Question 2 of 10: Gamma Ray Log Character by Lithology

Question 2: Gamma Ray Log Character by Lithology (7 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The exam’s figure is NOT a serrated, stylised sketch: it is a clean box-car trace — ten beds, each a flat vertical plateau joined by horizontal steps, with depth increasing downwards and radioactivity increasing to the right. The plateau positions (deflections normalised so the leftmost plateau is 0% and the rightmost is 100% of the trace’s full swing) give only seven distinct levels for the ten beds:

That structure makes the question exact rather than a matter of taste. One level (82–84%) recurs on four separate beds — beds 1, 3, 5 and 10 — and a level that repeats between every other bed is, by construction, the enclosing shale the other lithologies are interbedded with. That fixes zone 1 immediately and leaves exactly six remaining beds for the six remaining lithologies, which then sort uniquely by increasing radioactivity:

• Anhydrite (6) — bed 2, 0%, the lowest plateau on the log. CaSO₄ contains no K, U or Th at all, so it reads the minimum of the whole trace.
• Limestone (3) — bed 6, 4%. Clean calcite is almost as radioactivity-free as anhydrite, but the printed trace separates them by a small, consistent step; the figure therefore DOES distinguish the two, and both must be located individually.
• Sandstone (2) — bed 8, 12%. Clean quartz sand sits just above the two evaporite/carbonate end-members: trace feldspar, mica and heavy-mineral content give it a small but real GR above zero.
• Shaly sand (4) — bed 9, 45%. Sand-dominated with dispersed/laminated clay: a clearly intermediate plateau, roughly half-way to the shale level.
• Sandy shale (5) — bed 4, 64%. The mirror-image mixture, shale-dominated with a sand fraction, so it reads higher than shaly sand but still visibly below the clean shale level.
• Shale (1) — beds 1, 3, 5 and 10, 82–84%. The recurring background level, from K-bearing clay minerals.
• Black marine shale (7) — bed 7, 100%, the maximum of the trace. Organic-rich marine shale deposited under reducing bottom water adsorbs uranium on top of its ordinary clay K and Th, so it reads ABOVE the ordinary shale level — the only bed on the log that does.