Abstract: Anyue gas field in Sichuan Basin is known as an ancient large gas field of oil cracked gas with high thermal evolution, where the large amount of oil cracked gas and gas reservoirs were formed in the Jurassic to Cretaceous period. According to the accumulation characteristics of large gas fields in China and due to small natural gas molecules and easy diffusion, the loss of natural gas in large gas fields with early accumulation will increase with the increase of diffusion time. In view of the problem that the Anyue gas field can be preserved as a large gas field even though it was formed in the early stage, the analysis of deep gas diffusion model and the evaluation of the diffusion capacity of ancient gas reservoirs are the key to understand whether the deep ancient gas field can be effectively preserved. The multi-layer caprock model uses the comprehensive diffusion coefficient of the overlying strata of the gas field and takes into account the comprehensive sealing effect of the multiple strata overlying the deep gas field. Taking Anyue gas field as an example, single-layer and multi-layer caprock diffusion models are used to calculate the diffusion capacity of the Cambrian Longwangmiao Formation gas reservoir, and the total gas diffusion capacity is calculated by the gas density ratio between the ancient gas reservoir and the present gas reservoir. The results show that according to the classical single-layer caprock diffusion model, the cumulative diffusion capacity of the gas field after 90 Ma is 3 340×10⁸m³, far exceeding the upper limit of the total gas diffusion capacity (1 020×10⁸m³), indicating that the single-layer model is not applicable. The cumulative diffusion capacity calculated by the multi-layer caprock diffusion model is 920×10⁸m³, which is much lower than that calculated by the single-layer diffusion model. In fact, the deep gas field is covered with multiple regional caprocks, which can greatly reduce the gas diffusion. These results provide a method to evaluate the diffusion capacity of deep gas reservoirs and help to deepen the understanding of the preservation conditions of deep ancient gas fields.

Key words: deep layer, natural gas, caprock, diffusion, Sichuan Basin, Anyue gas field

摘要： 四川盆地安岳气田是古老高热演化程度原油裂解气大气田,其原油大量裂解生气及气藏定型时间在侏罗纪-白垩纪。根据中国大气田成藏特征以及对天然气分子小、易扩散特点的认识,早期成藏的大气田随着扩散时间增长,其天然气散失量也会不断增加。针对安岳气田在早期成藏但仍能保存为大气田的问题,开展深层天然气扩散模型分析和古老天然气藏扩散量评价,目前已成为认识深层古老气田能否有效保存的关键问题。多层盖层模型利用了气田上覆地层的综合扩散系数,考虑了深层气田上覆多套地层的综合封盖效应。以安岳气田为例,采用单层盖层和多层盖层扩散模型分别计算寒武系龙王庙组气藏的扩散量,并通过古气藏与现今气藏的天然气密度比例计算天然气散失总量。按照经典的单层盖层扩散模型进行计算,气田经历从90 Ma至今的累积扩散量(3 340×10⁸m³)将远远超过天然气散失总量的上限(1 020×10⁸m³),这表明单层模型不适用。而多层盖层扩散模型计算的累积扩散量为920× 10⁸m³,比单层盖层扩散模型的计算结果大幅降低。深层气田上覆多套区域盖层,可大幅降低天然气扩散量。研究结果提供了一种评价深层气藏扩散量的方法,有助于深化对深层古老气田保存条件的认识。

关键词: 深层, 天然气, 盖层, 扩散, 四川盆地, 安岳气田