Siliciclastic Rocks and Diagenesis PART 1

Question 1

What are the ideal temperatures and depths for oil reservoirs? (Oil window) What kind of material does it come from?

Answer 1

100-150 degrees
1-3.5 km (concentrated between 2-3km)
Organic - kerogen

Question 2

What are the ideal temperatures and depths for gas reservoirs? (Gas window) What kind of material does it come from?

Answer 2

150-220 degrees
3-4 kms (particuarly after 4)
Woody material

Question 3

What happens to the porosity of shale after 1km depth?

Answer 3

Loses most of its porosity

Question 4

What three factors cause changes in siliciclastic rocks during burial?

Answer 4

Increased pressure, temperature (geothermal gradient)

Changes in fluid porosity, fresh/salt water

Question 5

What impact does an increase in pressure have on a rock during burial?

Answer 5

Compaction and pressure solution, decreases porosity and permeability

Question 6

How does increasing temperature and changes in pore fluid chemistry impact a rock during burial?

Answer 6

Precipitation of new minerals fill pores and replace other minerals. Decrease porosity and permeability
Dissolution of minerals increases porosity and permeability

Question 7

What is the geothermal gradient of the North Sea?

Answer 7

20-40 degrees/km

Question 8

Why is it more desirable to have a high geothermal gradient?

Answer 8

High geothermal gradient = shallower depth that hydrocarbons are produced ,cheaper to drill and extract. Greater chance of high porosity and permeability

Question 9

What is meant by absolute porosity?

Answer 9

Total volume of voids in the rock

Question 10

What is meant by effective porosity?

Answer 10

Total volume of interconnected pores

Question 11

What is permeability?

Answer 11

Ability of the rock to transmit fluids (oil,gas,water)

Question 12

What is permeability dependent on?

Answer 12

Effective porosity, pore size, pore interconnections and fluid properties

Question 13

What is a good hydrocarbon reservoir porosity and how do sandstones compare?

Answer 13

20-35% is good with depositional porosity of sandstones around 40-50%.

Question 14

Why do deep buried sandstones have a high porosity?

Answer 14

Retention of some primary porosity

Development of secondary porosity

Question 15

Why is primary porosity lost during compaction?

Answer 15

Gravitational loading by overlying saturated sediment causes porosity to be lost.

Question 16

Describe the three stages that primary porosity is lost during compaction

Answer 16

Stage 1 - dewatering and rearrangement of grains (cubic packing = 48% porosity, rhombohedral compaction = 26%)
Stage 2 - Ductile deformation (mica and lithics) and brittle deformation (quartz and feldspars). Ductile deformation occurs easier so rocks rich in mica and rock fragments compact faster. Sandstones: Quartz rich 5% porosity loss per km
Stage 3 - Chemical compaction - pressure solution affects brittle grains, solubility increases with pressure. Grains with similar hardness/solubility ( two quartz grains) = sutured boundaries, grains with contrasting hardness/solubility = concavo-convex boundaries

Question 17

How can primary porosity be preserved? (5 ways)

Answer 17

1. Rock of high strength grains (well sorted quartz rich) limits early ductile compaction.
2. Make the rock from irregular grains which compact unevenly (e.g. shelter pores)
3. Grow enough early cement to hold grains apart
4. Prevent cements from infilling pores by: coat grains with a cement inhibitor (e.g. chlorite+microquartz), rock with few soluble minerals so no ions for a cement, early hydrocarbon invasion - expels water and wets grains
5. Fluid overpressure

Question 18

What is meant by overpressure?

Answer 18

A rock is overpressured if water within the rock under greater pressure than it would be if underneath continuous water column.

Question 19

How does overpressure preserve porosity?

Answer 19

The water takes some stress that the rock would have been under by supporting it from inside reducing mechanical compaction. Ideally develops before significant compaction.

Question 20

What causes overpressure in the North Sea?

Answer 20

1. Escape of water from a rock is prevented as it compacts - occurs during rapid burial of sandstones interbedded with mudrocks, found in Tertiary or Quaternary sandstones, overpressure not retained for millions of years.
2. Water enters the rock at a greater rate than it escapes.

Question 21

Magnitude of secondary porosity is determined by the mineralogy. How is it formed?

Answer 21

Produced during burial/uplift by influx of acidic pore fluids

Question 22

What are the four types of secondary pores in siliciclastic rocks and how do they form?

Answer 22

Intraparticle - Dissolution of parts of a detrital grain.
Mouldic - Dissolution of an entire grain or group of grains
Interparticle - Dissolution of intergranular cement. Porosity higher than when deposited if the cement corroded detrital grain edges (catenary).
Fracture - Brittle deformation