The Scanning Electron Microscope (SEM), which became available commercially in the mid 1960s, has added a new dimension to exploitation during the decade of the 1970s. Of primary importance are studies of pore geometry and diagenetic history, which influence the type, distribution, and flow of fluids in the lithosphere.

The SEM provides a means of evaluating the abundance and location of micropores relative to macropores, which influence fluid distribution. If a rock contains a significant percentage of authigenic clay minerals or other fine particles, bound water may be retained in the micropores and cause a high irreducible water saturation. The reservoir may produce water-free hydrocarbons, but wireline-log calculations may indicate water saturations greater than 60%.

The SEM is useful for examining the effect of fluids and chemical additives on rocks during enhanced oil recovery. For example, laboratory tests have shown decreasing permeability during flow tests using a specific micellar fluid. SEM examination of "before and after" rock samples revealed that smectite, an expandable clay, was reacting to the fluid and plugging pore throats, yielding reduced permeability.

Reservoir studies using the SEM have shown that varying distributions and morphologies of clay minerals can be directly related to productivity of sandstones. Clay distribution, in order of decreasing reservoir quality and decreasing mean-pore-aperture size, is (1) discrete loosely packed clay particles, partially filling pores; (2) clay lining pores; and (3) clay bridging from one sand grain to an adjacent sand grain. Reservoir quality due to clay distribution types 1 and 2 is exemplified by Mesaverde sandstones in southwestern Wyoming. Measured porosities for the two types of reservoirs are similar, but permeability and, hence, productivity are markedly dissimilar.

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