Abnormal fluid pressures caused by deposition and erosion of sedimentary basins
Gravitational loading or unloading due to deposition or erosion in sedimentary basins is one of the most common mechanisms responsible for abnormal fluid pressures in geologic formations. This paper examines the integrated response of fluid pressures to both deposition and erosion using vertical one-dimensional numerical models. The modeling results show that vertical loading and unloading of sedimentary basins containing thick interbeds of low permeability can give rise to abnormal fluid pressure-depth trends that closely resemble those observed in the field. On a geological time scale, an under-pressured formation may become normally pressured or over-pressured and vice versa because of transient flow induced by vertical geologic movements. A case study in the Guymon area of Texas County, Oklahoma, shows that there are two underpressured reservoirs in this area. The burial curve constructed from geologic information indicates that this area has undergone multiple subsidence and uplift events. Based on stratigraphic information, a vertical geologic section of this area is represented by a numerical model consisting of two reservoir formations and two shale (confining) units. The numerical model is able to reproduce the observed pressure abnormalities based on the drill-stem test (DST) data. The hydraulic conductivities of the two shale formations are estimated to be 4.2 × 10 and 5.1× 10 m s from model calibration.q 1998 Elsevier Science B.V.
Possible effects of erosional changes of the topographic relief on pore pressures at depth
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.
Physiographic Development of the Northern Great Plains
Introduction GENERAL STATEMENT This paper is a summary of the results of a study, not yet completed, of the Tertiary and Pleistocene gravel terraces and associated glacial deposits and erosional phenomena of the plains of Montana, Big Horn Basin, Wyoming, and adjacent areas. The writer’s studies, which have extended over a large part of six field seasons, are in the nature of a rapid reconnaissance of this vast area for the purpose of adding to and correlating data observed by a large number of other geologists who have worked in this region. The deposits mapped and the correlations made are shown on plate 11 as well as can be done on this small scale. It is hoped to present a better map, more detailed descriptions, and discussion of the evidence in an official paper now in preparation. Most of the detailed mapping, where such has been done, is the work . . .