CHAPTE R 5 DIAGENETIC MECHANISMS OF POROSITY AND PERMEABILITY REDUCTION AND ENHANCEMENT

This segment of the notes presents an overview of diagenetic processes of porosity and permeabil­ ity 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 par­ ticular interest. Major advances have been made in the under­ standing of clastic diagenesis in the last fifteen years. Many of these advances can be attributed directly to the intense efforts of oil industry re­ search, 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 de­ voted 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 reservoir 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 controls in most reservoir sandstones (e.g., Nagtegaal, 1979; Weber, 1980), it may also influence reservoir prop­ erties in an irregular manner, or even reverse depo­ sitional controls (e.g., Wescott, 1983; Stonecipher and others, 1984). Despite the large reductions of porosity and permeability produced by di agenetic alterations, the influences of depositional controls are invariably still recognizable. For example, relative levels of porosity and permeability in deeply buried (5,000 m, 16,000 ft) Hosston Group sandstones from cen­ tral Mississippi correlate closely with variations in grain size and matrix content (Figure 5-1). These depositional controls are recognizable despite the fact that absolute levels of porosity are now a third to a tenth of their original values, and permeabili­ ties are three to four orde:rs of magnitude lower than initial values. The persistence of depositional controls illustrated in this example is frequently overlooked. A diagram indicating ihe general depth (tem­ perature) regimes of the major diagenetic processes is presented in Figure 5-2. This chart is based ex­ tensively on studies by Curtis (1978, 1985), AGAT Consultants (1982, 1984), Land and others (1987), Palmer and Barton (1987), McBride (1989), Tada and Sevier (1989), Pittman and Larese (1991), and the North Sea case history presented in these notes (Chapter 21). The conditions which apply to the sequence shown are: 1. Thermal gradient of 30°C/km (1.65°F/100 ft), normal pressure gradient, and connate pore waters of normal marine salinity. 2. Nonspecific sandstone composition. The ex­ tent of a particular process is governed largely by specific framework mineralogy. 3. Large scale meteoric influx is excluded. 4. The widths of the bars indicate the relative rates at which the processes operate.