洞庭盆地第四系极浅层天然气成因类型及地质意义
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摘要
洞庭盆地自然出露地表的天然气苗众多,预示盆地内可能具有比较丰富的浅层天然气资源。通过对洞庭盆地野外地质调查及对沅江县共华镇发现的天然气进行气体组成及同位素分析,显示天然气组分主要为CH_4,且不含C_2H_6以上的重烃,CH_4碳同位素值平均为-79.6‰,表现出典型的生物成因气特征。造成和兴村与宪成垸村天然气碳氢同位素值差异较大的原因可能是存在同位素分馏现象。根据CH_4的碳氢同位素值及CO_2的碳同位素值分布关系,判断该生物气为CO_2还原途径生成。推测全新世由于掀斜式构造沉降使江汉盆地大量硅藻随湖水流入洞庭盆地,为生物气的形成提供了丰富的物源及有利的沉积环境。洞庭盆地DQ1与DQ2浅井揭示了纵向上均为上细下粗的正韵律沉积序列,生物气的埋深极浅,分布于10～120m之间,为自生自储式组合类型。由于埋藏浅,开发成本低廉,如合理勘探、开发、利用,可造福一方地区。
There are many natural gas seepages on the surface of Dongting Basin,indicating that there may be abundant shallow gas resources in this basin.By analyzing the composition and isotope of nature gas collected from Gonghua town,Yuanjiang county during field geological investigation in Dongting Basin,it was discovered that this natural gas belonged to biogenic gas.The main composition of this natural gas was methane,without other alkane gases.Moreover,the average carbon isotopic ratio of methane was-79.6‰.The reason for the difference in carbon isotope and hydrogen isotope of natural gas between villages of Hexing and Xianchengyuan may be due to the isotope fractionation.It was concluded that the biogas formed through the pathway of carbon dioxide reduction on the basis of the distribution relation of carbon isotopes of methane and carbon dioxide.We speculated that a large number of diatoms from Jianghan Basin flowed into Dongting Basin with lake water due to the tilted tectonic subsidence during Holocene,which provided abundant material source and favorable sedimentary environment for the formation of biogas.The data of Wells DQ1 and DQ2 in Dongting Basin all presented positive rhythmic sedimentary sequence of upper and lower coarse in vertical direction.Biogas has a very shallow depth and is usually distributed between 10 and 120 meters.Moreover,its type is generally positive reservoir-forming combination of authigenic and self-accumulation type.If biogas could be properly explored,developed,and utilized,it would benefit one area due to its shallow burial depth and low development costs.
There are many natural gas seepages on the surface of Dongting Basin,indicating that there may be abundant shallow gas resources in this basin.By analyzing the composition and isotope of nature gas collected from Gonghua town,Yuanjiang county during field geological investigation in Dongting Basin,it was discovered that this natural gas belonged to biogenic gas.The main composition of this natural gas was methane,without other alkane gases.Moreover,the average carbon isotopic ratio of methane was-79.6‰.The reason for the difference in carbon isotope and hydrogen isotope of natural gas between villages of Hexing and Xianchengyuan may be due to the isotope fractionation.It was concluded that the biogas formed through the pathway of carbon dioxide reduction on the basis of the distribution relation of carbon isotopes of methane and carbon dioxide.We speculated that a large number of diatoms from Jianghan Basin flowed into Dongting Basin with lake water due to the tilted tectonic subsidence during Holocene,which provided abundant material source and favorable sedimentary environment for the formation of biogas.The data of Wells DQ1 and DQ2 in Dongting Basin all presented positive rhythmic sedimentary sequence of upper and lower coarse in vertical direction.Biogas has a very shallow depth and is usually distributed between 10 and 120 meters.Moreover,its type is generally positive reservoir-forming combination of authigenic and self-accumulation type.If biogas could be properly explored,developed,and utilized,it would benefit one area due to its shallow burial depth and low development costs.
引文
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[4]Lin Chunming,Jiang Weisan.Favorable geological condition of out-shallow biogenic gas in Qiantang River Mouth Region[J].Natural Gas Geoscience,1996,7(3):23-27.林春明,蒋维三.钱塘江口超浅层生物气成藏的有利地质条件[J].天然气地球科学,1996,7(3),23-27.
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[7]Bai Daoyuan,Gao Feng,Ma Tieqiu,et al.Quaternary deposits and palaeogeographic evolution in the Lixian Depression,Dongting Basin,Hunan[J].Sedimentary Geology and Tethyan Geology,2009,29(4):10-21.柏道远,高峰,马铁球,等.洞庭盆地澧县凹陷第四纪沉积特征与古地理演化[J].沉积与特提斯地质,2009,29(4):10-21.
[8]Zhu Wei,Cao Zijian,Wang Hongyun,et al.Evaluation of shallow biogenic gas accumulation conditions and potential traps in Dongting Basin[J].Special Oil and Gas Reservoirs,2014,21(4):17-21,151.朱伟,曹子剑,王红云,等.洞庭盆地浅层生物气成藏条件及潜力圈闭评价[J].特种油气藏,2014,21(4):17-21,151.
[9]Lin Chunming,Zhu Sizhao,Huang Zhicheng,et al.Features and reservoir formation conditions of shallow biogenic gas of Quaternary in the Zhejiang coastal plain[J].Journal of Nanjing University:Natural Sciences,1999,35(3):286-295.林春明,朱嗣昭,黄志诚,等.浙江沿海平原第四系生物气特征和成藏条件[J].南京大学学报:自然科学版,1999,35(3):286-295.
[10]Zhang X,Lin C M.Characteristics and accumulation model of the Late Quaternary shallow biogenic gas in the modern Changjiang delta area,eastern China[J].Petroleum Science,2017,14(2):261-275.
[11]Chen Wei.Character and effect on the engineering construction of shallow layer natural gas in an area of Yangtze River coastwise[J].Journal of Guilin University of Technology,2006,26(2):181-183.陈卫.长江沿岸某区浅层天然气特征及其对工程建设的影响[J].桂林工学院学报,2006,26(2):181-183.
[12]Li Xianqi,Zhang Shuichang,Zhu Guangyou,et al.Types and research direction of biogenic gas in China[J].Natural Gas Geoscience,2005,16(4):477-484.李先奇,张水昌,朱光有,等.中国生物成因气的类型划分与研究方向[J].天然气地球科学,2005,16(4):477-484.
[13]Dai Jinxing,Chen Yin.Carbon isotope characteristics and identification criteria of alkane of biogas in China[J].Science in China:Series B,1993,(3):303-310.戴金星,陈英.中国生物气中烷烃组分的碳同位素特征及其鉴别标志[J].中国科学:B辑,1993,(3):303-310.
[14]Chen Ying,Dai Jinxing,Qi Houfa.On some theoretical and methodological topics of biogenic gas studies[J].Petroleum Geology&Experiment,1994,16(3):209-219.陈英,戴金星,戚厚发.关于生物气研究中几个理论及方法问题的研究[J].石油实验地质,1994,16(3):209-219.
[15]Wang Xiaofeng,Xu Yongchang,Shen Ping,et al.Geochemical characteristics and identification indexes of low-mature gases[J].Natural Gas Geoscience,2010,21(1):1-6.王晓锋,徐永昌,沈平,等.低熟气地球化学特征与判识指标[J].天然气地球科学,2010,21(1):1-6.
[16]Hou Dujie,Feng Zihui.Oil and Gas Geochemistry[M].Beijing:Petroleum Industry Press,2011:288-294.侯读杰,冯子辉.油气地球化学[M].北京:石油工业出版社,2011:288-294.
[17]Qin Shengfei,Zhou Guoxiao.The effect of gas field water on hydrogen isotope fractionation of methane[J].Natural Gas Geoscience,2018,29(3):311-316.秦胜飞,周国晓.气田水对甲烷氢同位素分馏作用[J].天然气地球科学,2018,29(3):311-316.
[18]Song Yan,Liu Shaobo,Hong Feng,et al.Geochemical characteristics and genesis of coalbed methane in China[J].Acta Petrolei Sinica,2012,33(supplement 1):99-106.宋岩,柳少波,洪峰,等.中国煤层气地球化学特征及成因[J].石油学报,2012,33(增刊1):99-106.
[19]Whiticar M J,Faber E,Schoell M.Biogenic methane formation in marine and freshwater environments:CO2reduction vs.acetate fermentation-isotope evidence[J].Geochimica et Cosmochimica Acta,1986,50(5):693-709.
[20]Schoell M.The hydrogen and carbon isotopic composition of methane from natural gases of various origins[J].Geochimica et Cosmochimica Acta,1980,44(5):649-661.
[21]Shen Ping,Xu Yongchang,Wang Xianbin,et al.Research on Geochemical Characteristics of Gas Source Rock,Natural Gas and Mechanisms of Gas Generation[M].Lanzhou:Gansu Science&Technology Press,1991:43-47.沈平,徐永昌,王先彬,等.气源岩和天然气地球化学特征及成气机理研究[M].兰州:甘肃科学技术出版社,1991:43-47.
[22]Shen Ping,Wang Xiaofeng,Xu Yin,et al.Carbon and hydrogen isotopic compositions:Generation pathway of bacterial gas in China[J].Acta Sedimentologica Sinica,2010,28(1):183-187,207.沈平,王晓锋,徐茵,等.我国生物气藏碳、氢同位素特征、形成途径及意义[J].沉积学报,2010,28(1):183-187,207.
[23]Rice D D,Claypool G E.Generation,accumulation,and resource potential of biogenic gas[J].AAPG Bulletin,1981,65(1):5-25.
[24]Whiticar M J.Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane[J].Chemical Geology,1999,161(1-3):291-314.
[25]Whiticar M J.A geochemical perspective of natural gas and atmospheric methane[J].Organic Geochemistry,1990,16(1-3):531-547.
[26]Bai Daoyuan,Li Changan,Ma Tieqiu,et al.Quaternary tectonicsedimentary characteristics and environmental evolution of Anxiang Sag in Dongting Basin and its west periphery[J].Journal of Earth Sciences and Environment,2010,32(2):120-129.柏道远,李长安,马铁球,等.第四纪洞庭盆地安乡凹陷及西缘构造-沉积特征与环境演化[J].地球科学与环境学报,2010,32(2):120-129.
[27]Liang Xing,Zhang Renquan,Pi Jiangao,et al.Characteristics of tectonic movement of Dongting Basin in The Quaternary Period[J].Geological Science and Technology Information,2001,20(2):11-14.梁杏,张人权,皮建高,等.洞庭盆地第四纪构造活动特征[J].地质科技情报,2001,20(2):11-14.
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