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Article Navigation >  Acta Sedimentologica Sinica  > 2025  >  43(1): 182-197

YANG FengYing, ZHANG YunFeng, ZENG Qi, PAN QingChuan, FAN CunHui, LI YiWei, YIN Hong, ZHANG BenJian. Formation and Types of Tuff Reservoirs: A case study of Permian Wujiaping Formation at Daping section, northwestern Sichuan Basin[J]. Acta Sedimentologica Sinica, 2025, 43(1): 182-197. doi: 10.14027/j.issn.1000-0550.2023.011
Citation: YANG FengYing, ZHANG YunFeng, ZENG Qi, PAN QingChuan, FAN CunHui, LI YiWei, YIN Hong, ZHANG BenJian. Formation and Types of Tuff Reservoirs: A case study of Permian Wujiaping Formation at Daping section, northwestern Sichuan Basin[J]. Acta Sedimentologica Sinica, 2025, 43(1): 182-197. doi: 10.14027/j.issn.1000-0550.2023.011
shu

Formation and Types of Tuff Reservoirs: A case study of Permian Wujiaping Formation at Daping section, northwestern Sichuan Basin

doi: 10.14027/j.issn.1000-0550.2023.011
  • 1.

    School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China

  • 2.

    Branch of Chuanxibei Gasfield, Southwest Oil and Gasfield Company, PetroChina, Jiangyou, Sichuan 621700, China

  • 3.

    Academy of Science and Technology Development, Southwest Petroleum University, Chengdu 610500, China

  • 4.

    BGP Southwest Geophysical Branch, PetroChina, Chengdu 610000, China

  • 5.

    Exploration and Development Research Institute, Southwest Oil and Gasfield Company, PetroChina, Chengdu 610041, China

Funds:

National Natural Science Foundation of China 41702122

Science and Technology Cooperation Project of the CNPC-SWPU Innovation Alliance 2020CX010301

  • Received Date: 2022-11-09
  • Accepted Date: 2023-03-19
  • Rev Recd Date: 2023-02-13
    • Available Online: 2023-03-19
  • Publish Date: 2025-02-10
  • Objective Marine carbonates of the Permian Wujiaping Formation commonly occur in tuffs in northwest Sichuan. The tuff beds vary in thickness, and therefore have different natural gas storage capacities. The Wujiaping Formation at the Daping section is 45.02 m thick, and subdivided into two members. Individual tuff beds are 5⁃55 cm thick. Measurements of geochemical parameters were conducted on tuff samples, taking their reservoir types into account. Methods A synthetic database of microfacies was developed for major and trace elements and zircon U-Pb using XRD and scanning electron microscopy （SEM）. Results The tuffs are mainly volcanic dust composed of glass chips smaller than 0.05 mm. The mineral composition of the tuffs is mainly illite and illite/montmorillonite mixtures, and they contain small amounts of quartz and gypsum. The Wu 1 member contains a high Al2O3/TiO2 ratio （23.36-34.56）, a low SiO2/Al2O3 ratio （1.84-2.75） and obvious negative anomalies of Eu, Ba, Sr and Ti. The Wu 2 member has a high Al2O3/TiO2 ratio （21.140-53.049）, a low SiO2/Al2O3 ratio （1.829-2.682） and a moderately negative Eu anomaly, with strong depletion of elements such as Nb and Ti. Conclusions These element profiles indicate that the volcanic debris has undergone strong secondary alteration. The tuffs were sourced from multiple acid volcanic eruptions in Emeishan large igneous province, and were formed in an intraplate extensional environment. The Wu 2 member tuffs were also affected by crust-mantle mixing. Reservoir space types include cavities, irregular dissolution pores, intra- and intergranular dissolution pores, organic matter pores, fractures, etc. The U-Pb age of hydrothermal zircon is dated at 132.37±0.53 Ma. The tuff reservoirs have also been influenced by various other factors such as Lower Cretaceous hydrothermal fluid and tectonic activity.
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  • Received:  2022-11-09
  • Revised:  2023-02-13
  • Accepted:  2023-03-19
  • Published:  2025-02-10

Formation and Types of Tuff Reservoirs: A case study of Permian Wujiaping Formation at Daping section, northwestern Sichuan Basin

doi: 10.14027/j.issn.1000-0550.2023.011
  • 1. School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
  • 2. Branch of Chuanxibei Gasfield, Southwest Oil and Gasfield Company, PetroChina, Jiangyou, Sichuan 621700, China
  • 3. Academy of Science and Technology Development, Southwest Petroleum University, Chengdu 610500, China
  • 4. BGP Southwest Geophysical Branch, PetroChina, Chengdu 610000, China
  • 5. Exploration and Development Research Institute, Southwest Oil and Gasfield Company, PetroChina, Chengdu 610041, China
Funds:

National Natural Science Foundation of China 41702122

Science and Technology Cooperation Project of the CNPC-SWPU Innovation Alliance 2020CX010301

  • Received Date: 2022-11-09
  • Accepted Date: 2023-03-19
  • Rev Recd Date: 2023-02-13
  • Available Online: 2023-03-19
  • Publish Date: 2025-02-10

Abstract: Objective Marine carbonates of the Permian Wujiaping Formation commonly occur in tuffs in northwest Sichuan. The tuff beds vary in thickness, and therefore have different natural gas storage capacities. The Wujiaping Formation at the Daping section is 45.02 m thick, and subdivided into two members. Individual tuff beds are 5⁃55 cm thick. Measurements of geochemical parameters were conducted on tuff samples, taking their reservoir types into account. Methods A synthetic database of microfacies was developed for major and trace elements and zircon U-Pb using XRD and scanning electron microscopy （SEM）. Results The tuffs are mainly volcanic dust composed of glass chips smaller than 0.05 mm. The mineral composition of the tuffs is mainly illite and illite/montmorillonite mixtures, and they contain small amounts of quartz and gypsum. The Wu 1 member contains a high Al2O3/TiO2 ratio （23.36-34.56）, a low SiO2/Al2O3 ratio （1.84-2.75） and obvious negative anomalies of Eu, Ba, Sr and Ti. The Wu 2 member has a high Al2O3/TiO2 ratio （21.140-53.049）, a low SiO2/Al2O3 ratio （1.829-2.682） and a moderately negative Eu anomaly, with strong depletion of elements such as Nb and Ti. Conclusions These element profiles indicate that the volcanic debris has undergone strong secondary alteration. The tuffs were sourced from multiple acid volcanic eruptions in Emeishan large igneous province, and were formed in an intraplate extensional environment. The Wu 2 member tuffs were also affected by crust-mantle mixing. Reservoir space types include cavities, irregular dissolution pores, intra- and intergranular dissolution pores, organic matter pores, fractures, etc. The U-Pb age of hydrothermal zircon is dated at 132.37±0.53 Ma. The tuff reservoirs have also been influenced by various other factors such as Lower Cretaceous hydrothermal fluid and tectonic activity.

YANG FengYing, ZHANG YunFeng, ZENG Qi, PAN QingChuan, FAN CunHui, LI YiWei, YIN Hong, ZHANG BenJian. Formation and Types of Tuff Reservoirs: A case study of Permian Wujiaping Formation at Daping section, northwestern Sichuan Basin[J]. Acta Sedimentologica Sinica, 2025, 43(1): 182-197. doi: 10.14027/j.issn.1000-0550.2023.011
Citation: YANG FengYing, ZHANG YunFeng, ZENG Qi, PAN QingChuan, FAN CunHui, LI YiWei, YIN Hong, ZHANG BenJian. Formation and Types of Tuff Reservoirs: A case study of Permian Wujiaping Formation at Daping section, northwestern Sichuan Basin[J]. Acta Sedimentologica Sinica, 2025, 43(1): 182-197. doi: 10.14027/j.issn.1000-0550.2023.011
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0.   引言

  • 凝灰岩是火山喷发过程中产生细粒碎屑物质,通过风或者水的搬运作用及固结压实作用形成的一类火山碎屑岩,因其在大气中滞留的时间较短（1~3年）,且横向上具有连续性,是极好的地层对比标志层[1],其纵横向分布亦可约束火山活动时限及规模[2⁃3]。四川盆地及华南地区晚二叠世凝灰岩广泛分布[4],成分以伊利石、伊蒙混层、高岭石等黏土矿物为主,含少量石英、黄铁矿等[5⁃6]；主要分布于四川盆地的西北部、西部、南部[7⁃9]、重庆南川[10]等。前人基于锆石U-Pb年龄、矿物组分、微量元素分析等,认为上二叠统凝灰岩成因与峨眉山大火成岩省酸性火山活动[11⁃12]、华南板块西南缘古特提斯二叠纪大陆岩浆弧[13⁃14]、西伯利亚大火成岩省相关的基性火山活动[15]等相关,并广泛用于火山喷发活动时间及期次的确定。

    受沉积及成岩作用的影响,凝灰岩中普遍存在不同尺度原生孔隙及次生孔隙,前者包括收缩孔、晶间孔等,其发育程度受岩性及火山喷发旋回的控制[16⁃17],后者包括溶蚀孔洞、粒内溶孔、粒间溶孔、有机质孔、脱玻化孔、裂缝等,发育程度受热液作用、脱玻化作用、多期溶蚀、破裂作用[18⁃19]等的影响。国内目前已在四川盆地[20]、三塘湖盆地[21]、鄂尔多斯盆地[22⁃23]等多个地区发现凝灰岩储层及油气藏。近年来,川西北地区S101、元坝7、YS1等井的上二叠统凝灰岩发现规模凝灰岩孔隙型、溶洞型、裂缝型储层及工业气流[24⁃26],显示了较大的储集能力及天然气工业产出价值；与此同时,由于钻井实物资料的缺乏,该区凝灰岩成因、储层形成机制等缺乏系统研究。

    选取出露条件好、特征典型的川西北大坪剖面二叠系吴家坪组凝灰岩作为研究对象,基于剖面实测及密集采样,开展了岩石学、地球化学等方面的测试,获取了凝灰岩发育位置、厚度、矿物类型、微量元素、锆石U-Pb年龄、不同尺度储集空间类型等数据,明确了凝灰岩的岩石类型及矿物组成、成因、不同岩石类型的储集性等,以期为川西北地区上二叠统凝灰岩成因、分布、储集性及天然气勘探提供支撑。

1.   地质背景及剖面特征

  • 川西北大坪剖面构造上位于四川盆地的西北部川北坳陷低缓处的龙门山断层北段的东缘（图1a）,地理位置位于旺苍县双汇镇大坪村西河边公路一侧（图1b；起点坐标30°40′33.79″ N,104°03′32.39″ E,终点坐标30°40′33.58″ N,104°03′32.85″ E）,剖面顶底界线清楚,无植被覆盖。受中—晚二叠世峨眉地裂运动影响,整个四川盆地及云南部分地区发育厚度巨大的玄武岩[11,27⁃29]（图1a）,并在四川盆地表现为中下部铁玄武岩、顶部酸性火山岩的火山序列[30]。晚二叠世吴家坪阶在川西北称为“吴家坪组”,为一套浅海相碳酸盐岩夹凝灰岩地层[30],自下而上可分为吴一段—吴三段三个岩性段,碳酸盐岩中珊瑚、海百合、有孔虫等普见,具备一定的造礁能力；四川盆地及周缘吴家坪期凝灰岩在各段均有不同程度的分布,主要分布在吴一段—吴二段,呈多个薄层状产出（图1c）[31],具多个形成年龄,如宾川地区峨眉山火山序列酸性熔结凝灰岩年龄为259.1±0.5 Ma[32]和259.2±0.3 Ma[33],广元朝天剖面吴家坪组凝灰岩年龄为260.1±2.8 Ma[8]等锆石U-Pb年龄,指示了峨眉山大火成岩省的火山活动的多阶段性。

    Figure 1.  Location map of northwestern Sichuan and Wujiaping Formation stratigraphic profile at Daping section

    川西北大坪剖面二叠系共实测22层,其中第1~7层为茅口组,第8~22层为吴家坪组（厚45.02 m）。剖面呈近南北走向、南西西倾向、倾角40°左右,分布稳定。岩性以泥晶灰岩、生屑泥晶灰岩、泥质泥晶灰岩、泥岩为主,夹薄层凝灰岩,共识别出12层凝灰岩,单层厚度5~54 cm（图1d）。

2.   样品采集及分析方法

  • 基于野外剖面实测,对该剖面进行了分层,描述了各层岩石类型、沉积构造、颜色等。对凝灰岩发育层位、层厚、颜色等进行了详细描述,采集凝灰岩样品22件（DP1~22,采样位置见图1d）,制作薄片22件,全岩及黏土矿物含量、主量元素、微量元素、扫描电镜各20件,锆石U-Pb年龄样品11件。

    薄片制备执行石油天然气行业标准《岩石薄片制备方法》（SY/T 5913—2021）；显微成分、组构及储集空间观察采用偏光显微镜（Zeiss AXIO SCOPE.A1）；超微成分、组构鉴定及能谱分析采用场发射环境扫描电镜（Quanta650FEG）；主量元素测试采用电感耦合等离子体发射光谱仪（PE5300V）；全岩及黏土矿物测定采用X射线衍射仪（Ultima IV）,电压为40 kV,电流为40 mV,取样品磨成粉末放在载物台进行XRD分析,全岩矿物鉴定与含量计算采用矿物特征衍射峰值和K值法[34],黏土矿物具体步骤为取样品磨成粉末,用蒸馏水进行脱气,再进行沉淀和离心法分离小于2 μm的颗粒,接着对黏土矿物样品在空气干燥、乙二醇溶剂化24 h和在490 °C下加热2 h后,进行三次XRD测试[35],X射线衍射数据处理使用软件HighScore在乙二醇曲线上进行,不同物质的X射线吸收指数不一样,黏土矿物吸收指数较低,黏土矿物的相对含量主要使用（001）晶面衍射峰的面积比,伊利石使用1 nm（001）晶面,蒙脱石使用1.7 nm（001）晶面,高岭石（001）和绿泥石（002）采用0.7 nm叠加峰,计算出来的矿物相对含量误差约为5%[34]。关键矿物的鉴定依据单晶矿物卡片库与实测谱图对比,一一对应就能检索出样品的全部物相；微量元素、稀土测试均采用电感耦合等离子体质谱仪（Aglient Technologie s7700 Series）,测试方法与文献[36]相同；以上测试在西南石油大学油气藏地质及开发工程国家重点实验室和天然气地质四川省重点实验室完成。

    锆石U-Pb年龄测试过程包括锆石样品挑选、清洗、热液锆石分选、测试等步骤：首先将凝灰岩样品清洗晾干、全岩碎样和锆石挑选,挑选出晶形完好和色泽度较好的锆石用环氧树脂固定于样品靶上,样品靶抛光后在超纯水中超声清洗至表面可能的污染除去；其次对锆石进行阴极发光图像观察及拍照,根据热液锆石无明显震荡环带、晶形通常为半自形—他形、其棱线也不显著等特征[37⁃38]挑选出热液锆石；最后以NIST610作为外标、91Zr作为内标,采用激光剥蚀—电感耦合等离子体质谱仪（Resolution SE+Agilent7900）进行年龄测试；数据处理用Iolite程序[39],协和图绘制采用Isoplot R程序[40],同位素比值误差为2σ,年龄采用207Pb/235U和206Pb/238U比值和年龄；锆石U-Pb年龄测试在南京宏创地质勘查技术服务有限公司微区分析实验室完成。

3.   凝灰岩岩石学及分布特征

  • 通过野外剖面实测,将第8~9层划分为吴一段,第10~20层划分为吴二段,第21~22层划分为吴三段,根据取样点位置（图1d）,将DP1~6,DP11~18,DP20~22称为吴一段凝灰岩样品,将DP7~10,DP19称为吴二段凝灰岩样品。

    大坪剖面二叠系吴家坪组中凝灰岩除第8层厚度相对较大（54 cm）外,其他层厚度一般小于20 cm,夹于灰岩地层之间,展布稳定。颜色呈灰黑色、土黄色、灰绿色等,且在同一层内可见多种颜色变化,单层顶底多与临近地层颜色相近,可能是受多种蚀变或成岩作用影响所致（图2a1~c1,a2~c2）。薄片、扫描电镜及XRD测试结果表明,吴一段凝灰岩样品的矿物组成以黏土为主（69.9%~88.0%）（图3a）,同时存在数量不等的石英（3.2%~16.3%）、钾长石（0~7.2%）、黄铁矿（0~12.5%）、半水石膏（0~8.7%）、黄钾铁矾（2.0%~19.4%）、金红石（0.6%~2.3%）、石膏（0~1.9%）（图3c）等矿物（表1）；黏土矿物以伊利石（22.9%~73.7%）、伊/蒙混层（26.3%~77.1%）为主,伊/蒙混层比中蒙皂石层和伊利石层均为10%和90%（表2）；吴二段凝灰岩样品的矿物组成以黏土为主（32.9%~87.5%）（图3b）,同时存在数量不等的石英（5.3%~10.0%）、钾长石（0~3.8%）、斜长石（0~1.8%）、方解石（0~61.4%）、半水石膏（0~2.5%）、黄钾铁矾（0~6.2%）、金红石（0.4%~1.1%）等（表1）；黏土矿物以伊利石（0~71.8%）、伊/蒙混层（28.2%~100%）为主,伊/蒙混层比中蒙皂石层和伊利石层均为10%和90%（表2）。凝灰岩可分为晶屑凝灰岩、玻屑凝灰岩及火山尘凝灰岩,晶屑凝灰岩中晶屑类型主要为长石、石英晶屑等,粒径一般介于0.2~0.5 mm；玻屑灰岩中玻屑多呈气泡的浮岩状,部分呈镰刀状,粒度一般介于0.1~0.8 mm；火山尘主要由粒径小于0.05 mm玻屑组成,可含少量晶屑、玻屑。大坪剖面吴家坪组凝灰岩以火山尘凝灰岩为主,火山尘显微镜下整体为深灰色、尘点状,可见脱玻化作用的不均匀斑团以及形成微晶长石、石英等（图2a3,b3,c3）。矿物学以富含伊/蒙混层和伊利石的黏土为主,含量一般大于70%；长石、石英以晶屑、孔洞充填物、脱玻化作用产物等存在；黄铁矿、石膏、方解石等以溶蚀孔洞、裂缝中充填物存在（图2a1~a3）；黄钾铁矾系黄铁矿氧化分解后的次生矿物,可能是暴露期形成。

    Figure 2.  Field characteristics of tuffs at Daping section

    Figure 3.  Scanning electron microscopy and energy spectra of tuffs

    层位样品编号石英钾长石斜长石方解石白云石黄铁矿半水石膏黄钾铁矾金红石石膏黏土矿物
    吴一段DP16.0000001.84.61.0086.6
    DP216.300007.202.02.1072.4
    DP37.7000002.119.40.9069.9
    DP45.3000001.64.11.0088.0
    DP59.76.90001.203.50.7078.0
    DP67.53.100005.08.02.3074.1
    DP115.0000003.17.91.1082.9
    DP124.90000007.80.7086.6
    DP134.4000008.75.00.61.979.4
    DP148.11.900001.52.81.0084.7
    DP158.40000005.81.3084.5
    DP164.57.20003.23.710.30.7070.4
    DP173.200002.72.76.30.9084.2
    DP204.8000012.53.56.70.8071.7
    吴二段DP76.401.84.70006.21.0079.9
    DP85.30061.400000.4032.9
    DP910.0002.900001.0086.1
    DP105.43.801.90003.01.1084.8
    DP196.2000002.53.10.7087.5

    Table 1.  Whole⁃rock mineral composition and content of Permian tuffs at Daping section（%）

    层位样品编号黏土矿物含量/%伊/蒙混层比/%绿/蒙混层比/%
    蒙脱石伊利石高岭石绿泥石伊/蒙混层绿/蒙混层蒙皂石层伊利石层蒙皂石层绿泥石层
    吴一段DP1073.70026.301090——
    DP2061.70038.301090——
    DP3059.50040.501090——
    DP4062.30037.701090——
    DP5065.60034.401090——
    DP6056.30043.701090——
    DP11041.50058.501090——
    DP12047.50052.501090——
    DP13022.90077.101090——
    DP14026.40073.601090——
    DP15030.70069.301090——
    DP16029.20070.801090——
    DP17043.70056.301090——
    DP20045.40054.601090——
    吴二段DP7000010001090——
    DP8000010001090——
    DP9000010001090——
    DP10043.00057.001090——
    DP19071.80028.201090——

    Table 2.  Clay types and contents of Permian tuffs at Daping section (%)

    为分析研究剖面凝灰岩与邻近区域分布的共性及差异性,结合钻井岩屑鉴定及电性曲线特征,建立了钻井与野外剖面凝灰岩分布图（图4）。可以看出,层位上,凝灰岩均分布在吴一段、吴二段,垂向上发育6~10层,反映了凝灰岩分布的时限性；厚度上,大坪剖面西侧单井的凝灰岩单层厚度（1~8 m）及累积厚度均较大,与凝灰岩物源来自西南侧相吻合,大坪剖面和邻近的铁炉坝剖面凝灰岩单层厚度显著减薄；横向上,凝灰岩各层整体具有比较好的可对比性。

    Figure 4.  Stratigraphic correlation of typical tuffs at wells in northwest Sichuan

4.   凝灰岩元素地球化学特征及成因

    4.1.   全岩主量元素特征

  • 大坪剖面凝灰岩全岩主量元素变化较大（表3）,吴一段凝灰岩烧失量介于9.010%~14.454%；SiO2含量介于46.185%~55.748%,大多数在50%以上；Al2O3的含量介于20.26%~26.51%；SiO2/Al2O3介于1.84~2.75；TiO2的含量0.723%~1.069%；全碱（Na2O+K2O）含量介于4.802%~6.561%；TiO2/Al2O3介于0.028 9~0.045 6,Na2O/K2O介于0.008 7~0.056 5。吴二段凝灰岩样品烧失量介于9.521%~22.354%；SiO2含量介于32.677%~55.352%,大多数在50%以上；Al2O3的含量介于17.52%~25.47%；SiO2/Al2O3为1.829~2.682；TiO2的含量介于0.367%~1.204%；全碱（Na2O+K2O）含量介于3.375%~6.221%；TiO2/Al2O3介于0.019~0.047；Na2O/K2O介于0.015 5~0.040 3。凝灰岩主量元素TiO2-SiO2图解[41]（图5）显示,吴一段、吴二段凝灰岩数据均在火成岩区域,指示大坪剖面凝灰岩主要为火成岩特性。

    层位样品编号SiO2K2ONa2OCaOMgOAl2O3TFe2O3MnOTiO2P2O5LOITotal
    吴一段DP150.8665.990.0521.2502.83324.771.960.0020.8680.08010.77499.445
    DP255.7484.620.1820.2312.95620.264.890.0020.7490.0539.48899.179
    DP352.0375.940.0530.9513.33025.471.580.0020.7370.0889.02899.216
    DP448.7805.930.1220.7523.23526.512.060.0031.0690.10810.34198.910
    DP555.6325.170.1901.1193.02420.382.640.0020.7580.04410.39699.355
    DP650.3205.650.1690.7942.88224.162.010.0181.0010.24911.66998.922
    DP1150.8596.200.1160.7202.59624.583.260.0031.0520.3129.39598.093
    DP1252.2844.920.0580.7843.08422.352.430.0150.7230.06612.50099.214
    DP1348.7746.000.0651.8812.73424.493.940.0010.8800.14210.51599.422
    DP1450.2055.420.0822.3112.95622.373.800.0030.6730.12911.50899.457
    DP1549.2545.670.1670.7792.89325.012.040.0030.9770.29111.76898.852
    DP1651.1726.120.0600.5952.98025.402.860.0020.8150.1919.01099.205
    DP1746.1855.050.1910.9132.99522.116.610.0030.7470.06114.45499.319
    DP1849.2515.360.0550.9703.31222.942.410.0020.8760.05813.90099.134
    DP2047.3526.210.3511.0522.58723.156.860.0071.0570.06411.540100.231
    吴二段DP755.3524.670.1850.5742.26920.631.850.0120.8240.09813.35899.831
    DP853.2924.640.1702.1912.28025.471.250.0080.6410.2179.52199.680
    DP932.6773.270.10520.661.56717.520.930.0340.3670.10322.35499.584
    DP1051.7214.890.0761.4102.46024.883.880.0030.4690.0319.91699.736
    DP1946.5475.980.2412.8652.99525.451.860.0041.2040.04813.201100.412

    Table 3.  Content of major elements in whole⁃rock Permian tuffs at Daping section (%)

    Figure 5.  TiO2 vs. SiO2 content in igneous and sedimentary rocks (after reference [41])

    • 4.2.   微量及稀土元素特征

  • 根据微量元素数据绘制了原始地幔标准化微量元素蛛网图（图6a）[42⁃43],可知大部分样品大离子亲石元素处于波谷,有明显的Ba、Sr、Ti元素负异常。元素Ba、Sr的负亏损与斜长石有关,这是由于岩浆分离结晶作用过程中斜长石的强分离结晶作用[44]。吴二段样品DP9的Sr处于波峰,显示Sr的正异常和Nb、Ti负异常,DP10显示Nb、Ti负异常,可能源区受到地壳、地幔混合作用。

    Figure 6.  (a) Spidergram of primitive mantle⁃normalized trace elements; (b) chondrite⁃normalized REE patterns

    球粒陨石标准化稀土元素配分模式图（图6b）[42⁃43]显示,吴一段凝灰岩样品具明显的Eu负异常（Eu/Eu*值介于0.165~0.422）、无Ce异常。这是由于斜长石能对Eu分配,在岩浆分离结晶过程中,斜长石的大量晶出将导致残余熔体中形成明显Eu负异常[44],因此凝灰岩中Eu负异常可以认为与火山作用有关。（La/Sm）N值介于1.17~5.58,平均为3.46；（La/Yb）N值介于4.64~26.99,平均为12.97,（Gd/Lu）N值介于1.66~5.16,平均为3.24。稀土元素配分图显示吴一段所有样品的配分模式相似,总体表现为LREE富集、HREE亏损。吴二段样品显示具Eu负异常（Eu/Eu*值介于0.41~0.77）；（La/Sm）N值介于0.57~2.45,平均为1.21,其中DP9、DP10样品的（La/Sm）N值均小于1；（La/Yb）N值介于0.38~4.49,平均为1.86,DP9、DP10样品的（La/Yb）N值小于1,（Gd/Lu）N值介于0.95~1.37,平均为1.23。因此,吴二段DP9、DP10表现为LREE亏损、HREE平缓,DP8表现与吴一段相似的配分模式。

    • 4.3.   凝灰岩来源

  • 高场强元素（Nb、Ti、Zr、Ta、Hf、Sc、Th）具有抗蚀变性及Al2O3/TiO2的比值可以用来解释岩浆的来源[45],基性、中性、酸性岩浆Al2O3/TiO2值分别介于3~8、8~21、21~70[46]。大坪剖面吴一段凝灰岩样品Al2O3/TiO2值介于23.36~34.56（图7a）[6,11,42,47⁃50],吴二段凝灰岩样品Al2O3/TiO2值介于39.70~53.04,均高于峨眉山玄武岩,范围在酸性岩浆21~70内,与宾川酸性熔结凝灰岩、峨眉山流纹岩、上寺剖面下部凝灰岩相似,表明大坪剖面吴家坪组凝灰岩可能与酸性火山喷发有关。凝灰岩在蚀变和风化过程中,表现为Si的释放和Al的累积,因此SiO2/Al2O3用以指示火山玻璃转化为黏土矿物过程中受风化和蚀变作用影响的程度[51],大坪剖面吴一段样品SiO2/Al2O3含量介于1.84~2.75,吴二段样品SiO2/Al2O3介于1.829~2.682,明显低于宾川酸性熔结凝灰岩样品的比值（5.47~6.31）[47],表明其受到蚀变、风化程度较高。

    Figure 7.  (a) Al2O3/TiO2⁃Eu/Eu*; (b) Zr⁃Ti (base map from reference [52]); (c) Zr/Sc⁃Th/Sc (base map from reference [53]); (d) Nb/Y⁃Zr/TiO2-4 (base map from reference [54])

    Zr-Ti判别图（图7b）[6,11,42,47⁃50,52]中,大坪剖面吴一段、吴二段凝灰岩样品落在板内岩浆内,表明其与板内岩浆地球化学特征具有相似性；Zr/Sc-Th/Sc图解（图7c）[6,11,42,47⁃50,53]中,大坪剖面吴一段凝灰岩样品与宾川酸性熔结凝灰岩、峨眉山流纹岩、上寺剖面下部凝灰岩沿峨眉山大火成岩省成分变化线分布,吴二段凝灰岩样品也邻近峨眉山大火成岩省成分变化线,暗示其与这几者具有相同的来源；在Nb/Y-Zr/TiO2图解（图7d）[6,11,42,47⁃50,54]中,吴一段凝灰岩样品点大多落入响岩之间,说明其来源为酸性岩；吴二段凝灰岩样品落在流纹岩中,其原岩可能为中酸性流纹岩；进一步通过Rb、Sr、Nb、Ta、Nd、Th、Zr、Hf等具有源区指示意义的微量元素对吴二段样品进行源区分析,地壳的Nb/Ta平均值为11,地幔为17.8[55⁃56]；地壳的Rb/Sr大于0.5,地幔值小于0.05,壳幔混合为0.05~0.5；地壳的Nd/Th约为3[57],地幔则高于15[58]；地壳的Zr/Hf为33,而地幔的为37[55⁃56]。吴二段凝灰岩样品中Nb/Ta介于8.59~22.04,平均值为13.95,平均值介于地壳值和地幔值；Rb/Sr介于0.04~1.93,平均值为0.92,平均值更接近地壳值；Nd/Th介于0.17~1.30,平均值为0.57,平均值与壳源接近；Zr/Hf介于31.12~36.58,平均值为34.69,样品值既有接近地壳值,也有接近地幔值,表明吴二段样品为壳幔混合特征。

    在Y+Nb-Rb判别图中（图8a）,吴一段、吴二段凝灰岩样品均落在板内区域；在Yb+Ta-Rb判别图中（图8b）,吴一段、吴二段凝灰岩样品也均落在板内区域；在Y-Nb判别图中（图8c）,吴一段凝灰岩样品落在板内区域,吴二段落在板内与洋脊交界处、洋脊区域；在Yb-Ta判别图中（图8d）,吴一段凝灰岩样品落在板内区域,吴二段凝灰岩样品落在板内与洋脊交界处,由此显示吴一段、吴二段凝灰岩显示出与形成于板内伸展环境下花岗岩类岩石相似的特征。

    Figure 8.  Discriminant diagrams for tuff tectonic environments[59]

    综上,大坪剖面吴一段、吴二段凝灰岩源岩区为中酸性火山岩,但吴一段、吴二段凝灰岩的微量、稀土元素配分模式表示两者之间有一定的差异性,暗示两者的成因存在一定的差异。吴一段凝灰岩微量元素、稀土元素均与宾川酸性熔结凝灰岩、峨眉山流纹岩、上寺剖面下部凝灰岩特征相似,指示了这几者为同时同源产物,因此认为吴一段凝灰岩可能是峨眉山大火成岩省酸性火山活动的产物。吴二段凝灰岩样品Zr-Ti图中显示与上寺剖面下部凝灰岩、峨眉山大火成岩省相关岩石具有相似特征（图7b）,且样品沿峨眉山火成岩成分变化线分布（图7c）,指示该凝灰岩也可能是峨眉山大火成岩省酸性火山活动的产物,形成于板内伸展环境；但其微量元素和稀土元素与吴一段凝灰岩有不同的特征,微量元素表示富集Rb、Th、U,亏损Nb、Ti以及Eu弱亏损,可能受到大陆地壳的混染[60],稀土元素球粒陨石标准化配分模式呈较平坦的左倾型,与洋中脊岩石相似,且在Y-Nb、Yb-Ta判别图（图8c,d）中样品投在板内和洋中脊交界处,暗示其源区可能是DMM源的岩浆经分离结晶作用和受地壳混染的分离结晶混染作用[61]；Nb/Ta、Rb/Sr、Nd/Th、Zr/Hf等值也表明样品主要为壳幔混合作用的成因,对应的峨眉山大火成岩省酸性火山作用与壳幔混合有关。据此推测,吴家坪组吴一段、吴二段凝灰岩是峨眉山大火成岩省不同期火山作用的产物,均形成于板内拉张环境,但吴二段凝灰岩形成可能是DMM源的岩浆受到地壳物质的混合作用。

5.   凝灰岩储集空间类型及成岩作用

    5.1.   凝灰岩储集空间类型

  • 凝灰岩自身可存在晶间孔等原生孔隙,受成岩作用改造可形成各类溶蚀孔隙[16,18⁃19,62]。根据野外观察、薄片及扫描电镜等鉴定,在研究剖面凝灰岩中识别出洞、不规则溶孔、粒内溶孔、有机质孔、粒间溶孔、裂缝等储集空间类型。洞主要见于第8层,洞径3 cm×5 cm~30 cm×47 cm,呈不规则状,未充填或黄铁矿、石英、石膏、锆石等充填（图2a1~a3、图9a）,多与裂缝相伴生,形成缝洞复合体；从洞的形态、充填物判断其成因与埋藏期来自深部的热液溶蚀作用相关。裂缝为流体主要运移通道,形成沿缝分布的大小不等的不规则扩溶洞,部分被黄铁矿、石英、石膏等热液矿物充填。不规则溶孔一般常与裂缝相伴生,溶孔多被硅质、方解石等充填（图2a3、图9d,f）,发育较为普遍,第8、14、16、20层凝灰岩中均可见。粒内溶孔主要为长石晶屑、伊利石、石膏等粒内溶孔,多呈圆形—椭圆形、蜂窝形或不规则状,孔径细小,多呈微米级（图9g）。有机质孔系有机质生烃演化过程中体积收缩或者被破坏而形成的孔隙（图10h,i）,孔隙细小,常密集分布。粒间溶孔多形成于黏土、石英长石、石膏、黄铁矿等晶粒间,多为多边形或者不规则状,为纳米—微米级孔隙,矿物边缘部分具港湾状溶蚀边（图9j~l）。裂缝可见构造缝、扩溶缝,前者较细小平直,局部呈密集分布,宽一般小于100 μm（图2a3~c3、图9b,d,e）；扩溶缝系构造缝基础上溶蚀扩大形成,相较于构造缝张开度明显增大,可至厘米级,缝边缘不规则,港湾状溶蚀边显著（图9b,c,e,f）；裂缝未充填或硅质、方解石充填。

    Figure 9.  Types of tuff reservoir space at Daping section

    Figure 10.  Zircon chondrite⁃normalized REE patterns

    • 5.2.   成岩作用对储集性的影响

  • 1） 溶蚀作用

    如前所述,大坪剖面凝灰岩储集空间以次生成因为主,溶蚀作用是其主要成因,凝灰岩内部矿物被溶蚀,如石膏、石英等,根据溶蚀强度的大小,可形成包括洞、不规则溶孔、粒间溶孔、粒间溶孔以及沿缝发生扩溶形成扩溶缝等不同尺度的储集空间类型（图9）,这些孔洞、缝具有较强的储集性。从充填物类型来看,包括黄铁矿、石英、方解石、隐晶硅质等,指示溶蚀作用以埋藏期为主。

    2） 热液作用

    通过对11个样品的10个锆石颗粒（测试点位为9~18）的测试分析,确定凝灰岩受到热液作用的影响。稀土元素球粒陨石标准化配分模式表现为亏损轻稀土、重稀土异常富集的左倾型,Ce正异常和Eu负异常（图10）,Th、U值含量相对较高,U的含量介于422~3 293 μg/g,Th的含量介于25.4~115.6 μg/g,Th/U介于0.034~0.083,Th、U含量及Th/U值不能很好地指示热液锆石,锆石显示较强的Ce正异常、重稀土富集是热液锆石另一种不常见的热液锆石特征,以澳大利亚Mole花岗岩为代表[38],与澳大利亚Boggy Plain岩体微量元素具有不同的特征[63]。

    阴极发光图像显示,大坪剖面凝灰岩中锆石形状多为柱状,半自形—他形晶,锆石不发光,呈灰白色或黑色,锆石一般不发育岩浆的震荡环带或少数模糊环带,局部出现震荡环带可能是由于热液改造不彻底,且棱线不明显（图11a）,与岩浆型锆石明显震荡环带的特征存在差异,应为热液锆石特征[37⁃38],表明其具有热液锆石的特性。热液锆石的U-Pb同位素平均年龄为132.37±0.53 Ma（图11b）,显示凝灰岩热液流体活动为早白垩世（96~137 Ma）；该期热液在四川乐山[64]、滇东北[65]等地峨眉山玄武岩次生杏仁体及不规则溶蚀孔洞中均有响应,充填物包括黄铁矿、铜、绿泥石等。热液改造是大坪剖面凝灰岩形成规模性储层的重要因素,形成的储集空间类型以不规则洞为主,洞径3 mm×5 mm~30 mm×47 mm,常与构造缝相伴生,充填物包括黄铁矿、隐晶硅质等；该期溶蚀在川西北葛底坝剖面[31]、铁炉坝剖面[62]凝灰岩中均有分布,且溶蚀特征类似,经热液溶蚀改造的凝灰岩储集性能显著提升；该期热液亦是四川盆地原油大量裂解生气时间[66⁃67],对天然气运聚成藏具有重要影响。

    Figure 11.  (a) Zircon cathode luminescence image; (b) zircon U⁃Pb concordia diagrams

    3） 构造破裂作用

    区域上多次构造运动及其形成的如龙门山构造带、大巴山构造带等多条断裂带是大坪剖面凝灰岩中构造缝发育的主要作用力,裂缝多呈北东—南西向展布的特征与主构造力基本一致,表明构造带应力常对裂缝的展布具有控制作用。薄片中可见平直细小的多期构造缝（图9b~f）。裂缝既是重要的储集空间类型,又能连通先期形成的孔隙,改善了孔隙连通网络,并为后期流体改造提供了良好的通道,整体上较大提高了凝灰岩的储渗性能。

    4） 脱玻化作用

    火山玻璃受到外界环境的影响转化为晶体,形成微晶石英、斜长石,该过程会伴随着颗粒体积缩小,形成晶间微孔及脱玻化孔[68]。在埋藏前或者埋藏期,都可能发生脱玻化作用。研究区可见脱玻化作用的不均匀斑团（图2b3）以及脱玻化作用导致石英颗粒间形成石英间溶孔（图9j）。脱玻化作用形成较多细小孔隙,进一步改善凝灰岩的储集性能。

6.   结论

  • （1） 大坪剖面吴家坪组凝灰岩主要分布在吴一段、吴二段,呈薄层状；岩石类型以火山尘凝灰岩为主,矿物以伊利石、伊/蒙混层等黏土矿物为主,存在数量不等的石英、钾长石、斜长石、方解石、黄铁矿、石膏、黄钾铁矾、金红石等；区域上厚度向西南地区增大,发育期次与钻井剖面具有较好的可对比性。

    （2） 大坪剖面凝灰岩与峨眉山大火成岩省多期酸性火山活动有关,吴一段和吴二段凝灰岩均形成于板内拉张性构造环境,吴二段可能受到地壳地幔混合的影响。

    （3） 大坪剖面凝灰岩储集空间类型包括洞、不规则溶孔、粒内溶孔、有机质孔、粒间溶孔、裂缝等,裂缝与不同尺度的孔、洞形成性能良好的储渗体,储集空间未充填或黄铁矿、石英、方解石、隐晶硅质等充填；其次生储集空间受多期构造活动及热液流体等的综合改造,寻找二者联合作用区应是川西北地区二叠系吴家坪组凝灰岩储层勘探的关键。

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