Abstract We present a new porosity-depth relationship for clean, rigid grain (quartz, feldspar) sands under hydrostatic burial. This allows the prediction of porosity in uncemented sandstones to an accuracy of ±2.5 porosity units at 95% confidence levels. The relationship was derived using experimental data from laboratory compaction experiments and field data for buried uncemented sandstones from around the world. The equation is: where porosity (φ) is in percentages and depth ( z ) is in meters. By scaling this relationship in terms of effective stress rather than depth, it can be used to provide an equally accurate prediction of porosity for uncemented sands in overpressured settings. This is done using the following equation: where z ′ = effective burial depth (in meters); z = burial depth (in meters); ρ r = density of rock (in Kgm −3 [kilograms per cubic meter]) = typically 2650; ρ w = density of water (Kgm −3 ) = typically 1050; g = gravity (in ms −2 [meters per second squared]) = 9.8; φ Σ = average porosity of overburden = typically 0.2; and u = overpressure (in MPa [megapascals]). We propose that there is considerable value in a “compaction only” porosity-depth relationship. A compaction-only trend allows the accurate prediction of porosity in uncemented sandstones, and gives a maximum porosity baseline to which cement volumes, and resultant cemented sandstone porosities, can be compared. If both cemented and uncemented sandstone data are included to produce a “porosity loss-depth” relationship, the resultant scatter (typically ±5 porosity units for a given depth) in the relationship limits its usefulness. Prior to drilling, the new relationships may be used either to predict the porosity of sands that are known to be uncemented or to place an upper limit on the porosity estimated for sandstones either known or suspected to be cemented

Abstract Sandstones that share common detrital mineralogies, depositional environments, and burial histories also share common diagenetic histories. A survey of the diagenetic history of 100 sandstones from around the world has recognized five common, repetitive, and predictable styles of diagenesis in which similar diagenetic mineral assemblages have been observed. The five diagenetic styles are: (1) quartz, commonly with lesser quantities of neoformed clays (e.g., kaolinite and/or illite) and late-diagenetic, ferroan carbonate; (2) day minerals (illite or kaolinite) with lesser quantities of quartz or zeolite and late-diagenetic carbonate; (3) early diagenetic (low-temperature) grain-coating clay mineral cements such as chlorite, which may inhibit quartz cementation during later burial; (4) early diagenetic carbonate or evaporite cement, often localized, which severely reduces porosity and net pay at very shallow burial depths; and (5) zeolites, which occur over a wide range in burial temperature, often in association with abundant clay (usually smectite or chlorite) and late-diagenetic, nonferroan carbonates. The quartz diagenetic style is the most common and accounts for 40% of the sample set. It is also most likely to occur in mineralogically mature sand-stones, while early diagenetic carbonates and zeolites dominate in miner-alogically immature sandstones. Presence or absence of clay appears to be independent of both initial sand mineralogy and depositional environment. However, when clay is present, the type appears to vary as a function of ini-tial sand mineralogy and depositional environment. Large quantities of quartz are unusual cements in sequences that have never been hotter than ~75°C, while illite precipitation at temperatures below ~100°C is rare. Zeolite composition changes systematically from clinoptilolite at ~25°C to laumonite at temperatures >100°C. The repetitive nature and simplicity of these five styles can help predict modifications in reservoir quality due to burial. An accurate prediction of the reservoir quality in sandstones forms the basis of an accurate porosity and permeability prediction ahead of drilling wells in petroleum exploration, development, or production.