As kehiner touches on a conventional gamma ray log measures the...

As kehiner touches on a conventional gamma ray log measures the natural radioactivity of a shale (which in doing so helps to identify sands). Shales are naturally more "radioactive" due to higher clay concentrations which hold uranium, thorium and potassium, and so a "hot shale" is simply a shale with elevated concentrations of these minerals (which are perfectly normal for any clay).

Nothing to worry about in terms of process engineering, and certainly not something to stress about before we've even flowed our first exploration well.