鄂尔多斯盆地伊陕斜坡中南部高热演化地区古生界天然气地球化学研究
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摘要
苏里格南部和高桥高热演化地区古生界天然气具有乙烷等重烃组分含量低,大多数样品的湿气系数小于2%,天然气甲烷碳同位素组成显著偏重,乙烷等重烃组分碳同位素组成变化大等特征。热演化程度更高、甲烷碳同位素更重、湿气系数更低的东南部地区,古生界天然气均显示乙烷等重烃组分碳同位素异常偏轻、碳同位素倒转的特征。通常,当乙烷等重烃组分含量极低的高演化天然气混入少量乙烷等重烃组分含量高、碳同位素组成显著偏轻的天然气就可形成天然气的乙烷等重烃组分碳同位素异常偏轻、碳同位素倒转。早白垩世末盆地抬升阶段初期古地温降至相当于高-过成熟阶段的温度区间时,气源岩和储层中残余的少量液态烃的热裂解生成的天然气混入导致了乙烷等重烃组分碳同位素异常偏轻与碳同位素的倒转。因此,应以甲烷碳同位素组成作为主要指标、乙烷等重烃组分碳同位素组成作为参考指标,结合成藏地质条件进行高演化天然气的成因判识。提出了研究区古生界天然气以煤成气为主,并有一定数量油型气混入的认识。
Paleozoic gases from the South Sulige and Gaoqiao areas in the Ordos Basin are characterized by low contents of heavy hydrocarbon gases, less than 2% of wetness for nearly all of the gas samples, relatively heavy d~(13)C_1, and large variations in d~(13)C_(2+). Gas samples from the southeast of the studied area are of higher thermal evolution, heavier d~(13)C_1 value and lower wetness, and show abnormally light d~(13)C_(2+) values and carbon isotopic reversal. In theory, only the mixture of highly mature gas with extremely low contents of heavy hydrocarbon gases and a small amount of natural gas with higher contents and lighter carbon isotopic composition of heavy hydrocarbon gases can lead to the occurrence of abnormally light d~(13)C_(2+) and the carbon isotope reversal between methane and ethane. The mixed gas probably is derived from the thermal cracking of a small amount of residual liquid hydrocarbons contained in the source and reservoir rocks during the initial stage of the uplift movement at the end of Early Cretaceous. Therefore, d_(13)C_1 should be used as the principal index and d~(13)C_(2+) values can be used as reference indexes for the genetic identification of highly mature natural gas. In addition, the combination with comprehensive geological and geochemical studies suggests that the Paleozoic gas in the studied area is mainly coal-derived gas, mixed with some oil-related gas.
Paleozoic gases from the South Sulige and Gaoqiao areas in the Ordos Basin are characterized by low contents of heavy hydrocarbon gases, less than 2% of wetness for nearly all of the gas samples, relatively heavy d~(13)C_1, and large variations in d~(13)C_(2+). Gas samples from the southeast of the studied area are of higher thermal evolution, heavier d~(13)C_1 value and lower wetness, and show abnormally light d~(13)C_(2+) values and carbon isotopic reversal. In theory, only the mixture of highly mature gas with extremely low contents of heavy hydrocarbon gases and a small amount of natural gas with higher contents and lighter carbon isotopic composition of heavy hydrocarbon gases can lead to the occurrence of abnormally light d~(13)C_(2+) and the carbon isotope reversal between methane and ethane. The mixed gas probably is derived from the thermal cracking of a small amount of residual liquid hydrocarbons contained in the source and reservoir rocks during the initial stage of the uplift movement at the end of Early Cretaceous. Therefore, d_(13)C_1 should be used as the principal index and d~(13)C_(2+) values can be used as reference indexes for the genetic identification of highly mature natural gas. In addition, the combination with comprehensive geological and geochemical studies suggests that the Paleozoic gas in the studied area is mainly coal-derived gas, mixed with some oil-related gas.
引文
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[11]吴小奇,黄士鹏,廖凤荣,李振生.四川盆地须家河组及侏罗系煤成气碳同位素组成[J].石油勘探与开发,2011,38(4):418–427.Wu Xiao-qi,Huang Shi-peng,Liao Feng-rong,Li Zhen-sheng.Carbon isotopic compositions of coal-derived gas in the Xujiahe Formation and Jurassic in the Sichuan Basin[J].Pet Explor Develop,2011,38(4):418–427(in Chinese with English abstract).
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[2]戴金星,李剑,罗霞,张文正,胡国艺,马成华,郭建民,葛守国.鄂尔多斯盆地大气田的烷烃气碳同位素组成特征及其气源对比[J].石油学报,2005,26(1):18–26.Dai Jin-xing,Li Jian,Luo Xia,Zhang Wen-zheng,Hu Guo-yi,Ma Cheng-hua,Guo Jian-min,Ge Shou-guo.Alkane carbon isotopic composition and gas source in giant gas fields of Ordos Basin[J].Acta Pet Sinica,2005,26(1):18–26(in Chinese with English abstract).
[3]夏新宇,洪峰,赵林,张文正.鄂尔多斯盆地下奥陶统碳酸盐岩有机相类型及生烃潜力[J].沉积学报,1999,17(4):638–650.Xia Xin-yu,Hong Feng,Zhao Lin,Zhang Wen-zheng.Organic facies type and hydrocarbon potential of carbonates in Majiagou Fm.Lower Ordovician in Ordos Basin[J].Acta Sedimentol Sinica,1999,17(4):638–650(in Chinese with English abstract).
[4]张文正,徐正球.低阶煤热演化生烃的模拟试验研究[J].天然气工业,1986,6(2):1–6.Zhang Wen-zheng,Xue Zheng-qiu.Modelling experimental research of hydrocarbon generateon from the thermal evolution of low grade coal[J].Nat Gas Ind,1986,6(2):1–6(in Chinese with English abstract).
[5]张文正.有机质碳同位素的成熟分馏作用及地质意义[J].石油实验地质,1989,11(2):177–184.Zhang Wen-zheng.Fractionation of caobon isotopes of organic matter and its geological significance[J].Exp Pet Geol,1989,11(2):177–184(in Chinese with English abstract).
[6]彭平安,邹艳荣,傅家谟.煤成气生成动力学研究进展[J].石油勘探与开发,2009,36(3):297–306.Peng Ping-an,Zou Yan-rong,Fu Jia-mo.Progress in generation kinetics studies of coal-derived gases[J].Pet Explor Develop,2009,36(3):297–306(in Chinese with English abstract).
[7]帅燕华,邹艳荣,彭平安,张水昌,熊永强,刘金钟.乙烷碳同位素动力学模拟及地质应用[J].地球化学,2006,35(2):151–156.Shuai Yan-hua,Zou Yan-rong,Peng Ping-an,Zhang Shuichang,Xiong Yong-qiang,Liu Jin-zhong.Kinetic modeling of stable carbon isotope ratios of ethane from coal in confined system and its significance in geological application[J].Geochimica,2006,35(2):151–156(in Chinese with English abstract).
[8]杨华,刘新社,闫小雄.鄂尔多斯盆地晚古生代以来构造-沉积演化与致密砂岩气成藏[J].地学前缘,2015,22(3):174–183.Yang Hua,Liu Xin-she,Yan Xiao-xiong.The relationship between tectonic-sedimentary evolutionand tight sandstone gas reserveoir since the late Paleozoic in Ordos basin[J].Earth Sci Front,2015,22(3):174–183(in Chinese with English abstract).
[9]刘新社,席胜利,付金华,王涛,王欣.鄂尔多斯盆地上古生界天然气生成[J].天然气工业,2000,20(6):19–23.Liu Xing-she,Xi Sheng-li,Fu Jin-hua,Wang Tao,Wang Xin.Natural gas generation in the upper Paleozoic in Ordos Basin[J].Natl Gas Ind,2000,20(6):19–23(in Chinese with English abstract).
[10]戴金星,夏新宇,秦胜飞,赵靖舟.中国有机烷烃气碳同位素系列倒转的成因[J].石油与天然气地质,2003,24(1):1–6.Dai Jin-xing,Xia Xin-yu,Qin Sheng-fei,Zhao Jing-zhou.Causation of partly reversed orders ofδ13C in biogenic alkane gas in China[J].Oil Gas Geol,2003,24(1):1–6(in Chinese with English abstract).
[11]吴小奇,黄士鹏,廖凤荣,李振生.四川盆地须家河组及侏罗系煤成气碳同位素组成[J].石油勘探与开发,2011,38(4):418–427.Wu Xiao-qi,Huang Shi-peng,Liao Feng-rong,Li Zhen-sheng.Carbon isotopic compositions of coal-derived gas in the Xujiahe Formation and Jurassic in the Sichuan Basin[J].Pet Explor Develop,2011,38(4):418–427(in Chinese with English abstract).
[12]杨华,张文正,昝川莉,马军.鄂尔多斯盆地东部奥陶系盐下天然气地球化学特征及其对靖边气田气源再认识[J].天然气地球科学,2009,20(1):8–14.Yang Hua,Zhang Wen-zheng,Zan Chuan-li,Ma Jun.Geochemical characteristics of Ordovician subsalt gas reservoir and their significance for reunderstanding the gas source of Jingbian Gasfield,East Ordos Basin[J].Nat Gas Geosci,2009,20(1):8–14(in Chinese with English abstract).