天然气藏的封存条件研究——以冀中坳陷北部地区为例

OBTURATION FACTORS ASSEMBLING NATURAL GAS:TAKING NORTH REGION OF JIZHONG SAG AS THE EXAMPLE

由于天然气"易扩散"的个性,在拉张伸展的裂谷盆地成藏过程中,其封存条件(盖层)十分重要,特别是纯气藏。冀中北部地区发育类型丰富的天然气藏,除了具备良好的生气条件以外,还与封存条件密切相关。基于该区天然气藏实钻资料的统计,系统分析了盖层的封存特性,讨论了封存能力。首次将本区盖层划分为3个封存系统,归纳出3个封存特性(毛细管力作用、超压封存作用和烃浓度封存作用),确定出毛细管力封存标准。同时认为, - 类盖层为好—较好盖层, - 类盖层为较差—中等盖层,能够形成天然气藏。石炭—二叠系的本溪、山西组煤系地层的排替压力大于5MPa,具备 - 类毛细管力封存条件,属于较好盖层。下第三系孔店组—沙三段泥岩的排替压力大于0.5MPa,最大值达2.83MPa,欠压实带发育,也是主要烃源层段,具备良好的超压和烃浓度封存作用,评价为较好—中等盖层。影响盖层封存能力的因素主要有2个方面:一是岩石与成岩作用,由于泥岩砂质含量增加,封闭能力减小,单层泥岩厚度大可以降低天然气的扩散速度,故中等成岩程度的泥岩就具备好的封存能力,也就是说,本区具备早成岩B期—晚成岩A期阶段的盖层封存性能;二是地层与构造,由于地层压力高不利于盖层的封存,一般低幅度圈闭的成藏条件优越于高幅度圈闭。

Due to pervasion diffusibility of gas, obturation factors of gas is very importance in rift-basin with tension tectonic suffering more tectonization. A lot of difference types gas pools have been explored in north region of Jizhong sag, China northern, besides having excellent source rocks of generating gas hydrocarbon, and yet relation of obturation factors of gas. The paper generally discussed obturation factors (cover) assembling gas, expatiated main research results.The capillary pressure obturation standard of gas pool be founded in the region, withⅠ-Ⅱ types cover being good and better, Ⅲ-Ⅳ types cover being middling,for them being capable of obturating gas pools. The displacement pressure values from coal covers, Bengxi-Sanxi group of Carboniferous-Permian, can be more then 5 MPa, belong to Ⅱ-Ⅲ types of being better cover. The displacement pressure values of mud-rock covers, Ek-Es^(3) of Lower Terrtiary, commonly can be more then 0.5 MPa, the most value is the 2.83 MPa, with containing nor-coMPaction zone, being primary source rocks stratum. Therefore, superpressure-obturation-action and hydrocarbon-obturation-action are more important, covers of Ek-Es^(3) are to belong better covers. Factors of rocks and diagenesis: The obturation capability of cover is depressed along with increase of arenaceous rocks; thick single mud rocks can debase pervasion action of gas. The rocks of middling extent of diagenesis has the most excellent capability of obturation, in other word, with being early diagenesis B phase-late diagenesis A phase. Factors of stratum and tectonic: lower amplitude traps are better obturation action of gas then higher amplitude. Obturation acton of fault be engendered by character and scale of fault as well as stratum contact between ascend and lift stratum, but some small size late fault is destruct action to obturation after produced gas pools.