FIGURE 4.15 Three cross sections showing the development of the Alberta Basin; arrows show the sense of fluid flow in these sections.

hydrodynamic forces. Dominant fluid-driving forces are the gravitational head created by topographic relief above sea level and the compactional head generated by subsidence and compression below sea level. Oil and gas stop moving when they reach a capillary pressure equilibrium or when they are trapped by capillary seals that reduce or prevent further migration.

The Alberta Basin, including its southward extension into Montana, provides an example of how fluid flow has evolved as a sedimentary basin developed (Figure 4.15). Some 250-million-years ago the west coast of North America was the site of an Atlantic-type margin. Sediments were deposited and various fluids moved through the deposits, both processes occurring largely in response to sea level changes or down-warping of the coast. From about 200-million-years ago, a complex succession of island arcs colliding from the west built a range of mountains along that margin, and a foreland basin formed in front of the rising mountain belt in response to the loading of the crust by the mountains. Sediments eroded from the mountains, filling the basin, and fluids moved through the basin accordingly. Compressional compaction of deeply buried sediments beneath the shortening mountains further enhanced fluid migration.

During the first phase, thermally driven free convection of waters in contact with seawater dissolved some minerals and deposited others, leading to a variety of changes that together are called diagenesis. Within the Alberta basin these changes included replacing some of the calcium in calcite with magnesium to form dolomite, a process that often reduces rock volume and yields more pore space to form potential reservoirs, and cementing sands to form sandstones, a process that reduces porosity. Sulfide mineral deposits were formed locally in the sediment wedge by fluid flow under this regime. In areas where rapid deposition of impermeable sediments confined water-filled porous materials, overpressures—which eventually are dissipated through faulting or seepage—may have developed. Oil formation in the Atlantic-type margin took place as cold, organic-rich sediments were deeply buried and warmed in the Earth's conductive

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