Limestone Core Logging


I am used to log sedimentary rocks of silisiclastic origin such as sandstones, shales, siltstone, coal, carbonaceous shale, mudstones. I have occasionally logged small amount of limestones but for the most part, the limestones were less than couple of feet so far.

Recently I started logging limestone cores where we are searching for the hi-calcium zones. When I first looked at the limestone cores, I could not tell that I am actually looking at a limestone until I test the rock with diluted HCL and it fizzed. More I keep looking, clearer it became.

This post is just to give you a simple guide when looking at limestone rocks. I will update the page as I have more great and useful pictures of limestones.

Brief Introduction:
Limestone is a type of rock that is mostly composed of calcium carbonate (CaCO3). In most cases the limestones are very pure in nature. The impurities make us less than 8% for most of the limestone rocks. When we have a substantial amount of magnesium in the rock, we call it a dolomite. The intermediate rock between limestone and dolomite is called “dolomitic limestone”. However, in most of the formations all over the world, usually we see either limestone as the rock type or the dolomite as the rock type. Occurrence of both rock types are possibly within same formation but rare.

Easiest way to confirm a limestone:
- Test with diluted HCL.
- Look for fossil fragments (Visual identification)
- Check hardness of the rock (Hand Specimen)
- Stylolites – extremely common in limestone rocks; stylotiles are usually identified as dark, irregular thin laminations. Many times the overall orientation of stylolite is parallel the bedding plane of the rock.
- Calcite Crystals and voids: large rhombohedra calcite crystals are also common in limestones along with vugs or cavities.
- Note: Dolomite does not fizz with dilute HCL unless it has been powered.

Depositional Environment:
Limestone formation could take place in various ways:
• By action of organisms
• Precipitation from evaporating water
• Precipitation through chemical reactions

A large portion of the limestone formation is due to the secretions or shells of organisms like mollusks, foraminifera, corals etc.

Classification of Limestones:
There is actually multiple different classification system for limestone rocks. In older books very general classification scheme can be found. Such as –
1. Textural Classification (old)

  • Compact, dense, fine grained to granular limestone
  • Partly crystalline to crystalline limestone
  • Fossiliferous limestone
  • Chalky or “rotten” limestone
  • Oolitic and Pisolitic limestone
  • Brecciated limestone
  • Sandy or arenaceous limestone
  • Marl


2. Chemical Classification (old)

  • High – calcium limestone
  • Magnesium limestone
  • Dolomite
  • Argillaceous limestone
  • Siliceous limestone

However, the older classifications are no longer used for the detailed classification of limestone rocks, but some of the descriptions are still frequently used for core description.

There are two main classification schemes for limestones that are widely used today:
• Folk Classification (based on Allochems and Interstitial material)
• Dunham Classification (based on depositional texture)

Both of the classification schemes are used in all textbooks and could be found by simple google search.

Allochems basically represent the “large pieces” while “Interstitial material” represent the matrix. The name of the limestone is a combination of such allochems and interstitial materials in Folk Classification.

Types of Allochems: Fossils, Oolite, Pellets (pelloids), Intraclast
Types of Interstitials: Mictrite, Sparite (Spar cement)

Now lets just look at some limestone rocks. I will just point out some important features that will help you to identify limestone rocks.

You can click on the images for a better and bigger version.

Burrow in Limestone

Burrow in Limestone

1. Burrows: Burrowing activity is a very common feature in Limestone rocks. I found it more prominent in micrite-rich limestones. Micrite rich limestones represent a calm water deposition. So, the rock has not gone though high current or wave action. That saves the near surface microbial activities as sedimentary feature in the rock.
Calcite Crystals

Calcite Crystals

2. Calcite Crystals: Many times you can also find nice crystals of rhombohedral calcite as intraclasts.
Calcite Crystals and Voids

Calcite Crystals and Voids

3. It is also not uncommon to find nice large calcite crystals in side voids or vugs in limestone rocks.
Fossiliferous-limestone

Fossiliferous-limestone

4. Fossils: One of the most easiest thing to identify in limestone rocks. Many limestones are highly fossiliferous and you can find various different types of fossils in it.
Fssiliferous-limestone

Fssiliferous-limestone

- Some more different types of marine fossil clasts in limestone core.
Fossiliferous-Limestone

Fossiliferous-Limestone

Sometimes when you wash the core with water or drop some dilute HCL, more features become visible.
Rhombohedral-calcite-crystals

Rhombohedral-calcite-crystals

Rhombohedral calcite crystals prominent perpendicular to bedding plane. Always take a good look at your core.
Shaley-Limestone-laminated

Shaley-Limestone-laminated

Shale or Limestone? Sometimes you have to test with your acid for sure. The above picture is of a “shaley limestone” or may be called a “micrite”. It just looks like shale, laminated. The limina are usually difined by some thin fossil rich band or calcite.
Stylolite-Seam-in-Limestones

Stylolite-Seam-in-Limestones

Stylolites – extremely common in limestones.
Stylolite-surface

Stylolite-surface

Stylolite surface – usually dark.

Filed under: Limestones

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