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Clathrates are a special variety of inclusion compound in which the guest molecules fit into separate spherical or nearly spherical chambers within the host molecule, and when the host molecule is water and the guest molecules are largely gases or liquids with low boiling points found in natural gas, the clathrates are termed natural gas hydrates. They are solid compounds, resembling ice or wet snow in appearance, and form both below and above the freezing point of water under specific PT conditions. The water molecules form pentagonal dodecahedra, which can be arranged into two different structures, leaving interstitial space in the form of either tetrakaidecahedra or hexakaidecahedra. Methane and hydrogen sulfide can be accommodated in all the spaces, ethane and carbon dioxide can fit in both the tetrakaidecahedra and the hexakaidecahedra, but propane and isobutane fit only in the hexakaidecahedra. Normal butane, pentane, and hexane are not known to form hydrates. PT diagrams describing the initial conditions for hydrate formation indicate that, relative to methane, all common components of natural gas (except nitrogen and the rare gases) raise the hydrate formation temperature, propane and ethane being the most effective. The presence of dissolved salts in the water, or nitrogen and rare gases in the natural gas, depresses the temperature of initial hydrate formation.The most likely way to produce natural gas hydrates in sedimentary basins is through a reduction of temperature, rather than an approach to lithostatic pressures, and the most pertinent situation is that found in regions with relatively thick permafrost sections. Sedimentary basins with extensive areas of relatively thick, continuous permafrost, which may contain potentially commercial 195

The main variables affecting the fluid potential distribution are topography and geology, and the effect of topography is treated by consideration of the three-dimensional flow net by means of fluid potential slice maps and cross-sections and is shown to be adequate to explain the major flow net. The dominant fluid potential in any part of the basin corresponds closely to the fluid potential at the topographic surface in that part of the basin. Major recharge areas correspond to major upland areas, and major lowland regions are major discharge regions. Large river valleys commonly exert a drawdown effect on the fluid potential distribution, which may be observed to depths of up to 5000 feet. The presence of a thick sequence of highly permeable Upper Devonian and Carboniferous carbonate rocks in the medium-depth portion of the Alberta basin has resulted in the development of a low fluidpotential drain, which essentially channels flow from most of the Alberta basin towards the Athabasca oil sands and has modified the theoretical relation between local and regional flow systems. INTRODUCTION' The flow of fluids through porous media may be considered as a transient (unsteady-state) or steady-state phenomenon. Problems concerned with the flow of fluids from a borehole or small group of boreholes through confined layers or if released from storage are probably best solved if they are analyzed as transient phenomena.When consideration is given to studies of small drainage basins or to flow regimes across large sedimentary basins, such as that under study in this series of papers, it may be assumed that a relatively steady state has been reached. This does not imply static conditions, but rather that any recharge to the flow regime is negligible in relation to the vast amount of fluid in the system. In addition, the law of conservation is operative, and the recharge is equal to the discharge. In short, we are considering a case of dynamic equilibrium.The classic paper of Hubbert [1940] on the theory of ground-water motion is significant, because it was the first published account of the x Contribution No. 421, Research Council of Alberta.

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