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 printed figure is a clean box-car (step-function) gamma-ray trace — depth increasing down the page, radioactivity increasing to the right, no API scale and no serration. Digitising the printed trace gives ten beds at seven distinct radioactivity levels: one level recurs four times (the background the section keeps returning to), and six further levels each occur once. Because the tool measures only total natural radioactivity, the seven listed lithologies are located by RANKING those seven levels, and the recurring background level is necessarily the shale.

Normalising the printed deflections so that the least radioactive bed reads 0% and the most radioactive reads 100% of the trace width:

The reasoning behind the ranking, in order of increasing radioactivity:

• Anhydrite (6) — the extreme left-hand bed (0%). A pure sulphate evaporite contains no potassium, uranium or thorium at all, so it gives the lowest reading on any gamma-ray log; a clean salt or anhydrite bed is routinely used as the zero-API reference.
• Limestone (3) — 4%. Clean calcite is likewise essentially free of K, U and Th; it reads marginally above anhydrite only because of trace clay or organic matter in the matrix.
• Sandstone (2) — 12%. Clean quartz sand reads low but characteristically a little HIGHER than a clean carbonate or evaporite, because of the accessory feldspar, mica, glauconite and heavy minerals (zircon, monazite) that almost always accompany a quartz framework.
• Shaly sand (4) — 45%. A sand framework carrying dispersed or laminated clay reads roughly half-way between the clean-sand and shale levels, because its clay fraction is intermediate.
• Sandy shale (5) — 64%. The complementary mixture — shale-dominated with a sand fraction — therefore reads above shaly sand but still short of the full shale level.
• Shale (1) — 83%, the recurring background. The level the trace returns to four times is the ordinary, laterally continuous shale that encloses the whole section; its potassium-bearing clay minerals (illite, mixed-layer clay) set the shale base line.
• Black marine shale (7) — 100%, the single most radioactive bed. An organic-rich shale deposited under reducing bottom water adsorbs uranium from sea water, so it reads distinctly ABOVE the ordinary shale base line — the one bed on this trace that exceeds the background shale level.

[Figure not reproduced: The exam's gamma-ray trace redrawn to the printed geometry (bed boundaries and relative deflections read from the printed figure), with the seven lithologies located. Seven distinct radioactivity levels are present; the 83% level recurs four times and is therefore the enclosing shale. See the official exam paper.]