Siliciclastic Rocks and Diagenesis PART 1 Flashcards
What are the ideal temperatures and depths for oil reservoirs? (Oil window) What kind of material does it come from?
100-150 degrees
1-3.5 km (concentrated between 2-3km)
Organic - kerogen
What are the ideal temperatures and depths for gas reservoirs? (Gas window) What kind of material does it come from?
150-220 degrees
3-4 kms (particuarly after 4)
Woody material
What happens to the porosity of shale after 1km depth?
Loses most of its porosity
What three factors cause changes in siliciclastic rocks during burial?
Increased pressure, temperature (geothermal gradient)
Changes in fluid porosity, fresh/salt water
What impact does an increase in pressure have on a rock during burial?
Compaction and pressure solution, decreases porosity and permeability
How does increasing temperature and changes in pore fluid chemistry impact a rock during burial?
Precipitation of new minerals fill pores and replace other minerals. Decrease porosity and permeability
Dissolution of minerals increases porosity and permeability
What is the geothermal gradient of the North Sea?
20-40 degrees/km
Why is it more desirable to have a high geothermal gradient?
High geothermal gradient = shallower depth that hydrocarbons are produced ,cheaper to drill and extract. Greater chance of high porosity and permeability
What is meant by absolute porosity?
Total volume of voids in the rock
What is meant by effective porosity?
Total volume of interconnected pores
What is permeability?
Ability of the rock to transmit fluids (oil,gas,water)
What is permeability dependent on?
Effective porosity, pore size, pore interconnections and fluid properties
What is a good hydrocarbon reservoir porosity and how do sandstones compare?
20-35% is good with depositional porosity of sandstones around 40-50%.
Why do deep buried sandstones have a high porosity?
Retention of some primary porosity
Development of secondary porosity
Why is primary porosity lost during compaction?
Gravitational loading by overlying saturated sediment causes porosity to be lost.
Describe the three stages that primary porosity is lost during compaction
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
How can primary porosity be preserved? (5 ways)
- Rock of high strength grains (well sorted quartz rich) limits early ductile compaction.
- Make the rock from irregular grains which compact unevenly (e.g. shelter pores)
- Grow enough early cement to hold grains apart
- 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
- Fluid overpressure
What is meant by overpressure?
A rock is overpressured if water within the rock under greater pressure than it would be if underneath continuous water column.
How does overpressure preserve porosity?
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.
What causes overpressure in the North Sea?
- 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.
- Water enters the rock at a greater rate than it escapes.
Magnitude of secondary porosity is determined by the mineralogy. How is it formed?
Produced during burial/uplift by influx of acidic pore fluids
What are the four types of secondary pores in siliciclastic rocks and how do they form?
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
