煤成甲烷碳同位素演化的数学模型与应用

MATHEMATIC MODELS OF STABLE CARBON ISOTOPE COMPOSITION OF COAL-DERIVED METHANE AND THEIR APPLICATIONS

分别介绍了以统计为基础的甲烷碳同位素与源岩成熟度关系的关系模型、以 Rayleigh方程为基础的同位素分馏模型和以动力学为基础的同位素动力学模型等 3种模拟煤成甲烷同位素演化的数学模型 ,评价了其应用范围和应用效果。用这 3种模型恢复了塔里木盆地克拉 2气田煤成气的聚散史后认为 ,克拉 2气田的天然气可能是源岩成熟过程中长期累积形成的 ,早期形成的天然气仅有少量散失 ,而晚期形成的天然气因超压而无法充注到气藏中。

In this paper different mathematic models are discussed, including the relationship models connected the stable carbon isotope composition with source rock maturation, distillation models based on Rayleigh expression and kinetic models, and a relationship diagram between instantaneous and accumulation carbon isotope composition linked to source bed maturity is proposed on the basis of other researchers, which can be used to roughly estimate the accumulation/loss history of coal-derived gas pool. Applicable extents of different models are illustrated. The gas accumulation/loss history of Kela 2 gas pool in the Kuqua depression of Tarim Basin, the biggest gas field of China, is restructured as an example by using isotope kinetic model and others. The results suggest that the natural gas of Kela 2 pool, with the carbon isotopic value of -27.78‰^-27.73‰, cumulated from early stage of gas generation to about 2.0 Ma, which was hardly lost during the source rock maturation and gas generation because of coalbeds with strong absorbability. However, the late generated gas was not filled into this pool due to the overpressure development starting at about 2.0 Ma, which is supported by non\|hydrocarbon components in this gas pool.