GUIDE

How to pick formation tops on well logs

Picking and naming tops for correlation

In short

A formation top is the depth at which a formation begins, picked from a change in log response.
Use the same name for the same top in every well and keep one depth reference.
Tops picked in each well are what a correlation chart connects.

What a top is

A formation top (or marker) is the depth where a mapped formation or other stratigraphic unit begins in a well. Tops are picked by a geologist from log character, supported where available by cuttings, core and seismic or regional knowledge.

What to look for

A sharp change in gamma ray, for example clean carbonate below shale.
A step in resistivity or a change in the density–neutron relationship.
Distinctive marker beds that appear in many wells, such as a thin radioactive shale or a persistent limestone.

Picking steps

Start from a well with a good reference pick, or from published tops.
Choose the log response that best defines the contact and pick at a consistent point on it, such as the midpoint of the break.
Record the depth, the name and the depth reference (measured depth or subsea).
Move to the next well, find the same log character, and pick it with the same name.
Review the picks across the section for consistent thickness trends.

Consistency checklist

One naming convention for every well.
One depth reference and unit.
Notes where a top is uncertain, faulted or missing.

From tops to a correlation chart

Once tops are picked, a correlation chart connects the same top between wells and can flatten the section on a datum. See how to make a well correlation chart online. Picks are interpretations, so check them against the logs and other data.