Abstract This segment of the notes presents an overview of diagenetic processes of porosity and permeability destruction and enhancement. In the limited space available here, it is impossible to cover in any detail the vast literature on this subject. Thus, only the volumetrically important mechanisms of reservoir quality modification will be discussed, along with highlights of some recent studies of particular interest. Major advances have been made in the understanding of clastic diagenesis in the last fifteen years. Many of these advances can be attributed directly to the intense efforts of oil industry research, exploration, and production personnel, as well as academicians, who have been involved in reservoir quality studies centered on the North Sea and offshore Norway. These studies have been greatly enhanced by the extensive coring that has attended drilling operations. Several publications focusing on diagenesis and reservoir quality in the North Sea and adjacent areas have been used widely in the preparation of this text. These are listed in Table 5-1. Other major publications devoted to diagenesis and reservoir quality in clastic rocks are also referenced frequently, and these are listed separately in Table 5-2. In most reservoir sandstones, levels of resrervoir quality are far below initial (time of deposition) levels (porosities of 40 to 50% and permeabilities of 10 to 2,000 darcies). Although diagenesis tends to accentuate the effects of depositional control in most reservoir sandstones (e.g., Nagtegaal, 1979; Weber, 1980), it may also influence reservoir properties in an irregular manner, or even reverse depositional controls (e.g.

Abstract As discussed in Chapter 2, Empirical Methods of Reservoir Quality Prediction, porosity and permeability reflect the cumulative result of physical and chemical processes acting on a given volume of rock. These processes are constrained by the fundamental physical-chemical variables of Pressure (P), Temperature (T), Time (t), and Composition (X, where composition includes not only rock chemical composition expressed as mineralogy, but also fluid composition and textural variables such as size, sorting, orientation, and packing). Porosity at any given time for a given unit of rock at depth results from the complex interaction of these variables through time. Fundamental to the development of predictive models is the measurement of these independent variables (or alternate variables which reflect the influence of these fundamental variables). This and the next three chapters will discuss the measurement of these independent variables, as well as some examples of interactive variables which combine the influence of two or more of these basic variables.

Abstract Porosity prediction in an open system is extremely difficult because sources of pore-filling cements and grain- and cement-dissolving solutions are not readily delineated, nor are the volumes of these solutions easily quantifiable. A good example of the problems encountered is the case of creating secondary porosity by dissolution of carbonate cement. In this case, porosity prediction depends upon the ability to predict the conditions conducive to both the development of the porefilling cement and its subsequent dissolution. The occurrence of carbonate cement must be considered in terms of timing with respect to other porosity reduction mechanisms, abundance and distribution within the prospective reservoir sandstone, and the various chemical and biological controls on precipitation. Similarly, late-stage dissolution must be evaluated in terms of timing with respect to both structuring and porosity preservation mechanisms, extent and distribution of secondary pores, and the chemical controls on dissolution. In light of these difficulties, prediction of porosity in a reservoir formed by decementation is highly problematic. A prediction of maximum porosity may be a more easily attained, although highly speculative and less meaningful, value for the explorationist.