Thickness of Removed Sedimentary Rocks, Paleopore Pressure, and Paleotemperature, Southwestern Part of Western Canada Basin

Abstract

Abstract The thickness of sedimentary rocks removed by erosion in the geologic past can be evaluated by the use of shale-compaction data, such as the shale transit-time values recorded by sonic logs. The pore pressures in shales also can be calculated using compaction information. Integration of these shale-compaction data with homogenization temperatures of fluid inclusions in quartz infilling of fractures in sandstone, as discussed by Currie and Nwachukwu, makes possible the estimation of the paleotemperature and paleogeothermal gradients in a basin. Such an integrated study in the southwestern part of the Western Canada basin shows that (1) significant erosion in the geologic past removed as much as 4,600 ft (1,400 m) of the uppermost part of the sedimentary rocks; (2) significant undercompaction and overpressure were present in the deeper part of the Cretaceous section in the past; (3) the Cardium sandstone, which has a temperature range of about 140 to 160°F (60 to 71°C) at present, had much higher temperatures of about 300°F (149°C) at the time of maximum burial (before erosion); and (4) the geothermal gradient probably has not changed significantly in the geologic past.