Guide to Logging Wells


When completing a log of the materials penetrated during drilling, first check the rock type maps for your area to see what rock/sediment types you might expect to drill through. Nineteen different rock types are included. See the glossary for definitions and some defining properties of rock types. It is understood that drillers typically see only small rock chips, often being transported in a muddy soup. Therefore, it may be impractical to define rock types based on texture or other qualities that cannot be seen in such small pieces. However, attentive observations and some knowledge of rock types should allow for a reliable well log to be recorded.

Complete the description using the basic components of a well log as listed below.

1. Color - Indicate the color of the material, as well as whether it is light or dark. Most commonly used colors would be:

  • Red
  • Yellow/orange
  • Brown
  • Gray
  • Blue
  • Green
  • Black
     

2. Rock/Sediment type - Use the rock type list and county maps as guides to what rocks types to expect in your area. It can be very difficult to tell rock chips apart from others. For example, claystone, shale, and siltstone may look the same. Some crystalline rock types may be indistinguishable. However, keen observations that a driller makes can be important information to include on the log. The drilling resistance and rock hardness are important clues for drillers to use in determining the rock type. For definitions and some distinct properties of the geologic materials listed below, see the glossary on the rock types page.

  • Igneous or metamorphic (crystalline) rocks
    • Coarse-grained crystalline
    • Fine-grained crystalline
    • Foliated rock
    • Marble
    • Quartzite
       
  • Sedimentary rocks
    • Sandstone
    • Siltstone
    • Shale, claystone
    • Conglomerate
    • Coal
       
  • Unconsolidated sediments
    • Clay
    • Silt
    • Sand
    • Pebbles
    • Cobbles
    • Boulders
       

3. Carbonate mineral composition

A 10 percent solution of Hydrochloric Acid (HCl) is used to test for carbonate minerals like calcite. HCl can be obtained from chemical supply companies. The test uses a bottle dropper of HCl to place a few drops on the rock sample, which should be dry. A “fizz” indicates the presence of carbonate minerals. Testing presents a practical problem for the driller because the cuttings are typically wet. One possible solution is to save and mark a few rock fragments with the depth of the hole, and perform the test later on the dried samples. Then record the results on the log.

  • Calcareous – reacts to dilute HCl. Indicate whether it is a strong or weak fizz

 

4. Other descriptions

Any other observations should be included on the log. Such observations might include:

  • Mixtures of above descriptions, especially unconsolidated sediments
  • Other descriptors, such as voids, relative water quantity, mud, organic material, etc.
  • Physical characteristics of the water such as color, smell, etc. (e.g., sulfur smell, water cleared upon purging, etc.)

You may use other terminology. The above guide is not all inclusive.

Examples:

  • Light brown shale; water has smell of sulfur
  • Red, weakly calcareous sandstone
  • Gray, very calcareous sandstone
  • Dark blue-gray, calcareous clay with pebbles and pieces of wood
  • Dark crystalline rock, very hard; little water
     

Much more information about the rocks in general, rocks of Pennsylvania, and the minerals of which they are composed, can be found in our publication ES1: Rocks and Minerals of Pennsylvania. The 1982 publication, EG1, Engineering characteristics of the rocks of Pennsylvania by A. R. Geyer and J. P. Wilshusen. (300 p.) also provides information on the rocks of Pennsylvania by geologic unit.