E7 Fluid Geology

Topic

Fluids occupying the Earth's crust—whether water, carbon dioxide, or hydrocarbons—play a fundamental role within it: they affect its mechanical strength, influence the seismicity of the faults traversing it, and transport both heat and minerals from one point to another. A well-established relationship describes how a fluid flows through a porous medium: the flow is proportional to the driving pressure gradient and inversely proportional to both the fluid's viscosity and the permeability of the medium it traverses. This permeability varies enormously from one rock type to another—by as much as ten billion-fold between completely intact crystalline rock and highly porous sediment.

Fluid pressure within the crust is typically hydrostatic—close to ten megapascals per kilometer of depth—but it can approach lithostatic pressure (between twenty-five and thirty megapascals per kilometer of depth) in zones of very low permeability or where large amounts of new fluid are generated, such as during metamorphism or within a subduction zone. When the ratio of fluid pressure to lithostatic pressure approaches unity, the frictional resistance of nearby faults drops drastically; this reduction in frictional resistance can trigger an earthquake or, alternatively, facilitate slow, silent slip along the fault.

Hydrothermal systems transport heat and dissolved metals from their hot source rock toward the surface. Mineral solubility in hot water increases exponentially with temperature, allowing very hot hydrothermal fluid to dissolve and transport vast quantities of metal; as the fluid cools during its ascent, these dissolved metals—such as gold, copper, or zinc—concentrate and precipitate, forming mineral deposits. Finally, the diffusion of pore pressure through the rock explains why earthquakes induced by fluid injection into a well do not occur solely in the immediate vicinity of that well but instead migrate away from it over time as the elevated pore pressure itself spreads into increasingly distant zones—thereby closing the link between the fundamental role of fluids (with which this description began) and their capacity to trigger seismicity even far from the point of origin.

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