Volume 43 Issue 1
Jan.  2021
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FU Xiaodong, ZHANG Tianfu, WU Jianping, WANG Xiaofang, ZHOU Jingao, JIANG Tao, LI Bing, LI Changxin. Characteristics and main controlling factors of tight oil reservoirs in Cretaceous Tengger Formation, A'nan Sag, Erlian Basin[J]. PETROLEUM GEOLOGY & EXPERIMENT, 2021, 43(1): 64-76. doi: 10.11781/sysydz202101064
Citation: FU Xiaodong, ZHANG Tianfu, WU Jianping, WANG Xiaofang, ZHOU Jingao, JIANG Tao, LI Bing, LI Changxin. Characteristics and main controlling factors of tight oil reservoirs in Cretaceous Tengger Formation, A'nan Sag, Erlian Basin[J]. PETROLEUM GEOLOGY & EXPERIMENT, 2021, 43(1): 64-76. doi: 10.11781/sysydz202101064

Characteristics and main controlling factors of tight oil reservoirs in Cretaceous Tengger Formation, A'nan Sag, Erlian Basin

doi: 10.11781/sysydz202101064
  • Received Date: 2020-04-16
  • Rev Recd Date: 2020-12-24
  • Publish Date: 2021-01-28
  • A set of "special lithology" tight reservoirs related to volcanic activity developed widely in the lower submember of the first member of Cretaceous Tengger Formation (K1bt1x) in the A'nan Sag, Erlian Basin, which is the main target for tight oil exploration. Based on section observation, physical property analysis, scanning electron microscopy (SEM), mercury intrusion porosimetry, CT scanning and nuclear magnetic resonance (NMR), the characteristics and main controlling factors of "sweet spots" of tight oil reservoirs with different lithologies in the K1bt1x were studied. Tuffaceous rocks, sandstones, dolomites and mudstones developed in the K1bt1x, and tuffaceous rocks and sandstones are the favorable lithologies for the development of tight oil reservoirs. The reservoir space consists of dissolved pores, intercrystalline micropores, residual intergranular pores, drainage pores and microfractures. The main reservoir space of tuff includes devitrified intercrystalline pores and dissolution pores, whereas that of sandstone includes residual intergranular pores and dissolution pores. The reservoir pore structures can be divided into 3 types and 4 subtypes. The thin-layered tuff and fine sandstone with solution pores have better pore structures, but the sedimentary tuff and dolomite have poor pore structures. The reservoir has a strong oil content heterogeneity controlled by reservoir lithology, porosity and permeability. The development of "sweet spots" in tuffaceous tight reservoirs is mainly controlled by the volcaniclastic deposition, devitrification and dissolution, while the development of "sweet spots" in sandstone reservoirs is mainly controlled by sedimentary microfacies, rock structure and dissolution.

     

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  • [1]
    张文朝, 雷怀玉, 姜冬华, 等. 二连盆地阿南凹陷的演化与油气聚集规律[J]. 河南石油, 1998, 12(2): 1-5. https://www.cnki.com.cn/Article/CJFDTOTAL-SYHN199802000.htm

    ZHANG Wenchao, LEI Huaiyu, JIANG Donghua, et al. Evolution of the Arnan Sag in the Erlian Basin and hydrocarbon accumulation[J]. Henan Petroleum, 1998, 12(2): 1-5. https://www.cnki.com.cn/Article/CJFDTOTAL-SYHN199802000.htm
    [2]
    李顺才. 阿南凹陷油藏特征及形成条件[J]. 复式油气田, 1983(1): 17-23.

    LI Shuncai. Reservoir characteristics and formation conditions of A'nan Sag[J]. Multiple Oil-Gas Field, 1983(1): 17-23.
    [3]
    赵文智, 胡素云, 侯连华, 等. 中国陆相页岩油类型、资源潜力及与致密油的边界[J]. 石油勘探与开发, 2020, 47(1): 1-10. https://www.cnki.com.cn/Article/CJFDTOTAL-SKYK202001002.htm

    ZHAO Wenzhi, HU Suyun, HOU Lianhua, et al. Types and resource potential of continental shale oil in China and its boundary with tight oil[J]. Petroleum Exploration and Development, 2020, 47(1): 1-10. https://www.cnki.com.cn/Article/CJFDTOTAL-SKYK202001002.htm
    [4]
    孙龙德, 邹才能, 贾爱林, 等. 中国致密油气发展特征与方向[J]. 石油勘探与开发, 2019, 46(6): 1015-1026. https://www.cnki.com.cn/Article/CJFDTOTAL-SKYK201906002.htm

    SUN Longde, ZOU Caineng, JIA Ailin, et al. Development characte-ristics and orientation of tight oil and gas in China[J]. Petroleum Exploration and Development, 2019, 46(6): 1015-1026. https://www.cnki.com.cn/Article/CJFDTOTAL-SKYK201906002.htm
    [5]
    康玉柱. 中国北方石炭系-二叠系致密油气资源前景分析[J]. 断块油气田, 2020, 27(6): 681-683. https://www.cnki.com.cn/Article/CJFDTOTAL-DKYT202006002.htm

    KANG Yuzhu. Prospect analysis of Carboniferous-Permian tight oil and gas resources in northern China[J]. Fault-Block Oil and Gas Field, 2020, 27(6): 681-683. https://www.cnki.com.cn/Article/CJFDTOTAL-DKYT202006002.htm
    [6]
    彭金宁, 邱岐, 王东燕, 等. 苏北盆地古近系阜宁组致密油赋存状态与可动用性[J]. 石油实验地质, 2020, 42(1): 53-59. doi: 10.11781/sysydz202001053

    PENG Jinning, QIU Qi, WANG Dongyan, et al. Occurrence and recoverability of tight oil in Paleogene Funing Formation, Subei Basin[J]. Petroleum Geology & Experiment, 2020, 42(1): 53-59. doi: 10.11781/sysydz202001053
    [7]
    何崇康, 成良丙, 陈旭峰, 等. 鄂尔多斯盆地新安边油田长7致密油有效储层识别与甜点优选[J]. 石油实验地质, 2017, 39(6): 812-818. doi: 10.11781/sysydz201706812

    HE Chongkang, CHENG Liangbing, CHEN Xufeng, et al. Effective reservoir identification and "sweet spot" optimization of Chang 7 tight oil reservoir in Xin'anbian oil field, Ordos Basin[J]. Petro-leum Geology & Experiment, 2017, 39(6): 812-818. doi: 10.11781/sysydz201706812
    [8]
    林铁锋, 康德江. 松辽盆地三肇凹陷扶余油层致密储层分类精细评价[J]. 石油实验地质, 2017, 39(5): 686-693. doi: 10.11781/sysydz201705686

    LIN Tiefeng, KANG Dejiang. Classification and evaluation of tight reservoirs in Fuyu oil layer, Sanzhao Sag, Songliao Basin[J]. Petroleum Geology & Experiment, 2017, 39(5): 686-693. doi: 10.11781/sysydz201705686
    [9]
    崔维兰, 韩华峰, 张永, 等. 鄂尔多斯盆地靖边油田李家城则地区长6致密油储层微观特征与含油性[J]. 石油实验地质, 2019, 41(3): 390-397. doi: 10.11781/sysydz201903390

    CUI Weilan, HAN Huafeng, ZHANG Yong, et al. Microscopic characteristics and oil content of Chang 6 tight sandstone reservoirs in Lijiachengze area, Jingbian Oil Field, Ordos Basin[J]. Petroleum Geology & Experiment, 2019, 41(3): 390-397. doi: 10.11781/sysydz201903390
    [10]
    匡立春, 唐勇, 雷德文, 等. 准噶尔盆地二叠系咸化湖相云质岩致密油形成条件与勘探潜力[J]. 石油勘探与开发, 2012, 39(6): 657-667. https://www.cnki.com.cn/Article/CJFDTOTAL-SKYK201206004.htm

    KUANG Lichun, TANG Yong, LEI Dewen, et al. Formation conditions and exploration potential of tight oil in the Permian saline lacustrine dolomitic rock, Junggar Basin, NW China[J]. Petroleum Exploration and Development, 2012, 39(6): 657-667. https://www.cnki.com.cn/Article/CJFDTOTAL-SKYK201206004.htm
    [11]
    JIANG Yiqin, LIU Yiqun, YANG Zhao, et al. Characteristics and origin of tuff-type tight oil in Jimusaer Sag, Junggar Basin, NW China[J]. Petroleum Exploration and Development, 2015, 42(6): 810-818.
    [12]
    朱国华, 张杰, 姚根顺, 等. 沉火山尘凝灰岩: 一种赋存油气资源的重要岩类: 以新疆北部中二叠统芦草沟组为例[J]. 海相油气地质, 2014, 19(1): 1-7. https://www.cnki.com.cn/Article/CJFDTOTAL-HXYQ201401005.htm

    ZHU Guohua, ZHANG Jie, YAO Genshun, et al. Sedimentary volcanic dust tuff, an important kind of rock storing hydrocarbon resources: disscusion on the lithology of middle Permian Lucaogou oil-bearing rocks in the north of Xinjiang[J]. Marine Origin Petroleum Geology, 2014, 19(1): 1-7. https://www.cnki.com.cn/Article/CJFDTOTAL-HXYQ201401005.htm
    [13]
    焦立新, 刘俊田, 张宏, 等. 三塘湖盆地沉凝灰岩致密储集层特征及其形成条件[J]. 天然气地球科学, 2014, 25(11): 1697-1705. https://www.cnki.com.cn/Article/CJFDTOTAL-TDKX201411004.htm

    JIAO Lixin, LIU Juntian, ZHANG Hong, et al. Tight reservoir characteristics and formation conditions of tuff in Santanghu Basin[J]. Natural Gas Geoscience, 2014, 25(11): 1697-1705. https://www.cnki.com.cn/Article/CJFDTOTAL-TDKX201411004.htm
    [14]
    MA Jian, HUANG Zhilong, LIANG Shijun, et al. Geochemical and tight reservoir characteristics of sedimentary organic-matter-bearing tuff from the Permian Tiaohu Formation in the Santanghu Basin, Northwest China[J]. Marine and Petroleum Geology, 2016, 73: 405-418.
    [15]
    MA Jian, LIU Guoheng, HUANG Zhilong, et al. Tight tuff reservoir characteristics and its controlling factors: a comparative study of the Permian Tiaohu Formation and Carboniferous Haerjiawu Formation in the Santanghu Basin, NW China[J]. Journal of Petroleum Science and Engineering, 2020, 187: 106808.
    [16]
    陈旋, 李杰, 梁浩, 等. 三塘湖盆地条湖组沉凝灰岩致密油藏成藏特征[J]. 新疆石油地质, 2014, 35(4): 386-390. https://www.cnki.com.cn/Article/CJFDTOTAL-XJSD201404004.htm

    CHEN Xuan, LI Jie, LIANG Hao, et al. Hydrocarbon accumulation elements analysis of Tiaohu tuff reservoir of Middle Permian, Santanghu Basin[J]. Xinjiang Petroleum Geology, 2014, 35(4): 386-390. https://www.cnki.com.cn/Article/CJFDTOTAL-XJSD201404004.htm
    [17]
    蓝宝锋, 杨克兵, 彭传利, 等. 二连盆地阿南凹陷致密油勘探潜力分析[J]. 复杂油气藏, 2014, 7(2): 9-12. https://www.cnki.com.cn/Article/CJFDTOTAL-FZYQ201402003.htm

    LAN Baofeng, YANG Kebing, PENG Chuanli, et al. Analysis of the exploration potential of tight oil in Anan Sag, Erlian Basin[J]. Complex Hydrocarbon Reservoirs, 2014, 7(2): 9-12. https://www.cnki.com.cn/Article/CJFDTOTAL-FZYQ201402003.htm
    [18]
    李军, 王炜, 王书勋. 青西油田沉凝灰岩储集特征[J]. 新疆石油地质, 2004, 25(3): 288-290. https://www.cnki.com.cn/Article/CJFDTOTAL-XJSD200403019.htm

    LI Jun, WANG Wei, WANG Shuxun. Bedded tuff reservoir in Qingxi oilfield[J]. Xinjiang Petroleum Geology, 2004, 25(3): 288-290. https://www.cnki.com.cn/Article/CJFDTOTAL-XJSD200403019.htm
    [19]
    付小东, 吴健平, 寿建峰, 等. 渤海湾盆地束鹿凹陷古近系沙河街组湖相混积泥灰岩致密油储层特征[J]. 石油与天然气地质, 2019, 40(1): 78-91. https://www.cnki.com.cn/Article/CJFDTOTAL-SYYT201901009.htm

    FU Xiaodong, WU Jianping, SHOU Jianfeng, et al. Tight oil reservoir characteristics of lacustrine mixed marlstone in the Palaeogene Shahejie Formation of Shulu Sag, Bohai Bay Basin[J]. Oil & Gas Geology, 2019, 40(1): 78-91. https://www.cnki.com.cn/Article/CJFDTOTAL-SYYT201901009.htm
    [20]
    GRYNBERG M E, PAPAVA D, SHENGELIA M, et al. Petrophysical characteristics of the Middle Eocene laumontite tuff reservoir, Samgori Field, Republic of Georgia[J]. Journal of Petroleum Geology, 1993, 16(3): 313-322.
    [21]
    KALAN T, SITORUS H P, EMAN E. Jatibarang field, geologic study of volcanic reservoir for horizontal well proposal[C]//Indonesian Petroleum Association, 23rd Annual Convention Proceedings. Tusla: AAPG, 1994: 229-244.
    [22]
    TOMARU H, LU Zunli, FEHN U, et al. Origin of hydrocarbons in the green tuff region of Japan: 129I results from oil field brines and hot springs in the Akita and Niigata basins[J]. Chemical Geology, 2009, 264(1/4): 221-231.
    [23]
    王会来, 高先志, 杨德相, 等. 二连盆地下白垩统湖相云质岩分布及控制因素[J]. 现代地质, 2014, 28(1): 163-172. https://www.cnki.com.cn/Article/CJFDTOTAL-XDDZ201401017.htm

    WANG Huilai, GAO Xianzhi, YANG Dexiang, et al. Distribution characteristics and controlling factors of the Lower Cretaceous lacustrine dolomitic rock in Erlian Basin[J]. Geoscience, 2014, 28(1): 163-172. https://www.cnki.com.cn/Article/CJFDTOTAL-XDDZ201401017.htm
    [24]
    张以明, 付小东, 郭永军, 等. 二连盆地阿南凹陷白垩系腾一下段致密油有效储层物性下限研究[J]. 石油实验地质, 2016, 38(4): 551-558. doi: 10.11781/sysydz201604551

    ZHANG Yiming, FU Xiaodong, GUO Yongjun, et al. Petrophysical property limits of effective tight oil reservoirs in the lower part of the first member of Cretaceous Tengger Formation, A'nan Sag, Erlian Basin, North China[J]. Petroleum Geology & Experiment, 2016, 38(4): 551-558. doi: 10.11781/sysydz201604551
    [25]
    王宏语, 樊太亮, 肖莹莹, 等. 凝灰质成分对砂岩储集性能的影响[J]. 石油学报, 2010, 31(3): 432-439. https://www.cnki.com.cn/Article/CJFDTOTAL-SYXB201003013.htm

    WANG Hongyu, FAN Tailiang, XIAO Yingying, et al. Effect of tuffaceous components on physical property of sandstone reservoir[J]. Acta Petrolei Sinica, 2010, 31(3): 432-439. https://www.cnki.com.cn/Article/CJFDTOTAL-SYXB201003013.htm
    [26]
    赵海玲, 黄微, 王成, 等. 火山岩中脱玻化孔及其对储层的贡献[J]. 石油与天然气地质, 2009, 30(1): 47-52. https://www.cnki.com.cn/Article/CJFDTOTAL-SYYT200901012.htm

    ZHAO Hailing, HUANG Wei, WANG Cheng, et al. Micropores from devitrification in volcanic rocks and their contribution to reservoirs[J]. Oil & Gas Geology, 2009, 30(1): 47-52. https://www.cnki.com.cn/Article/CJFDTOTAL-SYYT200901012.htm
    [27]
    赵玉婷, 单玄龙, 王璞珺, 等. 松辽盆地白垩系营城组火山岩脱玻化作用及其储层意义: 以盆缘剖面为例[J]. 吉林大学学报(地球科学版), 2007, 37(6): 1152-1158. https://www.cnki.com.cn/Article/CJFDTOTAL-CCDZ200706013.htm

    ZHAO Yuting, SHAN Xuanlong, WANG Pujun, et al. Devitrification of the volcanic rocks of the Cretaceous Yingcheng Formation in the Songliao Basin and its reservoir significance: an example from a basin boundary section[J]. Journal of Jilin University (Earth Science Edition), 2007, 37(6): 1152-1158. https://www.cnki.com.cn/Article/CJFDTOTAL-CCDZ200706013.htm
    [28]
    赵海玲, 王成, 刘振文, 等. 火山岩储层斜长石选择性溶蚀的岩石学特征和热力学条件[J]. 地质通报, 2009, 28(4): 412-419. https://www.cnki.com.cn/Article/CJFDTOTAL-ZQYD200904004.htm

    ZHAO Hailing, WANG Cheng, LIU Zhenwen, et al. Characteristics of petrology and thermodynamics of selective dissolution of plagioclase in volcanic reservoir rocks[J]. Geological Bulletin of China, 2009, 28(4): 412-419. https://www.cnki.com.cn/Article/CJFDTOTAL-ZQYD200904004.htm
    [29]
    徐梅桂, 张哨楠, 伏美燕, 等. 不同淡水成岩体系下长石溶蚀的对比实验[J]. 现代地质, 2013, 27(4): 925-933. https://www.cnki.com.cn/Article/CJFDTOTAL-XDDZ201304021.htm

    XU Meigui, ZHANG Shaonan, FU Meiyan, et al. Contrast experiment about dissolution of feldspar in different freshwater diagenetic systems[J]. Geoscience, 2013, 27(4): 925-933. https://www.cnki.com.cn/Article/CJFDTOTAL-XDDZ201304021.htm
    [30]
    朱焕来, 曲希玉, 刘立, 等. CO2流体-长石相互作用实验研究[J]. 吉林大学学报(地球科学版), 2011, 41(3): 697-706. https://www.cnki.com.cn/Article/CJFDTOTAL-CCDZ201103011.htm

    ZHU Huanlai, QU Xiyu, LIU Li, et al. Study on interaction between the feldspar and CO2 fluid[J]. Journal of Jilin University (Earth Science Edition), 2011, 41(3): 697-706. https://www.cnki.com.cn/Article/CJFDTOTAL-CCDZ201103011.htm
    [31]
    黄思静, 杨俊杰, 张文正, 等. 不同温度条件下乙酸对长石溶蚀过程的实验研究[J]. 沉积学报, 1995, 13(1): 7-17. https://www.cnki.com.cn/Article/CJFDTOTAL-CJXB501.001.htm

    HUANG Sijing, YANG Junjie, ZHANG Wenzheng, et al. Experimental study of feldspar dissolution by acetic acid at different burial temperatures[J]. Acta Sedimentologica Sinica, 1995, 13(1): 7-17. https://www.cnki.com.cn/Article/CJFDTOTAL-CJXB501.001.htm
    [32]
    远光辉, 操应长, 杨田, 等. 论碎屑岩储层成岩过程中有机酸的溶蚀增孔能力[J]. 地学前缘, 2013, 20(5): 207-219. https://www.cnki.com.cn/Article/CJFDTOTAL-DXQY201305022.htm

    YUAN Guanghui, CAO Yingchang, YANG Tian, et al. Porosity enhancement potential through mineral dissolution by organic acids in the diagenetic process of clastic reservoir[J]. Earth Science Frontiers, 2013, 20(5): 207-219. https://www.cnki.com.cn/Article/CJFDTOTAL-DXQY201305022.htm
    [33]
    朱筱敏, 王英国, 钟大康, 等. 济阳坳陷古近系储层孔隙类型与次生孔隙成因[J]. 地质学报, 2007, 81(2): 197-204. https://www.cnki.com.cn/Article/CJFDTOTAL-DZXE200702008.htm

    ZHU Xiaomin, WANG Yingguo, ZHONG Dakang, et al. Pore types and secondary pore evolution of Paleogene reservoir in the Jiyang Sag[J]. Acta Geologica Sinica, 2007, 81(2): 197-204. https://www.cnki.com.cn/Article/CJFDTOTAL-DZXE200702008.htm
    [34]
    SURDAM R C, CROSSEY L J, HAGEN E S, et al. Organic-inorganic interactions and sandstone diagenesis[J]. AAPG Bulletin, 1989, 73(1): 1-23.
    [35]
    BARTH T, BJØRLYKKE K. Organic acids from source rock maturation: generation potentials, transport mechanisms and relevance for mineral diagenesis[J]. Applied Geochemistry, 1993, 8(4): 325-337. http://www.sciencedirect.com/science?_ob=ShoppingCartURL&_method=add&_eid=1-s2.0-088329279390002X&originContentFamily=serial&_origin=article&_ts=1483786154&md5=e21945b4fd4e2f0a248f76d25633c3c9
    [36]
    黄思静, 武文慧, 刘洁, 等. 大气水在碎屑岩次生孔隙形成中的作用: 以鄂尔多斯盆地三叠系延长组为例[J]. 地球科学(中国地质大学学报), 2003, 28(4): 419-424. https://www.cnki.com.cn/Article/CJFDTOTAL-SYSD202106013.htm

    HUANG Sijing, WU Wenhui, LIU Jie, et al. Generation of secondary porosity by meteoric water during time of subaerial exposure: an example from Yanchang Formation sandstone of Triassic of Ordos Basin[J]. Earth Science (Journal of China University of Geosciences), 2003, 28(4): 419-424. https://www.cnki.com.cn/Article/CJFDTOTAL-SYSD202106013.htm
    [37]
    丁晓琪, 韩玫梅, 张哨楠, 等. 大气淡水在碎屑岩次生孔隙中的作用[J]. 地质论评, 2014, 60(1): 145-158. https://www.cnki.com.cn/Article/CJFDTOTAL-DZLP201401016.htm

    DING Xiaoqi, HAN Meimei, ZHANG Shaonan, et al. Roles of meteoric water on secondary porosity of siliciclastic reservoirs[J]. Geological Review, 2014, 60(1): 145-158. https://www.cnki.com.cn/Article/CJFDTOTAL-DZLP201401016.htm
    [38]
    DE GROOT K. Experimental dedolomitization[J]. Journal of Sedimentary Research, 1967, 37(4): 1216-1220.
    [39]
    张杰, 寿建峰, 文应初, 等. 去白云石化作用机理及其对储集层的改造[J]. 古地理学报, 2012, 14(1): 69-84. https://www.cnki.com.cn/Article/CJFDTOTAL-GDLX201201011.htm

    ZHANG Jie, SHOU Jianfeng, WEN Yingchu, et al. Mechanism of dedolomitization and its rebuilding to reservoir[J]. Journal of Palaeogeography, 2012, 14(1): 69-84. https://www.cnki.com.cn/Article/CJFDTOTAL-GDLX201201011.htm
    [40]
    郭沫贞, 文川江, 苑国辉, 等. 三塘湖盆地牛东石炭系火山岩示顶底构造特征、成因及地质意义[J]. 海相油气地质, 2010, 15(3): 74-78. https://www.cnki.com.cn/Article/CJFDTOTAL-HXYQ201003015.htm

    GUO Mozhen, WEN Chuanjiang, YUAN Guohui, et al. Features, origin and geological significance of geopetal structures in Carboniferous volcanic rocks in Niudong block, Santanghu Basin[J]. Marine Origin Petroleum Geology, 2010, 15(3): 74-78. https://www.cnki.com.cn/Article/CJFDTOTAL-HXYQ201003015.htm
    [41]
    张丽霞, 李培俊, 朱国华, 等. 北三台石炭系火山岩示底构造及与油气的关系[J]. 新疆石油地质, 2009, 30(5): 569-570. https://www.cnki.com.cn/Article/CJFDTOTAL-XJSD200905009.htm

    ZHANG Lixia, LI Peijun, ZHU Guohua, et al. Volcanic geopetal structure of carboniferous and its relationship with petroleum in Beisantai field, Junggar Basin[J]. Xinjiang Petroleum Geology, 2009, 30(5): 569-570. https://www.cnki.com.cn/Article/CJFDTOTAL-XJSD200905009.htm
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