Question 8 of 12: Interpreting the SP and Gamma-Ray Deflections

Question 8: Interpreting the SP and Gamma-Ray Deflections (6 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 two curves answer different questions, and the value of the pair is that neither can be fooled in the same way as the other. The gamma ray is a lithology log: it responds to the potassium, thorium and uranium carried by clay and by a few accessory minerals, and is completely indifferent to what is in the pores or in the borehole. The SP is a fluid-and-permeability log: it exists only where an electrolyte contrast can drive current across a permeable membrane, and its magnitude is fixed by $\log(R_{mf}/R_w)$, its sign by which of the two fluids is fresher. The dashed arrow above the track shows the gamma ray increasing to the right, and the three horizontal grid lines divide the log into four sections; because the beds cross those lines, the log is read bed by bed from the top down and each bed is placed in its section:

Bed 1 — top of Section 1 (thin). The SP makes a sharp spike far to the left of the shale base line while, over the same depth, the gamma ray swings to the right, close to its shale level. Taken alone the gamma ray would call this bed shale, but a shale cannot generate an SP deflection — it is the membrane, not the permeable bed. The bed is therefore a permeable but radioactive sand (potassium-feldspar- or mica-rich arkosic sand, a glauconitic sand, or a sand carrying precipitated uranium). The negative sign means the filtrate is the fresher fluid: $R_{mf} > R_w$. The pointed shape of the spike, with no flat top, marks a thin bed, so the recorded deflection is smaller than the static SP the contrast could produce.

Bed 2 — lower Section 1 into upper Section 2 (thick). The gamma ray drops to a low value — a clean sand — and the SP crosses to the right of the shale base line with a broad, flat-topped deflection. A positive, or "reversed", SP is not an artefact: the diffusion cell has changed sense because the formation water is fresher than the mud filtrate, $R_{mf} < R_w$ — a fresh-water aquifer drilled with a comparatively salty mud. It is still a clean, permeable bed.

Bed 3 — lower Section 2 (short). The gamma ray returns to the right, to its shale level, and the SP sits exactly on the shale base line. This is a shale: impermeable, it acts as the membrane and defines the base line from which every other SP deflection is measured, so no $R_{mf}$–$R_w$ comparison applies to it.

Bed 4 — bottom of Section 2 into upper Section 3. The gamma ray swings to its lowest value on the whole log — the cleanest rock present — yet the SP stays on the base line with no deflection. Two explanations survive: the bed is clean but impermeable (a tight cemented sand, a dense limestone or dolomite, or an anhydrite, all of which read very low gamma ray), so no current can flow; or it is permeable but $R_{mf} \approx R_w$, so $\log(R_{mf}/R_w) \to 0$. A caliper showing mudcake, or a microlog separation, settles it: mudcake means permeable, hence $R_{mf} \approx R_w$.

Bed 5 — lower Section 3. The gamma ray moves back to an intermediate level, roughly midway between its clean and shale readings, and the SP remains on the base line. This is a shaly sand or siltstone: its clay content both lowers its permeability and suppresses the SP, so little or no deflection develops; if it is permeable at all, the absence of SP also allows $R_{mf} \approx R_w$.

Bed 6 — upper Section 4 (thick). The SP swings strongly negative with a broad flat top — a fully developed deflection in a thick bed — while the gamma ray is again high, near its shale level. As in Bed 1, a shale cannot produce this SP, so the bed is a thick permeable radioactive sand (arkosic, micaceous or glauconitic, or uranium-bearing) with $R_{mf} > R_w$ and a large contrast. This is the classic case for a spectral gamma ray (Question 6b), whose thorium-plus-potassium curve separates clay from radioactive matrix.

Bed 7 — bottom of Section 4. The gamma ray falls back to a low value and the SP returns to the base line, with at most a trace of positive deflection. The rock is clean but shows no SP: as in Bed 4, it is either impermeable or permeable with $R_{mf} \approx R_w$ (at most very slightly $R_{mf} < R_w$).