成藏后天然气组分与同位素的分馏效应研究

COMPOSITION AND ISOTOPE FRACTIONATIONS OF ACCUMULATED NATURAL GAS AND THEIR SIGNIFICANCE

天然气成藏后会发生扩散、溶解逸失、微生物降解和高温裂解等作用,这些作用会使天然气组分、同位素发生分馏。其中扩散分馏取决于不同分子的半径与不同方向上的浓度梯度,溶解分馏则受控于分子大小与水合能力,这2种分馏作用常使藏内小分子烷烃含量减少、同位素组成变重;微生物对天然气的降解受温压条件控制,并且不同细菌对烃类具有不同嗜好性,常使天然气重烃含量减少、同位素组成变重;热裂解作用可使天然气组成变干、同位素组成变重。认为通过化学反应动力学可以分析藏内天然气分馏作用的条件及其定量过程,恢复成藏初期天然气特征,并有助于混源气判别、资源散失量估算等。

Hydrocarbon composition and stable carbon isotope composition of natural gas are fractionated by diffusion, dissolution, microbial degradation and cracking in gas reservoirs. The diffusive fractionation is determined by radius and density grads of the hydrocarbon molecules, the dissolving fractionation is controlled by bulk volume and hydration capability of the dissolved molecules. The two kinds of fractionations result in enrichment of heavy molecules in the gases （C2^＋） and heavy carbon isotopic composition. Hydrocarbon gases could be biodegraded selectively by microbes in gas reservoirs when the reservoir temperatures less than 80C. The biodegradating fractionation leads to decrease of C2^＋ components and δ^13C value. When gas reservoir buried to enough depth and its temperature over 200 C,the C2^＋ molecules would be cracked, which increases δ^13C value also. Application of chemical dynamics to rebuilding the fractionation history, the primary hydrocarbon composition and carbon isotope composition at beginning of the gas accumulation will be characterized, which is useful in quantitative identification of a gas-pool formation and estimation of the source ratio for the gases derived from different sources.