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Experimental Pressure Solution of Gypsum

Sherilyn C. Williams

It is not always obvious that the porosity in an evaporite sequence is lost due to cementation during early burial. Because evaporite deposits often act as a seal against movement of fluids in petroleum reservoirs, it is of particular importance to know when porosity reduction makes the evaporites impermeable. Clastic and precipitated gypsum is an important component of evaporite deposits. After deposition and mechanical compaction, the two types of plastic deformation which are most likely to be responsible for loss of porosity are pressure solution and intracrystalline translation glide. Both mechanisms can be activated at low temperatures and shallow burial depths (< 1 km), with translation glide becoming more important at high rates of overburden accumulation. Pres ure solution is a more important mechanism for slow strain rates, which would correspond to a slow rate of accumulation of overburden. Pressure solution experiments on gypsum single crystals show that, unlike relatively insoluble minerals such as quartz, results are readily obtainable in time available for laboratory experiments. The rate of change in the shape of a grain is related to the change in length of the radius of a circular single crystal disk of gypsum. A geometrical solution of how the stress per unit area on a grain varies with shape change can be derived, which can then be related to overburden stress. By using average grain size and information about the geothermal gradient, the rate of dissolution of stressed grain elements and reprecipitation at unstressed grain elements will then give the time necessary for the porosity in a gypsum deposit to be sealed.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.