Carbonates

Question 1

What are carbonates?

Answer 1

A carbonate (rock) consists of 50% or more carbonate (mineral).

Question 2

Where do carbonates grow?

Answer 2

In warm, shallow, clean water

Question 3

What does carbonate poro-perm depend on?

Answer 3

• sedimentary template
• early diagenesis
• burial diagenesis (cementation)

Question 4

Name 2 diagenesis processes.

Answer 4

• cementation and leaching
• dolomitization (generally, early dolomitization generate good reservoirs, whereas late dolomitization is mainly degrading reservoir properties)

Question 5

What are the 6 pore types in carbonates?

Answer 5

• Primary porosity
• Secondary porosity
• Mouldic porosity (low k)
• Vugs and channels
• Fractures
• Intercrystalline

Question 6

What is primary porosity a function of and name the 3 types of primary porosity?

Answer 6

Primary porosity is a function of sedimentary template.

• Intergranular
• Intragranular
• Fine intergranular

Question 7

What is secondary porosity a function of and name the 3 types of secondary porosity?

Answer 7

Secondary porosity is a function of diagenesis.

• Solution-enhanced intergranular
• Solution-enhanced intragranular
• Solution-enhanced fine intergranular

Question 8

Name the 3 types of mouldic porosity.

Answer 8

• Skeletal mouldic
• Ooid/pellet mouldic
• Crystal mouldic

Question 9

Under the Lucia classification for non-vuggy carbonates, how is permeability calculated?

Answer 9

Step 1: Calculate RFN from core porosity and core k

Step 2: Calculate k from RFN

Question 10

Carbonate vs Siliciclastic

Answer 10

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).

Question 11

Why is porosity determination from density tool difficult in carbonates, and what can be used instead?

Answer 11

Matrix density can vary significantly in carbonate reservoirs.

Can use DEN-NEU instead, or even better NMR (gives effective porosity independent of lithology).

Question 12

What are some of the log responses of carbonates that are different from siliciclastics?

Answer 12

• 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

Question 13

What are some of the difficulties in deriving parameters from core?

Answer 13

• 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.