Structural permeability of fluid-driven fault-fracture meshes

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 volume must be well sealed to be effective. Fluid volume increases associated with aquathermal expansion and clay dehydration are too small to generate significant overpressure unless perfect sealing occurs. Hydrocarbon generation and cracking to gas could possibly produce overpressure, depending 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 potential 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 overpressure 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 ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received October 17, 1995; revised manuscript received September 4, 1996; final acceptance January 20, 1997. 2Department 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

Much of the world's oil and gas has been generated from source rocks inside deep (> 3,000 m or 9,840 ft) seal-bounded fluid compartments. The quantity and composition of the kerogen and the burial history of the source rocks determine the volumes of petroleum generated; however, the migration from the compartments in an oil and gas phase is a pressure-driven process in which the flow direction is controlled by the configuration and internal pressures of the fluid compartments. Many sedimentary basins contain layers of two or more superimposed hydrogeological systems. The shallow systems are usually basin wide in extent and exhibit normal hydrostatic pressures. The deeper systems, where the oil is generated, are not basin wide and are abnormally over pressured. They usu lly consist of a series of individual fluid compartments that are not in hydraulic pressure communication with each other nor with the overlying hydrodynamic regime. Tops of fluid compartments in currently sinking basins do not always follow a specific stratigraphic horizon. They frequently have planar tops and subsurface temperatures ranging from 90 degrees to 100 degrees C (194 degrees to 212 degrees F). The tops in clastic sediments appear to be caused by carbonate mineralization along a thermocline. In the North Sea, the depth to the top of the deepest seal changes with the geothermal gradient. The seal is deeper where the gradient is lower. The generation of oil and gas within the compartments plus the thermal expansion of pore fluids eventually causes fracturing of the top compartment seal during periods of basin sinking. Hydrocarbons and other pore fluids then move vertically into the overlying lower pressured sediments and accumulate in the nearest structural and stratigraphic traps. Seal fracturing causes a pressure drop with compartment fluids rushing to the breakout point. The compartment then re-seals and pressure builds to another breakout. This episodic process continues with resealing and breakout cycles probably occurring in intervals of thousands of years in rapidly sinking basins such as the United States Gulf Coast. This concept of episodic dewatering of deep-basin fluid compartments needs to be considered n any basin-modeling program where the bulk of the oil generation occurs in the compartmented overpressured section of the basin and the oil moves vertically into the normally pressured rocks above.

Generation and Migration of Petroleum from Abnormally Pressured Fluid Compartments: Reply

Abstract Differential stress exerts both static and dynamic effects on rock-mass permeability, modulating fluid flow in the Earth’s crust. Static stress fields impose a permeability anisotropy from stress-controlled features such as faults, extension fractures, and stylolites which, depending on the tectonic regime, may enhance, or counteract existing anisotropic permeability in layered rock sequences. Textural evidence from hydrothermal veins suggests, however, that fluid flow in fault-related fracture systems generally occurs episodically and that dynamic stress cycling effects are widespread. In the vicinity of active faults that undergo intermittent rupturing, permeability and fluid flux may be tied to the earthquake cycle through a range of mechanisms, leading to complex interactions between stress cycling, the creation and destruction of permeability, and fluid flow. Mechanisms for fluid redistribution include: (1) various forms of dilatancy (localized to the fault zone or extending through the surrounding rock mass) related to changes in shear stress and/or mean stress that occur during the fault loading cycle; (2) localized post-seismic redistribution around rupture irregularities, especially dilational jogs and bends which act as suction pumps; and (3) post-seismic discharge of fluids from overpressured portions of the crust through fault-valve action when ruptures breach impermeable barriers. All of these processes may be involved in fluid redistribution around active faults, but they operate to varying extents at different crustal levels, and in different tectonic regimes.

Crustal stress, faulting and fluid flow

Mechanisms for Generating Overpressure in Sedimentary Basins: A Reevaluation: Discussion

A coherent oil-migration model based on geomechanical considerations includes both the high-molecular kerogen structure and the capillary properties of source rocks. Oil is squeezed from kerogen by compaction following oil generation. This squeezing effect should be created by the differential stress (maximum compressive stress minus least compressive stress) acting on the kerogen which has been chemically broken up by oil formation. In sedimentary bodies whose water is at hydrostatic pressure, the migration of oil seems to involve two processes: (1) lateral transfer, by channeling into the more coarsely microporous layers of the source rocks, from the oil generation site toward the geologic structure or lower pressured zone; and (2) vertical transfer from source rock to eservoir by the opening or reopening of vertical fractures in the few areas, such as structural tops, where the least compressive stress is slightly greater than or equal to the pore pressure, and where the capillary pressure increment (2^ggr/R) of oil in the microporosity exceeds the tensile strength of the rock. In sedimentary bodies whose water is overpressured, the pore pressure should be governed by the least compressive stress and thus migration should begin by oil transfers in a system of small open fractures, and eventually in larger fractures. The theory demonstrates the impossibility of oil being transferred to the reservoir under true tensile conditions (negative effective compressive stress) and thus explains the large asphaltic veins of southeastern Turkey and the well-known bitumen veins of the Uinta basin.

Jean du Rouchet

Papers

Stress Fields, A Key to Oil Migration