A PHYSICAL EXPLANATION FOR THE POSITIONING OF THE DEPTH TO THE TOP OFOVERPRESSURE IN SHALE-DOMINATED SEQUENCES IN THE GULF-COAST BASIN, UNITED-STATES

A one-dimensional model of fluid pressure and porosity evolution is us ed to investigate the physical processes that control the development and maintenance of overpressure in a compacting sedimentary basin. We show that for shale-dominated sequences the variation of the hydraulic diffusivity in both space and time is such that it produces a minimum between 2 and 4 km depth, consistent with observations from the Gulf Coast basin. This minimum inhibits the upward flow of fluid by acting as a ''bottleneck'' and thus determines the shallowest position of the depth to the top of overpressure. Above this region of bottleneck, ov erpressure does not develop because the porosity is sufficiently large to maintain high values of hydraulic diffusivity that are conducive t o the rapid dissipation of excess fluid pressure. Within the overpress ured shales, compaction propagates downward through the section, relea sing fluids from the upper part of the section while continuing to res train the upward flow of fluids from deeper within the section. As suc h, overpressures are predicted to be maintained within the deeper regi ons of a basin for tens to hundreds of millions of years. Further, flu id viscosity plays an important role in defining the depth behavior of hydraulic diffusivity as a function of time. Assuming a temperature-d ependent fluid viscosity guarantees that the hydraulic diffusivity min imum will always exist during the development of the basin. On the bas is of our results, we find that the depth at which the porosity equals 14 +/- 4% correlates with the depth to the local hydraulic diffusivit y minimum and thus the depth to the top of overpressure. Moreover. we interpret that the 14 +/- 4% represents the threshold porosity for whi ch a shale actually begins to act as a seal. Within the Gulf Coast bas in, the gross sediment facies consists of lower massive shales across which deltaic systems have prograded allowing the deposition of an alt ernating series of sandstones and shales that grade vertically into ma ssive sandstones. The massive sandstones are highly permeable and are connected hydrologically to the surface. We conclude that these sandst ones play little role in the development of overpressure because of th eir high permeability except to the extent that the base of the massiv e sandstones marks the minimum depth possible for the top of overpress ure. In contrast, overpressuring is observed to develop within either the shale-dominated sequence or the region of interspersed/interfinger ing sands and clays. The clay-encompassed sands play only a passive ro le in the development and maintenance of overpressure because it is th e low-permeability clays that control the movement of fluids into and out of the sands.