渝东北田坝地区五峰-龙马溪组页岩矿物学特征及其油气地质意义

摘要

渝东北田坝地区五峰-龙马溪组泥页岩的矿物成分主要为石英和黏土矿物,其次为长石,此外还含有少量黄铁矿、硬石膏等含硫矿物,以及极少量的碳酸盐矿物.沉积学及矿物学研究表明,五峰-龙马溪组的沉积环境以浅海陆棚为主,而浅海陆棚属还原环境,有利于有机质富集和保存,可以更好地形成页岩气;页岩含石英量较多并且脆性指数较高,因此有利于形成裂缝以利于渗流.与相邻的涪陵地区相对比,两者可比性较高,并且渝东北田坝地区五峰-龙马溪组页岩的石英等脆性矿物含量大于涪陵地区,而黏土矿物含量、碳酸盐矿物含量远小于涪陵地区;渝东北田坝地区黏土矿物组合反映,五峰-龙马溪组已进入了晚期成岩作用阶段,该阶段所对应的有机质成熟度为高成熟-过成熟,表明其成熟度条件有利于形成页岩气藏;进一步研究还发现,五峰组及龙马溪组页岩具有较高的孔隙度和渗透性,能为页岩气储存提供较好的储集空间.%Shale samples are from Wufeng-Longmaxi Formation of Tianba area in northeast of Chongqing.The results showed that major components of the Wufeng-Longmaxi Formation mud shale were composed of quartz, clay minerals and feldspar.In addition, other mineral components include Sulfide mineral like pyrite, gypsum and few carbonate.The mineral assemblage revealed that the Wufeng-Longmaxi Formation was composed of neritic shelf deposits that was in favor of the preservation and enrichment of organic matter and provided a good depositional condition for the shale gas reservoirs.Shale reservoir contained much quartz, and brittleness index was high that were conductive to form crack.Comparing adjacent Fuling area and Tianba area, we would find they were similar and the quartz and brittleness mineral content of Tianba shale was greater that of than the Fuling area, however, the clay minerals content and carbonate mineral content of Tianba shale was less than that of the Fuling area.Clay mineral compositions characterization reflected that Wufeng-Longmaxi Formation had already been in a phyllomorphic stage and that was corresponding to a highly mature or postmature evolutionary stage of hydrocarbons and provided with a suitable maturity condition for shale gas formation.Moreover, the Wufeng Formation and Longmaxi Formation were highly poriferous and which provided a better place for the shale gas reservoirs.