Thin Section Petrography: Diagenesis
(NOTE to course participants in Integrated Reservoir Analysis in Pau: These several
pages show most of the thin sections that I displayed on the screen in Pau, and describe
the key objectives of the use of thin section petrography).
DESCRIPTION: To interpret the diagenetic history of reservoir rocks due to the impact
on reservoir quality. Reservoir quality may be either degraded by cementation and
deposition of authigenic clays, or enhanced by dissolution of unstable minerals and the
development of secondary porosity.
List the factors that are the dominant causes of diagenesis in sandstones and
carbonates. Describe the effects on reservoir quality, and particularly the effects of
clays.
Diagenesis refers to all those processes, both mechanical and chemical, that a
sedimentary rock undergoes after it is buried. When sediments are first deposited, they
have a maximum porosity that is filled with water (excluding eolian sands). As the
sediments are buried deeper and deeper in a subsiding basin, the interstitial water is
gradually squeezed out as the weight of the overlying sediments, or overburden,
compacts the rock.
A list of the various processes includes:
1. Packing of the grains and rearrangement of grains.
2. Compaction by the overburden.
3. Chemical dissolution from circulating fluids.
4. Pressure solution from a combination of physical compaction and chemical
processes.
5. Deposition of cement in the intergranular space.
6. Formation of clays in the pore structure due to chemical alteration.
This diagram shows the flow paths of fluids in the subsurface after sediment burial:
heric Circ Precipitation
Atmosp ula
tion
Evaporation Evapotranspiration el
ann
Ch w
Flo
Water Table
Infiltration
Meteoric
Water COMPACTIONAL
WATER Meteoric
Water
Petroleum
Fluids Zone of abnormal pressure
Isotherms
CH 4,CO 2,H2 S
4.4 - Page 1 of 7 Pages; 12/3/03 Subsidence
(modified from from Galloway and Hobday , 1983)
The various types of grain contacts and the packing effects of sandstones upon burial are
shown in this figure. Notice that sand grains are no longer always rounded, but some a
flat or even sutured. This means that quartz is being dissolved from the grains and going
into solution in the interstitial water. Notice also that some of the intergranular pore
space is filled in with clay matrix or with cement.
Grain Packing in Sandstone
Line of Traverse
(using microscope) 4 Types of Grain Contacts
Packing Proximity
Tangential Contact A measure of the extent to
which sedimentary particles
are in contact with their
Sutured Contact neighbors
Long Contact Packing Density
Cement
A measure of the extent to
which sedimentary particles
occupy the rock volume
Matrix
Concavo-Convex
(clays, etc.)
Contact
This Example
Packing Proximity = 40%
Packing Density = 0.8
(modified from Blatt, 1982)
Here is an example from a thin section of quartz grains that have been sutured together by
pressure solution.
(From Pluim, 1985)
With the weathering of igneous rocks that results in the sediment grains making up
sediments, chemical alteration proceeds along the following sequence of minerals,
4.4 - Page 2 of 7 Pages; 12/3/03
starting with the most easily weathered and ending with quartz, the most resilient to
weathering processes: Olivine, calcium plagioclase pyroxene, potassium feldspar,
sodium feldspar, muscovite, and finally quartz. This sequence shows that quartz is the
most resistant to weathering, so it is the dominant mineral in clastic sediments.
Because some minerals dissolve more easily than quartz in the subsurface, this
dissolution results in increased porosity, or secondary porosity developed after burial.
This Scanning Electron Microscope picture shows partially dissolved feldspar:
Partially
Dissolved
Feldspar
Pore
Quartz Detrital
Grain
The end products of feldspars include clays that tend to fill in the pore spaces. The
example on the left is pore-filling kaolinite, which reduces porosity, permeability, and
causes high irreducible water saturation.
The figure on the right shows an example of calcite cement (the dull red color), another
example of reduced porosity and permeability due to diagenesis.
(From Pluim, 1985)
4.4 - Page 3 of 7 Pages; 12/3/03
The first three pages above describe the basics. The additional material below includes
most of the specific thin sections that I showed you in Pau. They are all referenced to
Milliken et al., 2002. The complete CD is available from AAPG. Go to www.aapg.org
on the internet and find the following reference: Milliken, Kitty, Suk-Joo Choh and Earle
F. McBride, 2002, Sandstone Petrography, A Tutorial Petrographic Image Atlas,
AAPG/Datapages Discovery Series 6, Version 1.0.
Additional examples of dissolution of feldspar that creates secondary porosity.
(Milliken el al., 2002)
(Milliken el al., 2002)
4.4 - Page 4 of 7 Pages; 12/3/03
Example of quartz cement completely filling in the original pore space between the
rounded quartz grains. Photo on left is with plain light, and photo on right is with
polarized light. The same color of light blue indicates that the quartz overgrowth of
cement has the same crystal axes as the quartz grain from original deposition.
(Milliken el al., 2002)
Example of mica induced quartz dissolution
(Milliken el al., 2002)
4.4 - Page 5 of 7 Pages; 12/3/03
Horrent chlorite almost completely filling in pore space.
(Milliken el al., 2002)
Example of quartz sandstone with all primary porosity cemented with calcite (red to the
left) and with quartz (right side)
(Milliken el al., 2002)
4.4 - Page 6 of 7 Pages; 12/3/03
Example of chlorite cement that formed a rim of uniform thickness around each quartz
grain. This step during diagenesis helped preserve some of the primary porosity.
(Milliken el al., 2002)
4.4 - Page 7 of 7 Pages; 12/3/03
