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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Some sedimentary rocks, particularly those containing volcanogenic material, develop laumontite cements during burial with increasing temperature. The development of laumontite cements in sandstones is highly destructive to reservoir potential. As a result, in the search for hydrocarbons, laumontite-bearing sedimentary rocks commonly are considered economic basement. However, it is possible to develop secondary porosity after the formation of laumontite.
Theoretical considerations show that the stability of laumontite (Lm) is limited by the following equilibria: (1) Lm + CO2 = Calcite (Cc) + Kaolinite (Kao) + 2 Quartz (Q) + 2 H2O,
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and (2) Lm + 2 H + = Kao + 2 Q + Ca++. At diagenetic temperatures, equilibrium (1) has a steep slope on a T-XCO2 diagram, indicating that the boundary between zeolite and clay minerals during diagenesis is more strongly controlled by fluid composition than by temperature. Equilibrium (1) and (2) along with the complimentary equilibrium (3) Cc + 2 H+ = Ca++ + CO2 + H2O can be plotted on an isothermal projection. This projection shows that either an increase in XCO2 or a decrease in pH can cause the breakdown of laumontite. Destruction of Lm by reaction (1) leads to a volume decrease of 10%, whereas Lm elimination by reaction (2) leads to a volume decrease of 28%. During diagene is, as a result of decarboxylation reactions, previously formed laumontite can be destabilized, creating secondary porosity. In contrast, the development of secondary permeability and ultimately the recreation of reservoir potential will be a function of aluminum mobility. Whether secondary porosity and permeability develop in laumontite-rich sedimentary rocks will be dependent on the sequence, rates, and magnitude of the laumontite-forming and organic maturation reactions.
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