Influence of Pressure, Salinity, Temperature, and Grain Size on Silica Diagenesis and Reservoir Quality in Quartzose Sandstones: ABSTRACT

Abstract

Based on calculated silica solubilities (1-4 km depth range), there is a suggested predictable effect of pressure (P), temperature (T), and salinity (S) on silica diagenesis. Silica distribution under various P-T-S conditions is in turn influenced by grain-size differences. Quartz solubility related to increasing grain-contact pressure with depth is about 7 times greater at 4 km than at 1 km (mS = 1.5 × 10-3 vs. 2.0 × 10-4). The differences between solubilities based on hydrostatic and point-contact pressures (orthorhombic closest packing) increase with increasing depth (excluding overpressured areas). At greater depths, local precipitation of silica appears more likely, whereas at shallower depths, larger scale silica migration ay occur preceding precipitation. The effect of increasing salinity generally decreases solubility regardless of depth (e.g., at 3 km and 3 m, mS = 8.9 × 10-4; at 6 m, mS = 7.5 × 10-4); hence, reservoir-quality loss by pressure solution is impeded in those sandstones containing more saline pore fluids. Pressure solution is far more active in basins characterized by high heat flow. Very fine-, fine-, and medium-grained sand laminae (perfect sorting; several grain diameters thick) will experience 500, 64, and 8 times (all other factors equal) the amount of pressure solution compared to coarse-grained sand. Chemical potential gradients are suggested among centimeter-scale size laminated intervals, resulting in migration of silica to coar er grained areas before precipitation. Reservoir-quality loss in the finer laminae is caused primarily by pressure solution, whereas, in coarser intervals, silica precipitation is mainly responsible. End_of_Article - Last_Page 269------------