What is a shale? In a recent presentation to the Geophysical Society of Houston, Rune Holt attempted to answer the question by listing three attributes of a shale: (1) clay minerals should constitute the load-bearing framework; (2) shales have nanometer pore sizes and nanodarcy permeability; (3) surface area is large, and water is adsorbed on surfaces or bound inside clay platelets. This is a good definition of shales, but perhaps a typical geophysicist may not know how to apply it in practice with typical data sets. In particular, a geophysicist often has no core data and is limited to well logs. This article describes how petrophysicists traditionally distinguish sands from shales using typical well logs in clastic sections. Only lean shales with minimal organic content are considered here.

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