porosity
Primary porosity
Secondary porosity
0-5% - Negligible
5-10%- Poor
10-15%- Fair
15-20%- Good
>20% - Very good
oil cut off
Sandstone ~8%
Limestone ~5%
gas is lower cut off
This microphotographs illustrates the typical components of a sandstone reservoir: quartz grains (gray), secondary quartz cement (also gray), pore space (dark), and secondary minerals in the pore spaces (brownish). Notice that the original grains are well rounded, but the quartz cement is forming a polygonal crystal
This is an example of well-rounded, clean sandstone. The green area is open pore space. This rock has high porosity and probably high permeability also
Poorly sorted coarse sandstone. The spaces between the large, well-rounded grains are filled by small angular fragments in a dark clay-rich
matrix. This rock has very low porosity and permeability
This is a sandstone that has been completely cemented. It is now a quartzite: a metamorphic rock with no porosity left. Notice that irregular grain boundaries, like a jig-saw puzzle.This is the result of pressure solution under high stress conditions. Pressure solution causes the grains to indent each other at the points of contact.
This is an oolitic sandstone with most of the primary porosity (space between the round oolites) filled with secondary quartz crystals
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