摘要
四川盆地威远地区和川南长宁地区志留系龙马溪组页岩气化学组成以及碳、氢和氦同位素组成分析表明,该区页岩气CH4含量占绝对优势(94.0%~98.5%)、湿度低(0.3%~0.6%)、非烃气体含量较低(以CO2和N2为主、含微量的He、未检测出H2S)。烷烃气体碳同位素值随碳数增加出现局部反序分布特征:δ13C1>δ13C2;δ13CCO2=-2.4‰^-6.0‰,3 He/4 He=0.01~0.03Ra。威远和长宁2个地区页岩气碳同位素组成有所差异,威远地区页岩气的碳同位素值比长宁地区的偏低,δ13C1值偏低约8‰,δ13C2值偏低6‰;重复性实验结果表明2个地区页岩气碳同位素的差异可能源于其地质演化条件。川南长宁龙马溪页岩处于高—过成熟阶段,封闭性好,页岩气主要为干酪根初次裂解气和液态烃二次裂解气不同比例的混合,非烃气体中除有机质热裂解成因外含少量碳酸盐矿物分解产生的CO2。液态烃二次裂解可能是碳同位素值随碳数分布发生反序分布的原因。威远地区页岩中沥青含量较长宁地区多,同时后期的构造作用和抬升可能导致威远地区页岩气的烷烃气体的碳同位素值偏低。
Gas geochemical analysis has been conducted for the shale gas from Longmaxi Formation in Weiyuan-Changning areas,Sichuan Basin,China.Chemical composition,carbon,hydrogen and helium isotopic compositions were measured using an integrated method of gas chromatography combined with mass spectrometry.The results show that Longmaxi shale gases after are dominated by methane（94.0%-98.6%）with low wetness（0.3%-0.6%）,minor non-hydrocarbon gases,which are mainly CO2 and N2,and trace amount of He.δ^13CCO2=-2.5‰--6.0‰,3 He/4 He=0.01-0.03 Ra.The shale gases in Weiyuan and Changning areas have reversed carbon isotope between methane and ethane（13^C113C2）and distinct carbon isotopic compositions.Shale gas from the Weiyuan pilot has heavier carbon isotopic compositions for methane（δ13 C1：-34.5‰--36.8‰）,ethane（δ^13 C2：-37.6‰--41.9‰）and CO2（δ^13 CCO2：-4.5‰--6.0‰）than those in Changning pilot（δ13 C1：-27.2‰--27.3‰,δ^13 C2：-33.7‰--34.1‰,δ^13 CCO2：-2.5‰--4.6‰）.The Longmaxi shale was at thermally high or over mature stage of organic matter with good sealing condition.The shale gas after hydraulic fracturing could be originated from the thermal decomposition of kerogen and the secondary cracking of liquid hydrocarbons which caused the reversal pattern of carbon isotopes.Some CO2 could be derived from decomposition of carbonate.The differences in carbon isotopes of gases between Weiyuan and Changning areas could be derived from different mixing proportion of gas from the secondary cracking of liquid hydrocarbons caused by the specific geological and geochemical conditions.
出处
《天然气地球科学》
EI
CAS
CSCD
北大核心
2015年第8期1604-1612,共9页
Natural Gas Geoscience
基金
国家自然科学基金项目(编号:41502143
41472070
41372095)
国家重点基础研究发展计划(编号:2012CB214701)
教育部科学技术研究重大项目(编号:311010)联合资助
关键词
稳定同位素
化学组成
页岩气
龙马溪组
四川盆地
Stable isotope
Chemical composition
Shale gas
Longmaxi Formation
Sichuan Basin
