晋城地区煤层甲烷碳同位素特征及成因探讨

Carbon isotope characteristics and its origin of coalbed methane in Jincheng area

对取自沁水盆地南部晋城地区的煤芯样中的解吸气进行了甲烷碳同位素测定.结果表明,随着解吸过程的进行,1δ3C1值逐渐变重,1δ3C1值和解吸时间呈对数关系,1δ3C1值变重趋势具有先快后慢的阶段性特点.取样条件和取样时间对煤层甲烷碳同位素值有较大影响,在某一个时间点所取气样的同位素值不一定代表该井原地气体的同位素值.在采样进行同位素测定时,煤样全部解吸气体的碳同位素的平均值才能代表该井煤层气的原地气同位素值.在实际操作中,可以用罐装煤样气体解吸半量时间点所取气样的同位素值来代表全部解吸气体的同位素平均值.与煤岩热模拟实验所得到的经验公式计算结果比较,晋城地区实测的煤层甲烷碳同位素值偏轻.晋城地区煤层甲烷碳同位素的组成特点受解吸-扩散-运移过程中发生的分馏效应以及其他多种因素的共同制约.

The desorbed methane carbon isotope of coal core samples collected from Jincheng area, South Qinshui basin was measured. The results indicate, the δ^13C1 value becomes heavier gradually as coalbed methane desorbs. The δ^13C1 value and the desorption time displays a logarithm relationship. The tendency of the δ^13C1 value becoming heavier has the interval feature of firstly quicker and then slower. The conditions and the time of sampling have strong influence on the δ^13C1 value, and the δ^13C1 value of gas sample collected at some time may not represent the isotope value of the in-place gas. When collecting samples to measure the isotope, the average carbon isotope of all the desorbed gas of coal sample can represent the isotope value of the in-place gas of the well. In the practical measuring, the isotope value of the sample collected at the time 50% desorption gas of canister desorption of coal can be used to represent the average isotope value of all desorption gas. Comparing with the calculated results based on the empirical formula of hydrocarbon-generation modeling experiment of coal, the actual measured δ^13C1 value of coalbed methane in Jincheng area is lighter. The composition characteristics of methane carbon isotope of Jincheng area is controlled by fractionation effect in the process of desorption-diffusion-migration and other factors.