Possible effects of erosional changes of the topographic relief on pore pressures at depth


  • J. Tóth,

  • R. F. Millar


Formation fluid pressures are said to be anomalous or abnormal if they differ from hydrostatic for the depth considered. Explanations of the origin and maintenance of anomalies seldom include the possibility of delayed adjustment of subsurface pore pressures to erosional modifications of the topographic relief. Yet the present paper shows that it is entirely possible for subaerial erosion to generate pore pressure changes at great depths by cross-formational energy transfer in realistic field conditions. Conversely, pressures may survive changes in the topography and be in a transient state for hundreds to millions of years, thus appearing anomalous at the time of observation. According to calculations for the Red Earth region in Alberta, Canada, present pore pressures in the Devonian immediately above the Precambrian basement at a depth of approximately 1400 meters may have been induced originally by erosionally shaped topography during Pliocene times 12 to 1 m.y. B.P. but be adjusting now to new boundary conditions created by the erosional exposure of the sub-Cretaceous unconformity. A 12% adjustment may already have taken place during the last 300 thousand years, leaving nearly 90% of Pliocene age pressures to survive to date as slowly decaying transient pressure relicts of the geologic past. In view of the possible erosional effects on pore pressures demonstrated in this paper, the question of anachronous formation pressures may deserve consideration in problems related to deep basin hydrology, including regional groundwater resource development, migration and accumulation of hydrocarbons, radioactive waste isolation, dating of various geologic events, and so on.