A MODEL FOR POROSITY REDUCTION IN QUARTZITE RESERVOIRS BY QUARTZ CEMENTATION

Quartz cementation is an important process of porosity reduction in sandstones. A mathematical model is presented for predicting the distribution and amount of quartz cement in a sandstone reservoir as a function of time and space. The approach combines kinetics and transport mechanism and incorporates the following assumptions: (1) no compaction during cementation, (2) transport of dissolved silica by advection, (3) surface-controlled growth, (4) sands simplified to a framework of spheres, and (5) isothermal system. The model predicts that low flow rates, small grain size, and higher temperatures favor limited volume of quartz cement because the pores are rapidly obstructed. Pervasive cementations are predicted in the presence of coarse-grain sands, high fluid flows, and low temperatures. Substantial volumes of quartz may occur within 250,000 years under conditions of shallow burial (T = 20 degrees C, u = 10 m/y). During conditions of deeper burial and higher temperatures (T = 80 degrees C), quartz cement should form during geologically rapid events rather than during the entire history of burial.