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Diagenetic alterations of sandstone occur in a continuous system. As a result, equilibrium thermodynamics cannot be strictly used to describe the equilibrium composition of the diagenetic system and the resulting course of diagenesis. If a geologist is to predict the course of diagenesis in a meaningful way, he must determine those factors which serve to control the various diagenetic pathways.
Geologic evaluation of sandstone fabric and texture is an integral part of most regional studies. These data are often critical in understanding diagenesis as well. Sediment grain size, roundness, sorting, and packing factors determine the ability of a sandstone to transmit fluid during the course of burial and diagenesis. These geologic factors can be used to evaluate the paleohydrology
of the sand bodies under investigation, and provide insight to a major control of diagenesis.
The fluid flow rate through a sandstone controls the residence time of the various chemical components in solution. When the residence time is sufficiently long with respect to the time scale of the diagenetic reaction, the time invariant condition of a continuous system approaches chemical equilibrium. Thus it is possible to have different diagenetic reactions occurring within a sand body due to local changes in fluid flux (or flow velocity) and the resultant varying degrees of approach to equilibrium.
In systems where fluid flow is high and residence time is small with respect to diagenetic reaction rate, the fluid chemistry is largely controlled by the chemistry of external fluid source. This situation results in either introduction of new material (or minerals) into, or removal of existing material or minerals from the sandstone. If the fluid is saturated or nearly saturated with respect to some specific mineral this mineral is added. Conversely, leaching of material occurs when the fluid is undersaturated.
In systems where fluid flow is slow and residence time is large with respect to diagenetic reaction rates, the fluid chemistry is largely controlled by the chemistry of the host rock. In this case, chemical equilibrium is approximated and the material originally present in the rock is redistributed by solution/precipitation reactions. Only small amounts of materials are introduced to or removed from the host rock. Understanding this control on sandstone diagenesis is important in delineating trends in diagenetic alteration and projecting those trends into new areas, and in identifying the trends in differential cementation that produces some types of diagenetic traps.
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