Mechanisms that Generate Abnormal Pressures: An Overview
Normally pressured reservoirs have pore pressures which are the same as a continuous column of static water from the surface. Abnormal pressures occur where the pore pressures are significantly greater than normal (overpressure) or less than normal (underpressure). Overpressured sediments are found in the subsurface of both young basins from about 1.0 to 2.0 km downwards, and in older basins, in thick sections of fine-grained sediments. The main mechanisms considered responsible for most overpressure conditions can be grouped into three broad categories, based on the processes involved: (1) ineffective volume reduction due to imposed stress (vertical loading during burial, lateral tectonic processes) leading to disequilibrium compaction, (2) volume expansion, including porosity increases due to changes in the solid:liquid ratios of the rock, and (3) hydraulic head and hydrocarbon buoyancy. The principal mechanisms which result in large magnitude overpressure are disequilibrium compaction and fluid volume expansion during gas generation. Disequilibrium compaction results from rapid burial (high sedimentation rates) of low-permeability rocks such as shales, and is characterized on pressure vs. depth plots by a fluid retention depth where overpressure commences, and increases downwards along a gradient which can closely follow the lithostatic (overburden) gradient. Disequilibrium compaction is typical in basins with a high sedimentation rate, including Tertiary deltas and some intracratonic basins. In older basins, disequilibrium compaction generated earlier in the basin history may be preserved only in thick, fine-grained sequences, but lost by vertical/lateral leakage from rocks with relatively high permeabilities. Gas generation from secondary maturation reactions, and oil cracking in the deeper parts of sedimentary basins, can result in large fluid volume increases, although the magnitudes are uncertain. In addition, the effect of increased pressures on the reactions involved is unknown. We doubt that any of the other mechanisms involving volume change can contribute significant regional overpressure, except in very unusual conditions. Hydraulic head and hydrocarbon buoyancy are mechanisms whose contributions are generally small; however, they can be easily assessed and may be important when additive to other mechanisms. The effects of transference of overpressure generated elsewhere should always be considered, since the present pressure distribution will be strongly affected by the ability of fluids to move along lateral and vertical conduits. Naturally underpressured reservoirs (as opposed to underpressure during depletion) have not been as widely recognized, being restricted mainly to interior basins which have undergone uplift and temperature reduction. The likely principal causes are hydraulic discharge, rock dilation during erosional unroofing, and gas migration during uplift.
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Abnormal pressures, pressures above or below hydrostatic pressures, occur on all continents in a wide range of geological conditions. According to a survey of published literature on abnormal pressures, compaction disequilibrium and hydrocarbon generation are the two most commonly cited causes of abnormally high pressure in petroleum provinces. In young (Tertiary) deltaic sequences, compaction disequilibrium is the dominant cause of abnormal pressure. In older (pre-Tertiary) lithified rocks, hydrocarbon generation, aquathermal expansion, and tectonics are most often cited as the causes of abnormal pressure.
The association of abnormal pressures with hydrocarbon accumulations is statistically significant. Within abnormally pressured reservoirs, empirical evidence indicates that the bulk of economically recoverable oil and gas occurs in reservoirs with pressure gradients less than 0.75 psi/ft (17.4 kPa/m) and there is very little production potential from reservoirs that exceed 0.85 psi/ft (19.6 kPa/m). Abnormally pressured rocks are also commonly associated with unconventional gas accumulations where the pressuring phase is gas of either a thermal or microbial origin. In underpressured, thermally mature rocks, the affected reservoirs have most often experienced a significant cooling history and probably evolved from an originally overpressured system.