Overpressure Detection Using Shear-wave Velocity Data - A Case Study from the Kimmeridge Clay Formation, UK CNS

The slowing of compressional-wave velocity (Vp) with increasing pore pressure has been used for overpressure detection in shales for many years, however, the success of which is highly dependent on the mineralogy, depth of burial, and the history of diagenesis of the shales. In the case of ‘hot’ shales, such as those >100oC, important additional mechanisms unrelated to change in confining stress may be occur such as (a) aquathermal fluid expansion; (b) hydrocarbon source maturation and fluid expulsion and, (c) clay diagenesis such as, smectite to illite conversion ( Swarbrick et al., 2002 ). In these particular cases the compressional-wave velocity will not uniquely indicate the pore pressure. There are techniques that exist that help predict the pore pressure where these addition mechanisms are active (e.g. Bowers 1995 ). However, these relationships use Vp as an input, which slows when gas is present, regardless of the overpressure regime. We propose to highlight this ambiguity of compressional-wave velocity data for pore pressure prediction in this situation, and seek to improve the reliability of overpressure detection in high pressure and high temperature (HP/HT) settings by using shear-wave velocity (Vs) data.