SP/GR Lithology and Fluid-Resistivity Comparison: 24-Pet-B1 Natural Gas Engineering, May 2014, Q8

Question 8: SP/GR Lithology and Fluid-Resistivity Comparison (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.

[Figure not reproduced: The SP and gamma-ray log of Question 8 as printed on source page 6. See the official exam paper or the cited reference text.]

The Question 8 log exactly as printed (source page 6): solid = SP, dashed = gamma ray (the dashed arrow above the track shows GR increasing to the right), and the labelled vertical line is the shale baseline. Eight beds are printed, and the SP takes all three of its possible signs across them — which is what the instruction to compare R_mf and R_w in each section is testing.

Reason for the deflections. The two curves respond to different physics, and reading them together separates lithology from fluid salinity.

Gamma ray responds only to natural radioactivity, which in these rocks is carried by the clay minerals (K, U, Th — Question 6b). Wherever the dashed curve swings right the bed is shale; wherever it swings left the bed is a clean, quartz-rich sand. GR says nothing about the pore fluid.
SP develops only where a permeable bed adjoins a shale AND the mud filtrate and the connate water differ in salinity (Question 3). In shale the curve is flat and defines the shale baseline (no membrane current can flow through an impermeable bed). In a permeable bed the deflection obeys $E_{SSP}=-K\log[(R_{mf})_{eq}/(R_w)_{eq}]$ , so its SIGN reads the salinity contrast directly: a deflection LEFT of the baseline (negative SP) requires $R_{mf}>R_w$ , a deflection RIGHT of it (positive SP) requires $$R_{mf}, and NO deflection in a bed the GR calls clean sand means $R_{mf}\approx R_w$.$

Section by section. Reading the printed log from top to bottom:

Bed (top → bottom)	GR (dashed)	SP (solid)	Lithology	R_mf vs R_w
1	High (far right)	Flat, on the baseline	Shale	— (impermeable; no SP can develop)
2	Low	Large deflection LEFT	Clean permeable sand	R_mf > R_w — fresh filtrate, saltier formation water
3	Low	Large deflection RIGHT	Clean permeable sand	R_mf < R_w — filtrate saltier than the formation water
4	High	Flat, on the baseline	Shale	—
5	Low	Essentially FLAT on the baseline	Clean permeable sand	R_mf ≈ R_w — no salinity contrast, so no EMF
6	High	Flat	Shale	—
7	Low	Large deflection LEFT	Clean permeable sand	R_mf > R_w
8	High	Flat	Shale	—

Beds 2 and 3 sit inside one continuous low-GR (clean) interval with no shale break between them, so they are either one thick sand whose formation-water salinity changes downward or two stacked sands in direct contact; either way the SP reverses sign within clean sand, which can only be a change in R_w relative to a fixed R_mf.

Putting the two together: lithology is fixed by the GR alone (four shales, four clean sands), and the SP then reads the fluid contrast bed by bed — saltier formation water than filtrate in beds 2 and 7, fresher formation water than filtrate in bed 3, and the two effectively equal in bed 5. Bed 5 is the instructive one: a clean, permeable sand with no SP deflection at all. Taken alone the SP would call it shale; the GR proves it is sand, so the flat SP has to be a salinity statement, not a lithology statement.

Topic: Combined SP/GR lithology and fluid-salinity interpretation.

Key relations: $E_{SSP}=-K\log[(R_{mf})_{eq}/(R_w)_{eq}]$ , $$K=61.3+0.133T$$ .

Why this works: GR and SP are sensitive to different physical properties (radioactive mineralogy vs. electrochemical/electrokinetic currents across a permeability boundary), so running them side-by-side lets an analyst cross-check lithology calls that a single curve alone could get wrong (e.g. a radioactive but permeable "hot sand" would fool GR alone but would still show a normal SP kick).

Common pitfall: Assuming the SP keeps one sign down a whole log, or that "no SP deflection" means shale. This paper's log deliberately prints all three cases — negative, positive and flat — in clean sands; the flat one (bed 5) is a permeable sand with R_mf≈R_w, not a shale, and only the GR distinguishes them.

Source: Bassiouni, Ch. 5; Schlumberger, Log Interpretation Principles, Ch. 3–4.

Question 8 of 12: SP/GR Lithology and Fluid-Resistivity Comparison

Question 8: SP/GR Lithology and Fluid-Resistivity Comparison (6 marks)