Question 8 of 10: (φ N −φ D )-vs-Gamma-Ray Crossplot and Fluid Typing

Question 8: (φ_N−φ_D)-vs-Gamma-Ray Crossplot and Fluid Typing (20 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.

Given. The plotted variable is the neutron–density SEPARATION, φ_N−φ_D, against gamma ray — not a φ_N-against-φ_D plot, and not a shale-corrected one. The two reference points the question supplies fix the whole interpretation:

Find. The crossplot of all selected zones in intervals A–E, and the fluid type of Zones F (9402 ft) and G (9599 ft).

Approach. Join the two reference points with a straight line. Any zone whose separation is caused purely by SHALE must fall on that line, because the shale fraction moves both the gamma ray and the separation in fixed proportion. Gas depresses φ_N without raising the gamma ray at all, so a gas zone plots BELOW the line. A zone on or above the line is liquid-bearing, however large its raw separation looks.

1. The shale line. Through (30, 0) and (85, −22): $$\left(\phi_N-\phi_D\right)_{\text{shale trend}}=\frac{-22-0}{85-30}\left(\gamma-30\right)=-0.40\left(\gamma-30\right)$$ Equivalently, the shale line is just the locus $V_{sh}\,(\phi_{N,sh}-\phi_{D,sh})$ plotted against $\gamma=\gamma_c+V_{sh}(\gamma_{sh}-\gamma_c)$, i.e. a linear shale index on both axes.
2. Digitise each zone off the log. Reading the gamma-ray curve in Track 1 (0–100 API; the smoother curve at the left of the same track is the caliper, 8–18 in) and the compensated formation-density and compensated-neutron curves in Tracks 2–3 (both on the 60–0 p.u. sandstone-matrix scale, density the left-hand curve of the pair):

   Zone	Depth (ft)	γ (API)	φD (p.u.)	φN (p.u.)	φN−φD	Shale line at that γ	Position
   F	9402	62	35	20	−15	−12.8	on the line
   A1	9416	32	37	14	−23	−0.8	far below
   A	9453	34	38	14	−24	−1.6	far below
   A2	9494	43	38	16	−22	−5.2	far below
   B	9506–9512	44	31	25.5	−5.5	−5.6	on the line
   C	9519	47	28	25.5	−2.5	−6.8	above
   D	9528	37	34	20	−14	−2.8	below
   E	9535–9549	46	28.5	25.5	−3	−6.4	above
   G	9599	72	29	16	−13	−16.8	above the line

3. Classify the A–E intervals. Zones A, A1 and A2 sit 17–22 p.u. BELOW the shale line at low gamma ray (32–43 API): far too clean for shale to explain a 22–24 p.u. separation, so the separation is gas. Zone D repeats the pattern more modestly (11 p.u. below the line at 37 API) and is also gas-bearing. Zones B, C and E fall on or above the line — their modest separations are fully accounted for by their shale content, so they are liquid-bearing.
4. Zone F (9402 ft) — liquid. Its raw separation of −15 p.u. looks large, but at 62 API it is a distinctly shaly interval, and the shale line already predicts −12.8 p.u. there. F plots 2 p.u. below the line, which is inside chart-reading tolerance: Zone F is a shaly, LIQUID-bearing (water or oil) sand, not gas. The contrast with Zone A is the whole point — A has a separation of the same order but at half the gamma ray.
5. Zone G (9599 ft) — liquid. At 72 API Zone G is shalier still, and the shale line predicts −16.8 p.u.; the log gives only −13 p.u., so G plots ABOVE the line. A point above the shale trend cannot be gas-bearing: Zone G is a shaly, LIQUID-bearing (wet) sand. The caliper corroborates it — the hole washes out sharply from about 10 in. to 14 in. through this interval, which degrades the pad-contact density reading and, if anything, exaggerates the separation rather than suppressing it.