Producing pore pressure profiles based on theoretical models in undrilled, deepwater frontier basins

Abstract

A working petroleum system was established on the shelf in offshore Labrador with the Bjarni H-81 discovery in 1973 in the Hopedale Basin. The same reservoirs as those targeted on the shelf are present in the deep water, which is currently receiving attention as the result of newly acquired seismic data. To date, only a very small number of wells have been drilled in the deep water, i.e., Blue H-28, Orphan Basin, and none off mainland Labrador. The wells that were drilled in the deep water had encountered significant overpressure, e.g., kicks that indicated overpressures of 26,850 kPa in the Mid-Cretaceous. Therefore, it was reasonable to assume that pore pressures be similarly high for any new deepwater prospects identified. To help reduce the risk in unexplored environments, we developed an approach that can be adopted to model pore pressure in deepwater settings, with Labrador as the main case study area featured, but also we discussed other global examples such as the Vøring Basin, Mid-Norway. Our results indicated, as a first approximation, that seismic velocity-based pore pressures in shale-rich intervals were similar to the geologic model down to the Lower Tertiary. Deep lithologies were, by regional analogue, likely affected by cementation that will act to preserve overpressure generated by disequilibrium compaction by reducing permeability but will not generate additional pore pressure. The cements (and any carbonate or volcanic lithologies) will, however, result in faster shales and will underpredict pore pressure by mimicking low porosity. A theoretical or “geologic modeling” approach can be used to sense-check any pore pressure interpretation from seismic velocity. The geologic approach also can be used to assess the risk for mechanical seal failure by allowing for estimates of the pore pressure, and related fracture pressure, to be made without the effects of cementation that affect the logs and seismic velocity data.