Effects of Smectite-Illite Reactions in Shales on Sandstone Diagensis, Gulf Coast

Park, Anthony J.¹ (1) Sienna Geodynamics & Consulting Inc, Melrose, MA

Shales and mudrocks are volumetrically dominant components in many sedimentary basins, and smectite is the most abundant of minerals composing them. However, smectite is a highly unstable mineral and readily undergoes thermal decomposition with increasing temperature, resulting in precipitation of illite and export of other solutes, such as Na, Ca, Fe, Mg, and Si. Sillica released from smectite-illite reactions are assumed to contribute significantly to quartz overgrowth. Observed temperature-dependent smectite-illite reaction trends can be effectively described using simple empirical rate laws.

Because of the volumetric abundance of shales, it is also easy to see that total masses of various chemical elements associated with smectite-illite reactions are significantly greater than the mass of elements involved in sandstone diagenesis. Thus, one can expect that sandstone diagenesis should be therefore strongly influenced by smectite-illite reactions.

To test the soundness of this expectation Balance diagenesis simulator was used to assess how smectite-illite reactions affect sandstone pore water chemistry, and thus diagenetic processes of sandstones. Published petrographic and water chemistry data of Oligocene Gulf Coast sediments were used in the simulations. Results show the reasonableness of the expectation, and along with it the inevitable observation that the ability to characterize sandstone diagenesis, and therefore reservoir property prediction, depends strongly on having accurate chemical and physical properties information of shales and mudrocks that surround the sandstones.