Controls on shale durability: the response of two Ordovician shales in the slake durability test

Abstract

The slake durability test has been proposed as a means of predicting the engineering performance of shales. Testing of sections in the two major shale units of Ontario show that the test is capable of making distinctions between apparently similar shale samples. Queenston Shale has generally lower durability than the Georgian Bay Formation samples. Inspection of the shale fabric shows that this is due partly to inefficient cementing by calcite in Queenston Shale, but is primarily because the microcracks in Queenston Shale are more curved than those in Georgian Bay Formation. Variation in shale durability within the shales is controlled by mineralogy. In Queenston Shale, it is controlled almost entirely by calcite cementation. In Georgian Bay, where hard bands (shaly limestone) are present, these dominate the test, giving a high durability. However, pure shale durability is controlled by clay content.Comments on the performance of the test are made that aim to make the testing process and interpretation of the results more reliable. The method appears suitable for shale index testing and should be used as widely as possible in order to establish reliable correlations.