Week 3

Question 1

Diagenesis =

Answer 1

change of sediments/existing sedimentary rocks to different ones during and after rock formation (lithification) at Ts lower than metamorphism

= Physical/chemical/biological alteration

Question 2

Sandstone diagenesis stages

Answer 2

1. Compaction and pressure dissolution
2. Silica cementation
3. Carbonate cementation
4. Clay mineral and feldspar authigenesis
5. Hematite formation

Question 3

1. Compaction and pressure dissolution

Answer 3

MECHANICAL COMPACTION

• point –> long contacts
• fractured/bent grains (N.B. soft e.g. muddy lithic grains squashed/deformed = pseudo matrix)

CHEMICAL COMPACTION
= dissolution at contact points
- can cause 2’ porosity

Question 4

How is mechanical compaction reduced?

Answer 4

Pore fluid overpressure = decrease Vertical Effective Stress (VES)

Grain framework strengthening cement

Poorly sorted

Small grains (less fracturing = less stress at each contact)

Low in calcite voids due to small amount of dissolution = increases grain contact area and decreases stress

Question 5

How is chemical compaction reduced?

Answer 5

Early cementation

Lots of matrix (load spreads and reduces contact pressure)

Question 6

Stylolite =

Answer 6

Chemical compaction once rock fully cemented = irregular/sutured planes

Question 7

2. Silica cementation

Answer 7

Most common = quartz overgrowth (~>2km)
Macroquartz 90-130’C
Microquartz from biogenic silica 60-80’C

= reduces porosity

Question 8

What do fluid inclusions in quartz overgrowth indicate?

Answer 8

Late stage deeper burial

Question 9

How is silica cementation reduced?

Answer 9

FeO/clay coatings

Question 10

3. Carbonate cementation

Answer 10

Commonly the 1st cement and in grain supported rocks

• quartz arenite
• arkose
• litharenite

Early carbonate cementation reduces silica cementation (2) and clay mineral/feldspar authigenesis (4)

Question 11

4. Clay mineral and feldspar authigenesis

Answer 11

Kaolinite indicative of fresh/brackish water (low K+:H+) = fluvial/deltaic/shallow marine sand

Chlorite forms 60-100’C
Illite forms 70-90’C

Question 12

How does illite form?

Answer 12

Kaolinite dissolves
Reacts with K-feldspar

THEREFORE a good reservoir has low levels of K-Feldspar and is mostly plagioclase

Question 13

5. Hematite formation

Answer 13

Red coatings

N.B. Look for if before/after quartz growth

Question 14

Factors affecting the path of diagenesis

Answer 14

Depositional environment and climate

Sediment composition/texture

Pore water chemistry

Burial depth

Uplift timing

Question 15

Diagenetic processes

Answer 15

Compaction

Burial

Decomposition

Alteration

Dissolution

Precipitation

Lithification

Question 16

Stages of limestone diagenesis

Answer 16

Cementation

Microbial micritisation

Neomorphism

Dissolution

Compaction

Dolomitisation

Question 17

Where does limestone diagenesis take place?

Answer 17

Marine zone

Meteoric zone

Burial zone

Question 18

Limestone diagenesis; microbial micritisation =

Answer 18

bioclasts altered on seafloor, holes filled with fine-grained sediment/cement

Question 19

Limestone diagenesis, neomorphism =

Answer 19

replacement/recrystallisation with change of mineralogy

Question 20

Limestone diagenesis; dissolution

Answer 20

Can increase secondary porosity

In meteoric zone

= UNPREDICTABLE RESERVOIR

Question 21

Limestone diagenesis; cementation

Answer 21

Marine zone

Question 22

Limestone diagenesis; compaction

Answer 22

Chemical > mechanical