Siliciclastic Rocks and Diagenesis PART 2

Question 1

Where do clay minerals come from?

Answer 1

Detrital - eroded from weathering soils

Authigenic - Cements grow into pore spaces during diagenesis

Question 2

What are kaolinite and illite and how do they differ?

Answer 2

Both authigenic clay mineral cements in sandstones.
They have different crystal habits (Kaolinite - stack of platy crystals, Illite - fine fibrous crystals) and different impacts on sandstone porosity and permeability.

Question 3

How do kaolinite and illite affect permeability and porosity in sandstones

Answer 3

Kaolinite reduces porosity and has little effect on permeability.
Illite has little effect on porossity but reduces permeability

Question 4

What causes smectite to transform into illite?

Answer 4

Temperatures ~70-95 degrees, input of K loss of water and Na, Mg, Ca, Si and Fe

Question 5

How does illite transform into kaolinite?

Answer 5

Slightly greater temperatures and depths and addition of chlorite.

Question 6

What effect does burial have on porosity of mudrocks?

Answer 6

Rapid decrease with burial, rocks deposited have 70-90% water, after 1km burial drops to 30%.

Question 7

How do hydrocarbons get out of mudrocks?

Answer 7

Compaction - pore sizes become smaller than hydrocarbon molecule and new pores opening with existing pores joined by fracturing.

Question 8

What causes new pores in a mudrock to open and existing ones to join?

Answer 8

Oil and gas generation from kerogen.

Volume increase following kerogen maturation Expulsion of interlayer water from clays.

Question 9

Where is shale gas found?

Answer 9

Methane adsorbed onto particles of organic matter or present as free gas in primary pores or fractures.

Question 10

Why are mudrocks important in hydrocarbon exploration? *(three)

Answer 10

Organic-rich mudrocks are main source of oil.
Clay mineral diagenesis can drive reactions to close primary porosity or create secondary porosity.
They can form an impermeable cap rock.

Question 11

What is hydraulic fracturing (fracking)?

Answer 11

Adding water at a rate to increase pressure within shale to greater than their fracture pressure. Fractures join up pores and allows methane to escape into the well.

Question 12

What four things are needed for shale gas formation?

Answer 12

High organic matter in mudrock (>2%)
Kerogen of planktonic marine origin
High burial depth (>1500m)
Porosity 4-7%

Question 13

What is a proppant?

Answer 13

Small particles added to fracturing fluid in fracking which hold open spaces once water pressure is relieved to enable gas to escape.

Question 14

What timescales and pressures are associated with eogenesis, mesogenesis and telogenesis?

Answer 14

Eogenesis - 1,000 - 1,000,000 years, depth up to km
Mesogenesis - 10’s of millions of years, depth 1-4km
Telogenesis - 10s-100s thousand years, depth less than to about 1km

Question 15

What causes red colour in arid terrestrial sandstones?

Answer 15

Red colour from haematite grain rims. Form by early diagenetic dissolution of unstable iron rich grains (e.g. pyroxene) and precipitation of iron oxide rims.

Question 16

What is the most important factor during eogenesis in terrestrial humid environments?

Answer 16

Dissolution - Dissolution of feldspars and mica in sandstones by acids produce seatearths.

Question 17

Where do calcretes form?

Answer 17

Environments where evaporation exceeds precipitation. Carbonate dissolved in groundwater as it is drawn to the surface by evaporation.

Question 18

Where does glauconite form?

Answer 18

In marine sandstones from iron bearing clay minerals.

Question 19

Where do ions within the water originate from? (5 things)

Answer 19

Thermal breakdown of organic matter freeing CO2
Smectite to illite reaction
Dissolution of evaporites and carbonates
Pressure solutions

Question 20

Where does water come from during mesogenesis?

Answer 20

Mudrock compaction, smectite to illite transformation and maturation of oil source rocks.

Question 21

Give three examples of replacement during mesogenesis

Answer 21

K-feldspar and plagioclase by albite
Quartz and feldspar by carbonate
Kaolinite by chlorite and smectite by illite

Question 22

How does secondary porosity occur during mesogenesis?

Answer 22

Dissolution of carbonates by carbonic acid.

Question 23

What are the requirements for albitization of feldspar?

Answer 23

Depth of 3.5km (~100 degrees)

Export of K from k-feldspar or Ca from plagioclase and import of Na.

Question 24

What are the sources of Na for albitization and the sinks of K?

Answer 24

Smectite to illite reaction provides a source of Na as it leaves when K enters. This entering of K is taken from the K-feldspar allowing it to turn into albte. Na also comes from evaporites elsewhere in basin.

Question 25

What effect does carbonate precitpitation during eogenesis and mesogenesis have on porosity?

Answer 25

Dissolution of acidic fluids produces a secondary porosity that is greater than the pre-carbonate primary porosity (caternary pores)

Question 26

What affect does quartz overgrowths have on sandstone porosity?

Answer 26

Prevents loss as thickly coats detrital grains.

Question 27

When do k-feldspar and quartz overgrowths form?

Answer 27

During eogenesis and mesogenesis

Question 28

Name three sources of silica during mesogenesis:

Answer 28

Pressure solution (quartz + feldspar), Replacement of feldspar by kaolinite and Recrystallisation of clay minerals (smectite to illite)

Question 29

Describe the microstructure of of quartz overgrowths

Answer 29

Syntaxial, cement in optical continuity with detrital grain substrate, crystal lattice of detrital and autigenic quartz line up perfectly

Question 30

Name two sources of silica during eogenesis:

Answer 30

Sponges, spicles, diatoms,

Feldspars, amphiboles, pyroxenes

Question 31

Describe the microstructure of feldspar overgrowths

Answer 31

Not syntaxial, crystal lattice of the cement is not aligned relative ot the detrital grains possibly as igneous/metamorphic feldspar is different from diagenetic feldspar.

Question 32

How does secondary porosity form during telogenesis?

Answer 32

Dissolution of detrital and authigenic feldspars, rock fragments, carbonate cements