ABSTRACT

The reservoir quality of sandstone is almost entirely controlled by diagenetic events. The chemical and physical processes responsible for diagenesis are complex and they influence sands during all stages of burial and, in some basins, during subsequent uplift. Petrographic studies in the past ten years by many workers provide the basis for formulating rules of sandstone diagenesis that help in predicting reservoir quality in different formations. The majority of the rules listed below are empirical, and causative factors are still poorly understood. The list is also not complete, but I present it so that the "rules" can be tested, challenged or added to by other workers. Few items in the list are original. Most have become part of the public domain by word of mouth or through numerous case studies in the literature. Major sources where some ideas herein appear in print are cited in the references, and the acknowledgements cite the individuals that have contributed to my thoughts on this problem.

1. The detrital mineral composition of a sand predetermines 50 to 90 percent of its diagenetic history. This rule is demonstrated both by chemical and physical diagenetic events. Physical compaction (omitting pressure solution) is influenced by the abundance of ductile and other easily deformable grains, whereas chemical processes of cementation, replacement, and dissolution are influenced by the original detrital composition. The composition and flow rate of formation waters that pass through sands after deposition are the major factors that are unrelated to original mineral composition.
2. Porosity lost by compaction cannot be regenerated during subsequent diagenetic events. Sands with abundant ductile grains (clay clasts, fecal pellets, shale clasts, micaceous rock fragments), flexible micas, crushable shell fragments, or clay introduced by bioturbation or other means can loose much primary porosity from the mechanical deformation of these grains during compaction. All primary porosity can be lost solely by compaction in sands that have 35 percent ductile grains (Rittenhouse, 1977).
3. The loss of primary porosity through compactional deformation of grains takes place chiefly during burial to 8,000 to 12,000 ft (2438 to 3658 m); loss of porosity by compaction at greater depths is mainly by pressure solution of detrital grains. Mechanical compaction of sands begins at shallow depths and proceeds until all pores are closed through ductile grain deformation and related processes or until cementation by quartz, carbonate or other pore-occluding cement strengthens the rock and stops it. Once the sand becomes strengthened by partial cementation, compaction will proceed chiefly through pressure solution. Pressure solution is identified by stylolites and grain contacts that are sutured, concavo-convex, or exceptionally long. Cathodoluminescence should be used to verify suspected pressure solution contacts.
4. Pressure solution of quartz grains is enhanced by the presence of grain coatings of illite and to a lesser degree by chlorite and other grain-coating clays. Clays promote pressure solution, in part, by compacting to permit detrital grains to come into contact with each other and in part as a source of absorbed water.
5. Quartz cement has an affinity for the coarser, more permeable sands in a formation. However, it rarely fills all pores in a sandstone except in some coarser-grained laminae or in pre-Tertiary quartzarenites. Because coarser-grained laminae and beds have greater permeability and will transmit greater volumes of formation water in a given time than finer-grained beds, the coarser sands will be cemented at a faster rate than the finer sands. The differential rate of cementation shows up as different amounts of cements in beds of different grain size.The small amount of silica in formation waters compared with the abundance of quartz grains in sands results in most sandstones having less than one-third of their primary pores filled by quartz cement. It is not clear at present why quartzarenites of pre-Tertiary age are the most abundant sandstones completely cemented by quartz.
6. Carbonate cement may have a patchy distribution in a bed, but where it is present it completely fills pores to produce tight sandstone. Calcite cement (and many other cements such as anhydrite) form in a different pattern from quartz. Quartz cement forms overgrowths on detrital grains, and all detrital quartz grains act as seed crystals. Cementation by quartz, thus, progresses fairly uniformly by the enlargement of overgrowths from many nucleation (grain) sites. In the commonest fabric of calcite cement, cementation also progresses by enlargement of crystals from nucleation sites, but such sites are widely separated fossil fragments or randomly formed cement crystals. Calcite cement crystals generally grow large enough to enclose several detrital grains of quartz or feldspar and develop poikilotopic fabric. If there is not sufficient calcite to completely cement the sand, the result is pores that are either totally free of cement or former pores that are totally plugged by cement.