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煤系地层致密砂岩气是中国陆相盆地主要的致密砂岩气类型,这种致密砂岩气源来自煤系烃源岩。中国煤系致密砂岩气以四川盆地须家河组和鄂尔多斯盆地上古生界最为典型。煤系烃源岩呈广覆式分布,大面积生排烃,进入大范围砂体后,天然气运移聚集过程中,砂体的非均质性控制了致密砂岩气局部富集和甜点形成。基于典型气区解剖和地质建模,开展了不同粒径砂与黏土相似材料模拟致密砂岩气运聚机理实验研究,以揭示煤系致密砂岩气充注、运移和聚集规律。实验结果表明,砂体渗透率级差控制天然气聚集的有效性、速率和含气饱和度。当砂体渗透率级差值为2.8时,即砂体非均质性较弱时,砂体内天然气不能有效聚集,而是运移和散失;当砂体的非均质性增强,渗透率级差值增加到5.4时,其中的天然气开始聚集成藏;当砂体非均质性进一步增强,渗透率级差值增加到9.0以上时,砂体内天然气发生大规模的有效聚集和成藏。在气源充足的条件下,大面积非均质性砂体中,物性相对越好的局部砂体,且该局部好砂体中均质性和连通性越好,含气饱和度越高。有断层连通的情况下,砂体的成藏速率增大。致密砂岩气运聚机理研究表明,勘探中应找分流河道、岩屑溶蚀相、断层裂缝和局部构造等部位物性较好的砂体,而其外围被物性较差的砂体或泥岩围限,则有利于天然气有效聚集和保存。
Tight sandstone gas in coalbed strata is the main tight sandstone gas in continental terrestrial basin in China. The tight sandstone gas source comes from coal-bearing source rocks. The tight sandstone gas of coal measures in China is characterized by the Upper Paleozoic in the Xujiahe Formation in the Sichuan Basin and the Ordos Basin. Coal-derived source rocks are distributed in a wide range of distribution, and large-area hydrocarbon generation and expulsion occur. After entering the large-scale sand body, the heterogeneity of sand body controls the local enrichment and dessert formation of tight sandstone gas during the migration and accumulation of natural gas. Based on the typical gas district anatomy and geological modeling, experiments on the mechanism of gas migration and accumulation in simulating tight sandstone with similar particle size of sand and clay were conducted to reveal the law of filling, migration and accumulation of tight sandstone gas in coal measures. Experimental results show that the permeability of sand bodies controls the effectiveness of gas accumulation, velocity and gas saturation. When the grade difference of permeability of sand body is 2.8, that is, when the sand body heterogeneity is weak, the natural gas in the sand body can not effectively aggregate but migrate and dissipate. When the sand body heterogeneity is enhanced, the permeability grade When the difference increases to 5.4, natural gas begins to accumulate in the reservoir. When the sand heterogeneity is further enhanced and the permeability difference is increased to 9.0, the natural gas in the sand body accumulates and accumulates in large-scale effectively. Under the conditions of sufficient gas supply, the local sand bodies with relatively good physical properties in the large-area heterogeneous sand bodies have better homogeneity and connectivity and higher gas saturation. In the case of fault connectivity, the rate of reservoir formation increases. The study on gas migration and accumulation mechanism of tight sandstone shows that the sand body with better physical properties such as distributary channel, lithic dissolution facies, fault fracture and local structure should be found in the exploration, while its periphery should be confined by sand body or mudstone with poor physical property, It is conducive to the effective accumulation and preservation of natural gas.