ABSTRACT Overpressure can be produced by the following processes: (1) increase of compressive stress, (2) changes in the volume of the pore fluid or rock matrix, and (3) fluid movement or buoyancy. Loading during burial can generate considerable overpressure due to disequilibrium compaction, particularly during the rapid subsidence of low- permeability sediments. Horizontal stress changes can rapidly generate and dissipate large amounts of overpressure in tectonically active areas. Overpressure mechanisms involving change in vol- ume must be well sealed to be effective. Fluid vol- ume increases associated with aquathermal expan- sion and clay dehydration are too small to generate significant overpressure unless perfect sealing occurs. Hydrocarbon generation and cracking to gas could possibly produce overpressure, depend- ing upon the kerogen type, abundance of organic matter, temperature history, and rock permeability; however, these processes may be self-limiting in a sealed system because buildup of pressure could inhibit further organic metamorphism. The poten- tial for generating overpressure by hydrocarbon generation and cracking must be regarded as unproven at present. Fluid movement due to a hydraulic head can generate significant overpres- sure in shallowly buried, “well-plumbed” basins. Calculations indicate that hydrocarbon buoyancy and osmosis can generate only small amounts of localized overpressure. The upward movement of gas in an incompressible fluid also could generate significant overpressure, but requires further investigation. Stress-related mechanisms are the most likely causes of overpressure in many sedi- mentary basins. INTRODUCTION Overpressure, sometimes termed “geopres- sure,” is common in subsurface rocks. A pore fluid is overpressured if its pressure exceeds that of the hydrostatic gradient at a specific depth. The hydrostatic pressure gradient is the pressure that would be exerted by a continuous column of static fluid (Figure 1), and will vary slightly depending upon the density of the pore fluid. It is important to understand how overpressure is generated if fluid pressures are to be predicted prior to drilling. In addition, overpressure observed in rocks today owes its distribution not only to the mechanisms of generation, but also to the redistribution of fluids during and after the creation of overpressure. The hydrodynamics of sedimentary basins are controlled by differentials of fluid pressure created by forces other than hydrostatic differences. A wide variety of mechanisms have been pro- posed for the generation of overpressure in sedi- mentary basins. These mechanisms can be divided into three categories: (1) increase in compressive stress (i.e., reduction of the pore volume) caused by disequilibrium compaction and tectonic com- pression; (2) fluid volume change caused by tem- perature increase (aquathermal pressuring), diage- nesis, hydrocarbon generation, and cracking to gas; and (3) fluid movement and processes related to density differences between fluids and gases caused by hydraulic (potentiometric) head, osmo- sis, and buoyancy. In addition, overpressure gener- ated at one site can be redistributed elsewhere in the rock succession. Previous reviews of overpressure mechanisms have concentrated on theoretical or mathematical aspects (Hall, 1993; Neuzil, 1995), or are more focused on overpressure prediction and detection (Mouchet and Mitchell, 1989). Our objective with 1023 AAPG Bulletin, V. 81, No. 6 (June 1997), P. 1023–1041. ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved. 1 Manuscript received October 17, 1995; revised manuscript received September 4, 1996; final acceptance January 20, 1997. 2 Department of Geological Sciences, Durham University, South Road, Durham DH1 3LE, United Kingdom. Osborne e-mail: M.J.Osborne@ durham.ac.uk; GeoPOP web site http://www.dur.ac.uk/~dgl0zz7/ We wish to thank the companies that support the Geosciences Project on Overpressure (GeoPOP) at the universities of Durham, Newcastle, and Heriot-Watt: Agip, Amerada Hess, Amoco, ARCO, Chevron, Conoco, Elf Exploration, Mobil, Norsk Hydro, Phillips Petroleum UK Company Limited, Statoil, and Total. We also thank Neil Goulty (Durham) for commenting on an earlier draft of this paper. Osborne thanks Gordon Macleod (Newcastle) for help with geochemical modeling. Mechanisms for Generating Overpressure in Sedimentary Basins: A Reevaluation 1 Mark J. Osborne and Richard E. Swarbrick 2