发育的小型洼槽,其中坳陷中段可能发育北东向展布的震旦系裂陷槽[15];三是少数正断层具有同沉降特征,控制中下寒武统沉积。从坳陷西段mx03-560和mx05-570二维地震剖面解释来看(图4),正断层活动控制着中下寒武统沉积,基底古地貌高部位中下寒武统沉积减薄或无沉积,基底或震旦系古地貌控制寒武系沉积;从坳陷中段mx10-556二维地震剖面解释来看[图5(a)],寒武系之下有震旦系发育,震旦系之下有一套反射层,夹持于正断层之间,笔者认为是震旦纪裂陷槽的反映,这套反射层可能为南华系,厚度超过了400m。从xn04-04二维测线来看[图5(b)],裂陷槽呈北东走向延伸直至昆仑山前,与山前的大洋相连通。对比四川盆地,在晚震旦世—早寒武世早期继承性发育“德阳—安岳”古裂陷槽,控制下寒武统烃源层分布[8]。目前塔西南坳陷中段地震测线分布较稀,后续随着地震部署和资料品质的提高,将有助于震旦系裂陷槽的深入认识。

图4     塔西南坳陷西段地震剖面(剖面位置见图1)
Fig.4     The seismic lines of western part in the southwest depression of Tarim Basin(profile position in Fig.1)

早加里东期正断层走向与塔西南坳陷磁异常图上的若干北东向高低相间的磁异常带相对应,磁异常特征(图6)与巴楚隆起高磁异常带不协调分布,反映了基底的磁性特征,基底断层也呈北东向分布。前人结合重力异常资料对塔西南坳陷的基底性质进行了讨论,认为塔西南坳陷北东向高低相间的磁异常不可能只是界面起伏变化引起,可能为一个残存的古老洋壳[16],海底扩张时由于地磁场的倒转,形成由正反向磁化的磁性岩体组成的洋壳,现今表现为正反磁异常,在前震旦纪的中、晚元古代,演化为克拉通部分。磁异常特征揭示可能发育的古洋壳与前述地震资料反映的震旦系裂陷槽有一定关联,是

图5     塔西南坳陷中段地震剖面(剖面位置见图1)
Fig.5     The seismic lines of middle part in the southwest depression of Tarim Basin(profile position in Fig.1)

图6     塔里木盆地剩余磁异常[16]
Fig.6     The residual magnetic anomaly map of the Tarim Basin[16]

有必要深入讨论的问题。一旦坳陷中部震旦系裂陷槽得以证实,其必然控制寒武系玉尔吐斯组烃源岩发育,将在坳陷中段与寒武系—奥陶系白云岩储层[17-20]构成“下生上储”的良好源、储组合。

4 结论

(1)早寒武世早期(玉尔吐斯组沉积期),塔中东部—巴楚隆起古地貌较高,沉积水体较浅,局部高部位海水未覆盖,形成中部浅水台地,周缘环水盆地的沉积格局。玉尔吐斯组的分布与震旦系或前震旦系古地貌有关,一般有震旦系沉积区域均有玉尔吐斯组发育,古地貌高部位玉尔吐斯组无沉积。隆起南北两侧玉尔吐斯组沉积稍厚一些。 (2)塔西南坳陷有玉尔吐斯组烃源岩分布,主要发育在早寒武统斜坡—盆地相中。其中碳酸盐岩台地内部(麦盖提斜坡区)分布局限,受早加里东期活动的张性正断层形成的洼槽或者震旦纪裂陷槽控制;靠近昆仑山前带的盆地相中发育较好烃源岩,可与满加尔凹陷烃源岩发育特征对比。 (3)由于缺钻井和露头数据,目前塔西南坳陷内玉尔吐斯组烃源岩的丰度较难评价,只能是一种推测。坳陷中段震旦纪裂谷控制下的烃源岩可能最为发育,要结合基底演化进一步认识,同时中段发育寒武系—奥陶系构造枢纽带和白云岩潜山、内幕储层,是有利的勘探区带,部署钻井也有助于证实震旦纪裂陷槽和寒武系烃源岩的存在。

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