Reply – On maintaining high fluid pressures in older sedimentary basins

Abstract

The creation of abnormal pressure, during the generation phase, involves a competition between the rate at which the pressure-generating mechanism creates a change in fluid pressure and the rate at which the fluids can flow into, or out of the area impacted by the generating mechanism. For the sake of argument, let’s assume the mechanism tends to create higher fluid pressures. If the surrounding material is sufficiently high in permeability there will be a small or negligible rise in fluid pressure. Conversely, if the permeability is relatively low, high pore pressures can be created. Once the pressure-generating mechanism ceases to operate, fluid pressures will tend to return to normal. The rate of the return depends, again, on the permeability of the surrounding material. As suggested in my article (Bredehoeft 2009), a geologic conundrum arises in older sedimentary basins where, as far as we know, no pressure-generating mechanism continues to operate, and yet, high abnormal pressures persist. One explanation is for the permeability of sediments that surround the area of high pressure to be effectively zero. Revil et al. (1998) and Cranganu & Deming (2000) suggested that capillary barriers were a possible mechanism to create very low, effectively zero permeability. They suggested that this could explain the zones of abnormally high pore pressure within the Anadarko Basin, as well as in other sedimentary basins. I made no reference or comment on this mechanism for creating low permeability, pro or con – it was not my intent. I made an entirely different suggestion. Recently, the oil and gas industry has focused on developing gas resources that are contained in situ within organic-rich shale. I was told by individuals at the Electric Power Research Institute (EPRI) that the presence of the adsorbed gas in the shale was well known; the problem was how to extract it. Recent advances in the technology of both horizontal drilling and hydraulic fracturing have made it commercially attractive to extract the gas from shale. This created a boom in leasing and drilling shale gas prospects throughout North America in the 21st Century. The gas production boom in shale demonstrated that all the organic-rich shale beds that are considered source beds for oil and gas are full of adsorbed natural gas – this is a new idea for most geologists. I pointed out that the quantities of adsorbed natural gas, even within Paleozoic age shale, is enormous. I suggested that none of the earlier models of mature-basin flow analysis explicitly included a source bed within which is absorbed an enormous quantity of natural gas. This gas will be desorbed as the fluid pressure in the basin declines. (It is analogous to coal-bed methane, except that the quantities of gas are much larger.) In other words, there is another major source of fluids in almost every sedimentary basin that most previous analyses have not accounted for. For example, a back-of-the-envelope calculation indicates that the volume of gas in the Anadarko Basin (at atmospheric pressure) is of the same order of magnitude as the pore water in the basin – it is not insignificant and cannot be ignored, although the gas is highly compressible. I hypothesized that explicitly including the shale gas will change our ideas of how low the permeability must be to maintain abnormal pressures for very long periods.