渭河盆地地热水水溶烃类天然气成因与来源研究

Origin and Source of Dissolved Hydrocarbon Gas in Geothermal Water,Weihe Basin

渭河盆地地热水富含烃类天然气,甲烷含量一般在10%左右,最高可达82.44%。碳同位素分析结果表明,烃类天然气具有正碳同位素系列特征,其中位于户县—西安新生代沉积凹陷的地热水伴生天然气δ13C1值小于-55‰,具有生物成因气的特征,而其他地区地热水伴生烃类天然气δ13C1值在-38.7‰～-27.2‰之间,为热解成因天然气。天然气源岩分析显示,渭河盆地古近系张家坡组深水湖相沉积岩系,是渭河盆地生物气的重要源岩。分布于渭河断裂之北、构成渭河盆地基底的古生界,与出露于鄂尔多斯盆地南缘铜川—韩城(即渭北隆起)一带的古生界相当,是渭河盆地热解天然气的主要气源岩。

The geothermal water in Weihe basin contains abundant soluble hydrocarbon gas with the main composition of methane, whose content is commonly about 10% and the highest of 82.44%. Carbon isotope analysis results of methane gas show that the carbon isotope of hydrocarbon gas features with positive carbon sequence, and the value of δ^13C increases with the carbon number of gas. This indicates that hydrocarbon gas dissolved in geothermal water is organic gas. Gas from the Huxian-Xi＇an Cenozoic depression is biological gas characterized with δ^13 C1 less than --55%. However, gas from other areas of geothermal water is pyrolysis gas with δ^13C1 of --38.7‰--27.2‰. Zhangjiapo Formation, a set of deep lacustrine deposition with good organic material and appropriate temperature and environment conditions for biological gas formation, is the important source rocks of biological gas. The Palaeozoic sediments of the basin basement, distributed in the north Weihe Fault, and the equivalent Palaeozoic carbonate and coal series occurred in the southern margin of Ordos basin, are the main source rocks of pyrolysis gas.