SANDSTONE POROSITY LOSS - A BIG PICTURE VIEW OF THE IMPORTANCE OF COMPACTION

Proper assessment of the relative importance of porosity loss by compaction and cementation is important to the prediction of porosity in sandstones, studies of mass transport during diagenesis, and the modeling of basin fluid flow. Quantitative estimation of the amounts of porosity loss by compaction and cementation can be made from standard point-count data on cement and pore space abundance. This method is discussed and its sources of error are evaluated. Error in the method arises mainly from uncertainty in initial sandstone porosities, and the amount of local grain dissolution during burial. Data on the porosity of modern sands indicate that assumed initial porosities of 40 to 45% will produce small average errors in the calculation of compactional porosity loss for clean sandstones. Grain dissolution, whether resulting in local porosity enhancement by export in solution of former grain mass or local precipitation of authigenic cement, will cause an underestimation of the significance of compaction. A large database of point-count data from diverse sandstones is used to provide a broad perspective on the importance of compaction and cementation to sandstone porosity loss. It is shown that the global importance of compactional porosity loss has generally been under-appreciated. In fact, compaction (mechanical and chemical) is probably the dominant mechanism of porosity loss in most sandstones. The extent of compactional porosity loss is related to sand grain composition and basin thermal structure.