Sandstone diagenesis and porosity modification during basin evolution

Abstract

The properties of sandstones as potential reservoirs and shales as source rocks depend on primary facies relationships and diagenesis. Porostiy loss due to mechanical compation and pressure solution is essentially a function of grain parameters (sorting, packing and composition) and net overburden stress. The porosity loss can be predicted to a certain extent. The importance of secondary porosity caused by dissolution of framework grains and cements has been fully recognized. The discussion has focused on the processes causing such dissolution and to what extent it can cause net increase in porosity. The most critical factor in clastic diagenesis is the nature of porewater flow and the degree of mass transfer taking place as a result of this. In the North Sea reservoir rocks, petrographic and geochemical evidence suggest that most of the leaching of feldspar and mica resulting in the formation of kaolinite occurred early during fresh wather flushing. Recent calculations indicate that »acids« derived from source rocks are inadequate to explain the secondary porosity observed in reservoir rocks. Mathematical modelling suggests that thermal convection is of limited importance in sedimentary basins, except where there are high lateral changes in geothermal gradients. Evidence from porewater geochemistry suggests that porewaters in sedimentary basins are often stratified or compartmentalized in a way which is inconsistent with large scale convection or compactional flow, making it necessary to assume that diagenetic reactions are relatively isochemical during deeper burial. A better understanding of the diagenetic reactions will help us to improve our predictions about porosity/depth relations, pore size, and pore geometry distribution in reservoir rocks. Porosity depth trends from offshore Norway and published data from other basins are discussed. Empirical linear best fit lines are found to illustrate the relationship quite well for depths between one and five km. Within a specific region, the linear porosity gradient is a function of mineral composition and of temperature and pressure gradients. Primary porosity tends to be best preserved in sandstones with high proportions of stable grains (e. g. in quartz arenites) down to about 3 or 4 km. At greater depth, porosity loss is accelerated due to increased pressure solution. Secondary and primary porosity adjacent to feldspar grains then tends to be selectively preserved relative to primary pores between quartz grains.