沁水盆地南部煤系地层吸附气与游离气共生成藏研究

Study on Coexistence of Absorbed Gas and Free Gas in Coal Strata South of Qinshui Basin

沁水盆地煤系地层天然气资源丰富,具有形成大中型气藏的物质基础。大量钻探成果表明,3#煤层与15#煤层是主要的吸附气含气层,而游离气一般赋存在煤层裂隙、煤层顶底板的围岩中,区内煤系地层吸附气和游离气之间普遍存在同源共生的特点,具有相近的温压条件,形成统一的吸附气—游离气共存系统;在系统内部,煤层吸附气与煤层游离气、煤层游离气与围岩中游离气之间发生动态交换作用,形成一个动态平衡体系。随着构造演化和温压条件的变化,会引起吸附气和游离气之间的不对称迁移。根据煤层与其他岩层接触关系,可将煤层吸附气—游离气共存系统划分为3类:煤岩—顶板型、煤岩—底板型、煤岩围限型。可根据围岩岩性进一步划分为煤岩—砂岩型、煤岩—泥岩型和煤岩—石灰岩型,不同的煤岩—岩性组合具有不同的煤层吸附气、游离气含气特征。沁水盆地南部煤系地层吸附气与游离气同源共生规律的发现,为实现煤层吸附气与游离气合理共采奠定了理论基础。

Coal strata in Qinshui Basin are rich in natural gas resources and they have better physical condition for forming medium-large gas pools.Plenty of drilling results show that 3# coal bed and 15# coal bed are major absorbed gas beds,and free gas mainly exists in surrounding rocks of coal-bed fracture and coal-bed roof and floor.Absorbed gas and free gas coexist in the Basin and they are of the same origin,with similar temperature and pressure.So a coexistence system of absorbed gas and free gas comes into being,within which the absorbed gas and the free gas in the coal strata,the free gas in the coal strata and the free gas in surrounding rocks exchange dynamically and form a dynamic equilibrium system.Tectonic evolution and the change of temperature and pressure may lead to asymmetric migration of absorbed gas and free gas.According to the contact relations between coal beds and other rocks,the coexistence system could be classified into three types:the coal rock-roof type,the coal rock-floor type,and the confined type of coal rock.According to the lithology of surrounding rocks,the system could be further classified into the coal rock-sandstone type,the coal rock-mudstone type,and the coal rock-limestone type.Different coal rock-lithology associations vary in gas-bearing features.The paragenesis regularity of absorbed gas and free gas of the same origin in the coal strata south of Qinshui Basin lays theoretical basis for their rational exploitation.