Geopressures in the Deep Smackover Of Mississippi

Abstract

Through field examples, this paper examines diverse origins of deep Jurassic geopressures in the interior basin of Mississippi, contrasting them with geopressures in the Gulf Coast Tertiary. Unlike the latter, which result from loading and undercompaction caused by rapid deposition, the Jurassic geopressures are related to geologically late. Introduction and Purpose Many concepts about geopressures have been developed relative to the Gulf Coast Tertiary where shale porosity variations often relate to overpressures. In porosity variations often relate to overpressures. In the deep Jurassic of Mississippi, where simply finding any porosity is perhaps anomalous, there is a more complex rock package dominated by quartzites, limestones, dolomites, evaporites, and mixtures of these. This rock package provides a very competent seal to contain any late, deep geopressures that may generate. Deep geopressures in Mississippi commonly are found in hostile environments characterized by high temperatures and sour or acid gases. This environment precludes some of the more standard pressure measurements; hence basic data are not so available as in other geological regions. The purpose of this paper is to examine diverse origins for geopressures. Late geopressures that have generated below competent Jurassic seals contrast with early, leaky types in the Gulf Coast Tertiary, A related purpose is to argue that deep Jurassic reservoirs are not a result of geopressures' propping open interparticle pores, as might occur in the Gulf Coast Tertiary, but are due to secondary porosity. This paper examines how late secondary porosity may paper examines how late secondary porosity may relate to late geopressures. Concept of Geopressure When Dickinson plotted Gulf Coast pressures with depth, he found no rectilinear increase of hydrostatic pressure with depth. There are, instead, disruptions pressure with depth. There are, instead, disruptions of hydrostatic equilibrium caused by faults and sand distribution that result in levels of unequal flow potential throughout the Tertiary section. In seeking potential throughout the Tertiary section. In seeking hydrostatic equilibrium, water is moving in response to gradients between zones of unequal flow potential and according to Darcy's law, but at a rate so geologically slow that the water is required to support part of the overburden weight. This weight or load part of the overburden weight. This weight or load of the fluid-saturated sediment is the driving force causing the water to seek hydrostatic equilibrium. Stuart coined "geopressure" as a genetic term to express this driving force: a geopressure is an overpressure generated by the overburden. Stuart's geopressure term has been used within the Shell organization since 1956. Hubbert and Rubey assert, "Anomalous water pressures invariably imply a hydrodynamic state. pressures invariably imply a hydrodynamic state. Unless rocks are ideally impermeable, it follows that away from any pressure greater than nominal pressure water must be flowing and must continue to do so until excess pressure is dissipated." Therefore, an important concept about a geopressure is that it involves a potentially dynamic fluid. Dynamic fluids are important in porosity-leaching mechanisms and as force fields for hydrocarbon trapping. Many overpressure classifications have focused on the type of seal that isolates the overpressure; however, the driving force is probably a more important consideration. P. 971