四川盆地高石梯—磨溪地区震旦系—寒武系气藏盖层特征及封闭能力评价

Caprock Characteristics and Sealing Ability Evaluation of Sinian-Cambrian Gas Reservoirs in Gaoshiti-Moxi Area,Sichuan Basin

四川盆地磨溪地区寒武系龙王庙组发现了迄今为止中国最大的碳酸盐岩整装特大型气田,新增天然气探明地质储量4 403.8×10^8 m^3,而且在高石梯—磨溪地区已初步证实了震旦系、寒武系气藏规模超过万亿立方米。经历长期演化和多次构造运动的古老碳酸盐岩地层,为何气藏能够很好地保存下来,以及震旦系灯影组和寒武系龙王庙组气藏压力系数为何分别为常压和超压（压力系数为1.06～1.13和1.53～1.67）。通过宏观与微观、地质解剖与模拟实验相结合的方法,详细研究了气藏盖层特征及封闭能力。结果表明：研究区主要发育区域分布的寒武系泥页岩盖层、二叠系泥岩超压盖层和中下三叠统膏盐岩盖层;盖层岩石孔隙度以小于3%为主,渗透率主要为（0.001～0.02）×10^-3μm^2,平均喉道半径为0.6～1.2μm,孔径多集中在400～500nm之间,饱和水突破压力多大于15MPa,扩散系数为1.03×10^-10～2.36×10^-7cm^2/s。这些特征反映了盖层岩石具有一定的物性封闭能力,但诸如寒武系龙王庙组特大型整装超压气藏能够长期保存,是物性和超压共同封盖的结果。上述认识可以合理解释研究区常压、超压气藏的成因,同时对有效预测大气田的有利分布区域也有重要的指导意义。

The reason why Sinian-Cambrian gas reservoirs in the Moxi area of the Sichuan Basin can survive well and why there is a great difference between pressure coefficient of Dengying reservoirs and Longwangmiao reservoirs have attracted more and more scientists.A detailed study of gas reservoir caprock characteristics and sealing ability by the combination of macroscopic and microscopic as well as geology dissection and simulation experiment has been carried out.Cambrian shale caprocks,Permian overpressure shale caprocks and Low-Middle Triassic gypsum caprocks are regionally distributed in the study area.The characteristics of caprocks are as follows,porosity is mainly less than 3%,permeability is（0.001-0.02）×10^-3μm^2,average throat radius ranges from 0.6μm to 1.2μm,pore size is mostly concentrated in 400-500 nm,breakthrough pressure is more than 15 MPa,and diffusion coefficient ranges from 1.03×10^-10cm^2/s to 2.36×10^-7cm^2/s.Caprocks have certain sealing ability,but the reason that Longwangmiao Gasfield can survive for a long history is the result of interaction of capillary sealing and overpressure sealing.Understandings above can be used to interpret formation reason of atmospheric and overpressure gas reservoirs in the study area reasonably.Meanwhile,it can be used for predicting favorable distribution of large gas field.