Jakob Heller, Stephen O ’Connor, Andrew Emery, Ed Hoskin, Jack Lee and Chris Ward
In many basins in SE Asia such as the Nam Con Son Basin, offshore Vietnam, East Java Basin, Indonesia, basins offshore Thailand and the North-West Shelf of Australia e.g. Browse and Carnarvon basins, we have observed unusual pore pressure profiles in thick shale packages that are beneath unconformities such as the Middle Miocene Unconformity (MMU). These pressure profiles are parallel to the regional hydrostatic gradient, although still overpressured. The amount of overpressure within these shales appears related to the post-unconformity magnitude of re-burial. Typically, pressure profiles through thick shale units, where disequilibrium compaction is generating the overpressure, are parallel to the overburden - the weight of the rock column and contained fluids. The profiles here therefore appear anomalous.
We present several examples of these “atypical” profiles. In the case from the Nam Con Son Basin, Vietnam, we have used the ‘Swarbrick Method’ to determine the overpressure created by recent loading. That is, from the depth of the MMU, we use sedimentation rates to calculate the shale pressure at unconformity - this then provides a guide to shale pressure (and overpressure) below the unconformity.
In the other main case, from the Dampier sub-basin, Carnarvon Basin, North-West Shelf, we note a relationship between the unconformity associated with the Walcott Formation whereby the thickness of the seabed to Top Walcott i.e. the loading by this interval, provides a guide to deep shale pressure by providing a limiting factor.
In summary, a combination of the ‘Swarbrick Method’ and an unconformity/loading model can be invoked to explain shale pressures in many basins. This geologically-based model for shale pore pressure has proved to be a successful forward-modelling approach in well planning in these basins, where the only inputs required are ages of seismic markers and depth of the relevant unconformity to give maximum shale pressures, matching kicks taken in wells such as Montague-1 and Angel-1.