What are carbonates?
A carbonate (rock) consists of 50% or more carbonate (mineral).
Where do carbonates grow?
In warm, shallow, clean water
What does carbonate poro-perm depend on?
- sedimentary template
- early diagenesis
- burial diagenesis (cementation)
Name 2 diagenesis processes.
- cementation and leaching
- dolomitization (generally, early dolomitization generate good reservoirs, whereas late dolomitization is mainly degrading reservoir properties)
What are the 6 pore types in carbonates?
- Primary porosity
- Secondary porosity
- Mouldic porosity (low k)
- Vugs and channels
- Fractures
- Intercrystalline
What is primary porosity a function of and name the 3 types of primary porosity?
Primary porosity is a function of sedimentary template.
- Intergranular
- Intragranular
- Fine intergranular
What is secondary porosity a function of and name the 3 types of secondary porosity?
Secondary porosity is a function of diagenesis.
- Solution-enhanced intergranular
- Solution-enhanced intragranular
- Solution-enhanced fine intergranular
Name the 3 types of mouldic porosity.
- Skeletal mouldic
- Ooid/pellet mouldic
- Crystal mouldic
Under the Lucia classification for non-vuggy carbonates, how is permeability calculated?
Step 1: Calculate RFN from core porosity and core k
Step 2: Calculate k from RFN
Carbonate vs Siliciclastic
1) Porosity
- porosity in siliciclastics may be severely reduced by clay overgrowths; rare in carbonates.
- porosity in carbonates may be reduced or increased by diagenesis.
- vuggy porosity common in carbonates; rare in clastics
2) Permeability
- permeability in siliciclastics may be severely reduced by clay overgrowths.
- permeability may increase as porosity decreases, or decrease as porosity increases.
- link between porosity and permeability is not straightforward in carbonates
3) Recovery
- Primary HC recovery is commonly lower in carbonates.
- Saturation and wettability are difficult to predict in carbonates (dolomites tend to be mixed-wet).
Why is porosity determination from density tool difficult in carbonates, and what can be used instead?
Matrix density can vary significantly in carbonate reservoirs.
Can use DEN-NEU instead, or even better NMR (gives effective porosity independent of lithology).
What are some of the log responses of carbonates that are different from siliciclastics?
- porosity from density is unreliable
- high resistivity can occur in rocks that are waterbearing (vugs)
- carbonates have low GR; so use spectral GR/porosity to depth match core
What are some of the difficulties in deriving parameters from core?
- Carbonate rocks tend to be more variable on different scales; small plugs may not be representative. Whole core analysis is preferred.
- The poro-perm relationship is not straightforward due to variations in pore type. Also, must distinguish good data from bad data (for e.g. induced microfractures).
- m not constant due to variations in pore type. Vugs give a curved behaviour on FRF plot.
- n not constant as carbonates have a larger likelihood to be oil wet than sandstones.
- For capillary pressure curve, samples with a significant fraction of mouldic porosity must be treated with special care. Vugs exposed to the outside of the sample must be corrected for.
