Question 8 of 10: Density-Neutron Crossplot and Fluid Typing
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Examinations, 17-Pet-B1, Well Logging and Formation Evaluation — December 2019, 3 hours, closed book (Casio or Sharp approved calculators permitted), 10 questions, all marked.
Reference texts: Bassiouni, Theory, Measurement, and Interpretation of Well Logs (SPE Textbook Series Vol. 4); Asquith & Krygowski, Basic Well Log Analysis, 2nd ed. (AAPG); Ellis & Singer, Well Logging for Earth Scientists, 2nd ed.; Schlumberger, Log Interpretation Charts / Log Interpretation Principles and Applications.
Question 8: Density-Neutron Crossplot and Fluid Typing (20 marks)
Gamma ray 0–100 API (with the CALIPER, 8–18 in, as the smoother left-hand curve in the same track); porosity tracks a single 60–0 p.u. sandstone-matrix scale, DENSITY the left (higher-porosity) curve of the pair
Find. The φN−φD versus gamma-ray crossplot of the zones picked within intervals A–E, and the fluid type of Zones F (9402 ft) and G (9599 ft).
Approach. Read the question literally: the ordinate is the SEPARATION $\phi_N-\phi_D$ and the abscissa is gamma ray, so no shale correction is applied to the plotted points. Instead the shale effect is carried by a straight SHALE LINE drawn between the two reference points the question supplies — the clean line (γ=30 API, zero separation) and the shale point (γ=85 API, $\phi_{N,sh}-\phi_{D,sh}=16-38=-22$ p.u.). Any zone lying ON that line has exactly the separation its own shaliness explains; a zone lying well BELOW it has more negative separation than shale alone can account for, which in a sandstone can only be gas.
The shale line.$$\phi_N-\phi_D = \frac{\phi_{N,sh}-\phi_{D,sh}}{\gamma_{sh}-\gamma_{clean}}\,(\gamma-\gamma_{clean}) = \frac{16-38}{85-30}(\gamma-30) = \boxed{-0.40\,(\gamma-30)}$$
Zone readings from the printed log. Depths are calibrated from the printed 9400 FT and 9600 FT ticks, which puts the F and G labels at 9402 ft and 9599 ft exactly as the question states.
Zone
GR (API)
φD (p.u.)
φN (p.u.)
φN−φD
Shale line at that GR
Position
F (9402 ft)
62
33
21
−12.0
−12.8
on the line
A1
32
37
14
−23.0
−0.8
far below
A
34
38
14
−24.0
−1.6
far below
A2
43
38
16
−22.0
−5.2
far below
B
44
31
25.5
−5.5
−5.6
on the line
C
47
28
25.5
−2.5
−6.8
above
D
37
34
20
−14.0
−2.8
far below
E
46
28.5
25.5
−3.0
−6.4
above
G (9599 ft)
72
29
17
−12.0
−16.8
above
Zone F (9402 ft). GR = 62 API is well up the shale trend, and the measured separation of −12.0 p.u. is within 1 p.u. of the −12.8 p.u. the shale line predicts at that gamma ray. The whole of F’s neutron-density separation is therefore explained by its shale content: Zone F is a shaly, LIQUID-bearing (oil or water) sand — no gas.
Zone G (9599 ft). GR = 72 API is the highest reading of the picked zones (the caliper shows the hole enlarging here, so the true formation GR may be a little lower), and the measured separation of −12.0 p.u. is almost 5 p.u. ABOVE the −16.8 p.u. the shale line predicts. G is on the liquid side of the line, not below it: Zone G is also a shaly, LIQUID-bearing interval — not gas.
The gas zones, for contrast. Zones A, A1, A2 and D plot 11–22 p.u. BELOW the shale line, i.e. they show far more neutron-density separation than their (low) gamma ray can explain — those are the genuine gas sands on this log, and they are exactly the intervals where the printed porosity tracks show the shaded density-neutron crossover. Zones B, C and E sit on or just above the line: shaly, liquid-filled.
The crossplot the question asks for: φN−φD against gamma ray, with the shale line drawn between the clean line (30 API, 0 p.u.) and the shale point (85 API, −22 p.u.). Zones A, A1, A2 and D fall far below the line (gas); B, C, E, F and G lie on or above it (liquid).
Zone
GR (API)
φN−φD (p.u.)
Distance from shale line
Fluid type
F (9402 ft)
62
−12.0
+0.8 (on the line)
Liquid (oil/water)
G (9599 ft)
72
−12.0
+4.8 (above the line)
Liquid (oil/water)
Check: the GR and porosity values above are read from the printed log against its own printed scales (gamma ray 0–100 API, porosity 60–0 p.u. sandstone matrix); treat them as engineering estimates with a ±2–3 p.u. / ±3 API tolerance. Two reading traps on this particular log are worth flagging because both change the answer: (1) track 1 carries the CALIPER as well as the gamma ray, and the caliper is the smoother, left-hand curve — reading it as GR puts Zone F near 40 API instead of 62; (2) the gamma-ray scale is 0–100 API, not 0–200. How much margin each conclusion has is worth stating plainly. The shale line reaches −12.0 p.u. at 60 API, so Zone F (62 API) sits only 2 API clear of it — F is ON the line within reading tolerance, which is itself the finding: its separation is fully explained by its shaliness, and it is 10–22 p.u. away from every genuine gas zone on this log. Zone G has a real margin: its GR would have to read below 60 API, i.e. be misread by more than 12 API, before it fell below the line.
