Abstract

INTRODUCTION

Fluid pressures in excess of hydrostatic can be generated in sedimentary basins by way of several mechanisms. Among the most common mechanisms proposed are gravitational loading (Dickinson, 1953), the montmorillonite-illite transformation (Burst, 1969), and the thermal expansion of water due to heating (Barker, 1972). Of these, gravitational loading has been singled out as being most susceptible to mathematical treatment, largely because the dissipation of such pressures is a problem in fluid flow for which the physics have been known for more than 50 years (Terzaghi, 1925). Examples here include the work of Bredehoeft and Hanshaw (1968) and of Smith (1971), who dealt with the problem of fluid pressure development In isothermal sedimentary basins receiving sediments over long periods of time. The primary motivation of this current study is to incorporate the effects of a geothermal gradient in theoretical models of deep sedimentary basins and to determine the conditions under which thermal effects provide an important contribution to the total excess pressure development. Because the cumulative effect of gravitational loading and water expansion can result in excess pressures of sufficient magnitude to initiate some sort of inelastic deformation, an opportunity is provided to qualitatively assess the significance of excess pressure generation insofar as it influences the initiation of fracture in the host rock.

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