试论长英质颗粒对湖相泥页岩脆性的控制条件

Control condition of felsic on brittleness of lacustrine shales

关键词:  渤南洼陷  长英质颗粒  细粒沉积岩  脆性  成岩作用  页岩气

Key words:  Bonan sag  felsic  fine-grained sedimentary rocks  brittleness  diagenic metamorphism  shale gas

摘要:

泥页岩的脆性是评价页岩油气开发潜力的重要指标,一般认为长英质颗粒含量与泥页岩的脆性指数具有正相关性。在渤南洼陷沙一段泥页岩矿物成分特征分析的基础上,利用三轴实验系统测试泥页岩的弹性模量与泊松比,计算泥页岩脆性指数,并分析泥页岩矿物成分与脆性指数之间的对应关系。结果表明:碳酸盐矿物含量与泥页岩的脆性指数呈正相关,而长英质矿物含量则与脆性指数基本不相关,这与常规认识有很大不同;长英质含量作为泥页岩的脆性指标是有条件的,研究区沙一段处于中成岩A期,成岩作用比较弱,泥页岩的固结程度差,陆源沉积物质之间的黏结程度低,造成长英质矿物含量与脆性指数不相关;从成岩作用过程的角度推测泥页岩在中成岩B期是长英质颗粒与脆性指数由不相关转为正相关的过渡阶段,因此较强的成岩作用是提高湖相泥页岩脆性的关键因素之一。

Abstract:

The brittleness of shale is one of the important indices for evaluating the potential of shale oil and gas development. In general, the felsic content is positively correlated with the brittleness index of shale. Based on mineral composition characteristic analyses of shale in the Sha 1 Member of Bonan sag, we applied triaxial testing system to measure the Young modulus and poisson ratio of shale, calculated the brittleness index and analyzed the corresponding relationship between the shale mineral composition and its brittleness index. The results indicate that the content of carbonate minerals is positively related to the brittleness index, whereas there is no such correlation between felsic minerals and the brittleness index, which is different from previous empirical conclusions. Therefore, it must be cautious to utilize the content of felsic mineral as the brittleness index of shale. Our study suggests that Sha 1 Member, Paleogene Shahejie Formation, is in the period A of middle diagenetic stage, resulting in the relatively weak diagenesis, poor degree of consolidation in shale, and weak bonding strength between terrigenous sediment, which leads to non correlative relation between the content of felsic and the brittleness index. From the perspective of diagenesis processes, it is speculative that the period B of middle diagenetic stage is a transition phase, in which the relationship between the content of felsic and the brittleness index transforms from non-correlative to positively correlative. As a summary, strong diagenesis is one of the key factors to promote the brittleness of lacustrine shale.