Inter– vs. Intra–

I got a question from the Peanut Gallery yesterday.
Mohammed asks:
“What is the difference between ‘inter–’ and ‘intra–’?”

The prefix inter– means between (or among, if there are more than two things).
The prefix intra– means inside or within.

Hence, the Internet (capital I) is a network connecting many companies, whereas the corporate intranet (lower case I) is within the firewall of a single company.

What if we are talking about shale?
If we follow this line of thinking, intershales will be shale layers between different named sand zones or major reservoirs, and intrashales will be shale fingers within a given stratigraphic layer. The adjective intershale would mean between two shale layers.

Example of intershale used as an adjective:
Intershale fluid flow between Shale A and Shale B is assumed to be negligible.

However, intra– can also mean “between the layers of,” and the dictionary gives the example of intradermal, which means between the layers of the skin. Therefore, intrashales could also be located between the layers of sand in a certain reservoir.

Since neither intrashale nor intershale is in the dictionary or even in the Schlumberger glossary, to dispel any lack of clarity, let’s use intershale to mean a shale layer between zones with different names, and let’s use intrashale to mean a shale layer within a single named zone. Either way, there is no hyphen.


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