Page 166 - Geology of Carbonate Reservoirs

DIAGENESIS AND DIAGENETIC PROCESSES 147

Figure 6.1 The stylolite just below the 1 - cent coin is typical of these features that are formed
by the combination of pressure and dissolution. Most of them are filled with insoluble mate-

rial, commonly including or limited to bituminous residue and most stylolites are baffl es or

barriers to flow. A few may remain open, or become opened during later dissolution, in which

case they may provide access for fluid migration (see the sketch in Figure 6.6 ). This stylolite,
like most, resembles a trace on a seismograph or an electrocardiogram. Note the small natural
fractures and dissolution vugs also in this core segment from a Mississippian - aged dolomitic
reservoir in Texas.
between rocks and water. Generally, as long as a continued supply of undersaturated
water is available to react with the rock, dissolution continues. Extensive dissolution
creates karst features, such as caverns and sinkholes, and enlarges pores to form
molds and vugs. Water moving down aquifers dissolves as it migrates so long as it
is undersaturated in CaCO 3 . If the aquifer water becomes saturated, dissolution
stops. If excess calcium carbonate is in solution and a trigger exists for precipitation,
then cementation will take place in the water - filled pores. Similarly, fresh water may

percolate from the surface downward through rock or soil, dissolving as it goes until
saturation equilibrium is reached or exceeded. As with aquifers, cementation can
occur when carbonate saturation is high and a trigger mechanism causes precipita-
tion. Stagnant aquifers are those in which water does not move or moves very little;
consequently, there is comparatively low potential for either dissolution or cementa-
tion (Longman, 1980 ).
Carbonate cementation binds loose grains and fills pores with one or more car-

bonate crystal growth forms. Cement crystals may occur in a variety of individual
shapes and forms depending on the crystallographic system in which the mineral
forms and on the environment in which crystallization occurs. For example, calcite
may crystallize as pointed scalenohedra ( “ dogtooth spar ” ) or as fl at, bladed