The undrained shear behaviour of mudrocks is important in petroleum engineering; for instance; when performing wellbore stability analyses. In this paper, the undrained shear behaviour of mudrocks is investigated experimentally, and the observed results are explained in terms of the petrographical properties of the tested materials. Tests were conducted on Kimmeridge clay, which was sampled from two sites in the UK. The two samples have undergone different burial histories and different diagenetic reactions. Rock mechanical tests under high confining stresses are performed to examine the undrained deformation properties, effective stress path, and peak- and post-peak shear strengths of the two types of madrock. The geological properties of the samples, including texture, mineralogy, and diagenetic minerals, have been investigated by X-Ray diffraction, various types of scanning electron microscopy, and optical microscopy. The results are compared with those from other UK mudrocks. The study shows that both the primary depositional mineralogy and the burial history affect the undrained shear behaviour of mudrocks. It is also seen, that,diagenetic reactions greatly increase both the undrained peak strength and shear stiffness of mudrocks.