Sandstone diagenetic processes

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Predicting the occurrence of oil and gas traps
Chapter Predicting reservoir system quality and performance
Author Dan J. Hartmann, Edward A. Beaumont
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Diagenesis alters the original pore type and geometry of a sandstone and therefore controls its ultimate porosity and permeability. Early diagenetic patterns correlate with environment of deposition and sediment composition. Later diagenetic patterns cross facies boundaries and depend on regional fluid migration patterns (Stonecipher and May, 1992). Effectively predicting sandstone quality depends on predicting diagenetic history as a product of depositional environments, sediment composition, and fluid migration patterns.

Diagenetic processes

Sandstone diagenesis occurs by three processes:

  • Cementation
  • Dissolution (leaching)
  • Compaction

Cementation destroys pore space; grain leaching creates it. Compaction decreases porosity through grain rearrangement, plastic deformation, pressure solution, and fracturing.

Diagenetic zones

Surdam et al.[1] define diagenetic zones by subsurface temperatures. Depending on geothermal gradient, depths to these zones can vary. The table below summarizes major diagenetic processes and their impact on pore geometry.

Zone Temperature Major diagenetic processes
Preserves or enhances porosity Destroys porosity
Shallow <80°C or 176°F (<5,00 to 10,00 ft)
  • Grain coatings (inhibit later overgrowths)
  • Nonpervasive carbonate cements that can be dissolved later
  • Clay infiltration
  • Carbonate or silica cement (in some cases irreversible)
  • Authigenic kaolinite
  • Compaction of ductile grains
Intermediate 80-140°C or 176–284°F
  • Carbonate cement dissolved
  • Feldspar grains dissolved
  • Kaolinite, chlorite, and illite precipitate as a result of feldspar dissolution
  • Ferroan carbonate and quartz cement
Deep > 140°C or 284°F
  • Feldspar, carbonate, and sulfate minerals dissolved
  • Quartz cement (most destructive)
  • Kaolinite precipitation
  • Illite, chlorite form as products of feldspar dissolution
  • Pyrite precipitation

Effect of temperature

Figure 1 Porosity–depth plot for sandstones from two wells with different geothermal gradients. Copyright: Wilson;[2] courtesy SEPM.

Depending on geothermal gradient, the effect of temperature on diagenesis can be significant. Many diagenetic reaction rates double with each 10°C283.15 K
50 °F
509.67 °R
increase (1000 times greater with each 100°C373.15 K
212 °F
671.67 °R
).[2] Increasing temperatures increase the solubility of many different minerals, so pore waters become saturated with more ionic species. Either (1) porosity–depth plots of sandstones of the target sandstone that are near the prospect area or (2) computer models that incorporate geothermal gradient are probably best for porosity predictions.

Figure 1 is a porosity–depth plot for sandstones from two wells with different geothermal gradients. The well with the greater geothermal gradient has correspondingly lower porosities than the well with lower geothermal gradient. At a depth of 7000 ft2,133.6 m, there is a 10% porosity difference in the trend lines.

Effect of pressure

The main effect of pressure is compaction. The process of porosity loss with depth of burial is slowed by overpressures. Basing his findings mainly on North Sea sandstones, Scherer[3] notes sandstones retain approximately 2% porosity for every 1000 psi6,894.757 kPa
6.895 MPa
68.046 atm
of overpressure during compaction. He cautions this figure must be used carefully because the influence of pressure on porosity depends on the stage of compaction at which the overpressure developed.

Effect of age

In general, sandstones lose porosity with age. In other words, porosity loss in sandstone is a function of time. According to Scherer,[3] a Tertiary sandstone with a Trask sorting coefficient of 1.5, a quartz content of 75%, and a burial depth of 3000 m9,842.52 ft probably has an average porosity of approximately 26%. A Paleozoic sandstone with the same sorting, quartz content, and burial depth probably has an average porosity of approximately 13%.

See also

References

  1. Surdam, R., C., Dunn, T., L., MacGowan, D., B., Heasler, H., P., 1989, Conceptual models for the prediction of porosity evolution with an example from the Frontier Sandstone, Bighorn basin, Wyoming, in Coalson, E., B., Kaplan, S., S., Keighin, C., W., Oglesby, L., A., Robinson, J., W., eds., Sandstone Reservoirs: Rocky Mountain Association of Geologists, p. 7–21.
  2. 2.0 2.1 Wilson, M., D., 1994a, Non-compositional controls on diagenetic processes, in Wilson, M., D., ed., Reservoir quality Assessment and Prediction in Clastic Rocks: SEPM Short Course 30, p. 183–208. Discusses the effect that variables such as temperature and pressure have on diagenesis of sandstones. A good reference for predicting sandstone reservoir system quality.
  3. 3.0 3.1 Scherer, M., 1987, Parameters influencing porosity in sandstones: a model for sandstone porosity prediction: AAPG Bulletin, vol. 71, no. 5, p. 485–491.

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