Prediction of Calcite Cementation in Sandstones Associated with Sandstone-Shale Interaction, Gulf of Mexico
Precipitation of calcite in sandstone requires two chemical elements: Ca and C. These elements and their aqueous solute equivalents can be derived from detrital carbonate material, feldspars and clays, or through diffusive and advective mass-transfer of solutes from neighboring (or deeper) sediment sources. However, while sources of Ca and C may be abundant, several conditions must combine to produce a calcite saturation condition in sandstone. These conditions include: temperature, mineralogical makeup of sediments, Ca and C in pore water, mass-influx rate of Ca and C (as bicarbonate, CO2(g), acetates, and other organic acids) into the sandstone, and in situ acid generation in shale associated with maturing organic material.
To demonstrate the complexity of the problem posed, Balance.CM diagenesis simulator was used to model Gulf of Mexico reservoirs. Using the reactive-transport modeling methodology, the program combines mass-transfer, kinetic and empirical reaction rate laws of minerals, and dynamic texture model to simulate diagenesis and associated changes in sediment mineralogy and petrophysical properties. Simulations were carried out for a meter-scale column of layered sandstone and shale system, using water composition, CO2 and acetate abundances, and smectite-illite reaction trends for Gulf of Mexico sediments derived from published resources. Sandstone mineralogy and composition data used were from a number of reservoirs, while the shale composition was approximated based on published literature. Known burial histories were used to set the temperature condition.
A series of simulations were carried out with incremental influx of CO2 into the sandstone and H production in the shale at different stages of burial for varying pore water Ca content. Results show that the abundance of Ca, in clastic minerals of both shale and sandstone and/or nearby sources, may determine the quantity of calcite cementation that can occur, but the abundance of Ca and interactions among CO2, organic acids and other solutes that can affect or buffer water pH may determine the condition and the timing when calcite precipitation may commence.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009