Sandstone diagenetic processes

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. 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; courtesy SEPM.

Depending on geothermal gradient, the effect of temperature on diagenesis can be significant. Many diagenetic reaction rates double with each temperature::10°C increase (1000 times greater with each temperature::100°C). 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 depth::7000 ft, 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 notes sandstones retain approximately 2% porosity for every pressure::1000 psi 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, a Tertiary sandstone with a Trask sorting coefficient of 1.5, a quartz content of 75%, and a burial depth of depth::3000 m 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

• Predicting sandstone porosity and permeability
• Effect of composition and texture on sandstone diagenesis
• Hydrology and sandstone diagenesis
• Influence of depositional environment on sandstone diagenesis
• Predicting sandstone reservoir porosity
• Predicting sandstone permeability from texture
• Estimating sandstone permeability from cuttings

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