页岩气解析过程中烷烃碳同位素组成变化及其地质意义——以鄂尔多斯盆地伊陕斜坡东南部长7页岩为例

Variation of Carbon Isotopic Composition of Alkanes during the Shale Gas Desorption Process and Its Geological Significance:A Case Study of Chang7 Shale of Yanchang Formation in Yishan Slope Southeast of Ordos Basin

以鄂尔多斯盆地伊陕斜坡东南部延长组长7页岩为例,通过30余块页岩样品的解析实验研究了页岩气解析过程中烷烃碳同位素组成的变化规律,同时探讨了可能导致这种现象的原因及其地质意义。研究发现,页岩气在解析过程中甲烷碳同位素值增大了9.2‰(从-50.1‰^-40.9‰),乙烷碳同位素值增大了2.8‰(从-35.5‰^-32.7‰),丙烷碳同位素值几乎不发生变化。在常温或恒温加热条件下,页岩气解析过程中甲烷、乙烷碳同位素值持续增大,当升高温度时甲烷、乙烷碳同位素值先突然减小,然后慢慢变大。造成这种现象的原因可能主要是页岩气的吸附/解吸作用和同位素的扩散运移分馏作用。此外,页岩气的甲烷、乙烷碳同位素组成的这种变化特性,可能是页岩气甲烷碳同位素值相对于源岩热演化程度较小的重要原因,同时可以应用于压裂效果的评价和页岩气剩余资源量的评价中。

Taking the Yishan slope southeast Chang7 shale of Yanchang Formation in the Ordos Basin as an example,with desorption experiments of more than 30 shale samples,this paper studied the variation of carbon isotopic composition of alkanes during the shale gas desorption process,also discussed the possible causes of this phenomenon and its geological significance. The study found that carbon isotopic composition become higher by 9.2‰ （ from-50.1‰ to -40.9‰） for methane of desorbed gas （δ13C1） ,become higher by 2.8‰ （ from-35.5‰ to -32.7‰） for ethane （δ13C2）,and nearly no change for propane during the de- sorption process. At room temperature 0r constant temperature, carbon isotopic composition of methane and ethane （δ13C1 ＆ δ13C2） continuously increases. When the temperature rises,δ13C1 ＆ δ13C2 values sud- denly become low first and then slowly become high during the desorption process of shale gas. The reason for this phenomenon is probably due to the main adsorption/desorption of shale gas and diffusion migration fractionation of isotope. In addition, these variation characteristics of methane and ethane carbon isotope composition of shale gas may be the important reason for the low carbon isotopic composition of methane