应用CH4／N2指标估算塔里木盆地天然气热成熟度

CH_4/N_2 ratio as a potential alternative geochemical tool to predict the thermal maturity of natural gas in Tarim basin

通过对塔里木盆地满加尔凹陷低成熟煤岩在开放在线程序升温体系的热模拟实验,获得煤岩在不同温度点的热分解产物CH4、CO和N2,并对模拟后残留产物进行了镜质体反射率测试,建立了镜质体反射率与对应受热温度之间的关系。煤岩镜质体反射率与热温度之间的二阶关系表达为:Ro=0.0014×T+0.109,r=0.9931(Ro<0.6%);Ro=0.0067×T-1.5855,r=0.9996(Ro>0.6%)。通过上述方程建立CH4/N2值与煤岩镜质体反射率之间的对应关系,并利用CH4/N2值对塔里木盆地库车坳陷天然气热成熟度进行了预测。预测结果与实际地质分析结果相吻合,说明CH4/N2值可以作为塔里木盆地煤成气热成熟度预测指标应用于油气勘探中。

In this paper, the bulk ratio of CH4/N2 is examined as a potential alternative geochemical parameter for the assessment of thermal maturity of natural gas and compared to other previously published data. Opensystem non-isothermal pyrolysis of the low-mature coal from the Manjiaer sag, Tarim basin, yielded generation curves for methane and nitrogen. The analysis of the change of vitrinite reflectance indicates a two-stage process of thermal maturation with increasing temperature. The relation between Ro and pyrolysis temperature T could be expressed by the following equations., stage I, Ro = 0. 001 4 T＋ 0. 109, r= 0. 993 1 （Ro 〈0. 6%） ; stage II, Ro=0. 006 7 T-1. 585 5, r=0. 999 6 （Ro〉0. 6%）. A kinetic interpretation of nitrogen and methane generation characteristics of humic coal during laboratory pyrolysis indicates that the bulk ratios of methane to nitrogen as a thermal maturity parameter can be applied to the assessment of the maturity of gassources. Thus, the maturity of the source rocks of natural gases in the Tarim basin was expressed in terms of the CH4/N2 ratios and compared with other geochemical natural gas parameters, such as stable isotopes. The predicted results were found to be consistent with the published data.