沁水盆地东南部寺头断层对煤层气藏的封闭性评价

An evaluation study on the sealing mechanisms of Sitou fault to coalbed methane reservoirs in the Southeast Qinshui Basin

寺头断层是沁水盆地东南部煤层气藏的主要控制断层,其封闭性对于煤层气富集成藏具有重要意义。根据煤、油气、煤层气勘探开发资料,在分析寺头断层发育史的基础上,从断层两侧岩性配置、泥岩涂抹效应,颗粒碎裂作用、胶结作用等方面对寺头断层的封闭机理进行了研究。结果表明:①当断层落差较小时,断层泥比率(SGR)和泥岩涂抹潜势(CSP)值较大,泥岩涂抹效应强烈,且与断层两侧煤层对接的岩石主要为泥岩类,此时起封闭作用的主要为泥岩涂抹作用和断层两侧岩性配置关系;②当断层落差较大,泥岩涂抹作用较弱时,对接越不利于封闭形成,但碎裂作用强烈,沿断层两侧形成的糜棱岩对煤层起到了封闭作用;③地下水活动导致的胶结作用对整个断层的封闭具有重要的意义;④断裂作用形成的糜棱煤沿断层两侧分布,此糜棱煤带是一种低渗透、高排替压力的特殊储层,其规模与断层的落差、性质、上下盘都有一定关系。对寺头断层而言,落差越大,糜棱煤的分布宽度越大。

The Sitou fault is the major fault controlling the coalbed methane reservoirs in the Southeast Qinshui Basin.Its sealing capacity is critical to the enrichment and accumulation of coalbed methane.According to the data in the exploration and development of coal,petroleum and coalbed methane,and based on an analysis of the growth history of the Sitou fault,its sealing mechanisms are studied in respect of juxtaposition,clay smear,cataclasis,and cementation.When the fault throw is relatively small,the shale gouge ratio(SGR)and clay smear potential(CSP)are relatively large and shale smear is strong.In addition,the coal layers on both sides of the fault are mainly juxtaposed against shale.Therefore,shale smear and juxtaposition play a key role in the sealing.In contrast,when the fault throw is relatively large and shale smear is relatively weak,juxtaposition is unfavorable for sealing.However,the mylonite resulted from strong cataclasis can act as a seal of the coals.The cementation resulted from underground water activity is of great significance to the sealing ability of the fault.Mylonitic coal layers resulted from faulting occur on both sides of the fault.They are unique reservoirs with low permeability and high displacement pressure and their scales depend,to some extent,on the fault throw,fault properties,the hanging wall and footwall.As to the Sitou fault,the larger the fault throw,the wider the mylonitic coal layers.