Neutron–Density versus Gamma-Ray Crossplot and the Fluid Type of Zones F and G: 24-Pet-B1 Natural Gas Engineering, May 2016, Q11

Question 11: Neutron–Density versus Gamma-Ray Crossplot and the Fluid Type of Zones F and G (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.

Quantity	Value
Log suite	caliper + gamma ray in track 1 (0–100 API); CNL neutron and FDC density porosity in tracks 2–3 (60–0 p.u., sandstone matrix)
Adjacent shale, density porosity $\phi_{D,sh}$	40 %
Adjacent shale, neutron porosity $\phi_{N,sh}$	18 %
Shale gamma ray, $\gamma_{sh}$	86 API
Clean-formation gamma ray, $\gamma_{clean}$	30 API
Zones to classify	F at 9402 ft, G at 9599 ft
Zones available for the crossplot	A, A₁, A₂, B₁, B₂, C, D, E₁, E₂, E₃

Find. The crossplot of $\phi_N - \phi_D$ against gamma ray for the labelled zones, and from it the fluid occupying zones F and G.

Approach. Both shale and gas separate the neutron and density curves, so a raw separation cannot distinguish them. The crossplot removes the ambiguity by giving shaliness its own axis: the two anchor points supplied in the question define a straight shale line along which a purely liquid-bearing, variably shaly rock must plot, and any zone displaced from that line by more than the reading uncertainty is displaced by something other than shale — that is, by gas.

Establish the two anchor points of the shale line. A clean, liquid-bearing formation on the correct matrix scale reads the same porosity on both devices, so its ordinate is zero at the clean gamma-ray level: $$(\gamma, \ \phi_N - \phi_D)_{clean} = (30\ \text{API},\ 0\ \text{p.u.})$$ Pure shale reads the given averages, so its ordinate is $$(\phi_N - \phi_D)_{sh} = 18 - 40 = -22\ \text{p.u.} \quad\text{at}\quad \gamma_{sh} = 86\ \text{API}$$
Draw the shale line between them. Shale volume enters both the neutron and the density response linearly, so the locus of liquid-bearing rock of any shaliness is the straight line joining the two anchors: $$(\phi_N - \phi_D)_{line} = (\phi_N - \phi_D)_{sh}\,\frac{\gamma - \gamma_{clean}}{\gamma_{sh} - \gamma_{clean}} = -22\,\frac{\gamma - 30}{86 - 30}$$ $$\boxed{(\phi_N - \phi_D)_{line} = -0.393\,(\gamma - 30)\ \text{p.u.}}$$ Equivalently, the ordinate of the line at any level is $-22\,V_{sh}$, where $V_{sh} = I_{sh}$ is the linear shale index from the gamma ray.
Read the log at each labelled zone. Reading the gamma ray against its 0–100 API track and the two porosity curves against the 60–0 p.u. sandstone-matrix scale gives the values below, together with the ordinate $\phi_N - \phi_D$ and the vertical displacement from the shale line, $\Delta = (\phi_N - \phi_D)_{obs} - (\phi_N - \phi_D)_{line}$.
Plot the points and classify by displacement. A zone sitting on the line has a separation fully explained by shale; a zone lying well below it has an extra separation that shale cannot account for. Taking a threshold of 6 p.u. — twice the reading uncertainty — three zones qualify as gas-bearing and the rest do not.

The required crossplot. The red line is the shale (liquid-bearing) trend joining the clean point (30 API, 0 p.u.) to the shale point (86 API, −22 p.u.). Zones A, A₂ and D fall far below it — gas. Zones F and G, the two the question asks about, fall on and above it.

Zone	γ, API	$\phi_D$, p.u.	$\phi_N$, p.u.	$\phi_N-\phi_D$, p.u.	Shale line, p.u.	Δ, p.u.	Verdict
A	42.8	37.3	16.2	−21.1	−5.0	−16.1	gas
A₁	48.5	31.0	21.1	−9.9	−7.3	−2.6	liquid
A₂	44.0	39.8	16.7	−23.1	−5.5	−17.6	gas
B₁	44.5	29.3	22.9	−6.4	−5.7	−0.7	liquid
B₂	44.9	31.8	29.3	−2.5	−5.9	+3.4	liquid
C	46.0	29.2	25.7	−3.5	−6.3	+2.8	liquid
D	36.3	34.7	19.3	−15.4	−2.5	−12.9	gas
E₁	47.0	28.4	24.8	−3.6	−6.7	+3.1	liquid
E₂	45.0	28.3	25.5	−2.8	−5.9	+3.1	liquid
E₃	48.1	26.8	22.8	−4.0	−7.1	+3.1	liquid
F (9402 ft)	64.0	35.4	20.4	−15.0	−13.4	−1.6	liquid-bearing shaly sand
G (9599 ft)	73.3	28.4	16.1	−12.3	−17.0	+4.7	liquid-bearing shale / shaly sand

Fluid type of the two zones asked about

Zone F, 9402 ft — water (liquid) bearing, not gas. Zone F shows a large raw separation of 15 porosity units, which on a raw neutron–density overlay would look like a promising gas crossover. The crossplot corrects that impression: at 64 API the zone is more than 60 % shale on a linear index, and shale alone accounts for 13.4 of the 15 units. The residual displacement, 1.6 p.u. below the line, is inside the reading tolerance of the log, so there is no evidence of gas. Zone F is a shaly, liquid-bearing (water-bearing) sand; its separation is a shale effect, and at most a residual gas saturation could hide within the uncertainty.

Zone G, 9599 ft — water (liquid) bearing, not gas. Zone G plots 4.7 p.u. above the shale line, i.e. on the liquid side of it: its separation is smaller than shale alone would produce at 73 API. There is no gas effect whatever. The caliper is enlarged over this interval, the gamma ray is the highest on the section, and the crossplot places the point close to the shale end of the trend — consistent with a water-bearing shale or very shaly sand of no reservoir interest.

What the crossplot does find. The method's discrimination is demonstrated by the zones it flags: A, A₂ and D lie 13 to 18 porosity units below the shale line, far outside any reading uncertainty, and those are the gas-bearing intervals in this well. Zone D is the cleanest of the three (36 API) and shows the effect most convincingly, because almost none of its separation can be blamed on clay. Zones B, C and E cluster on or just above the line: clean to slightly shaly, liquid-bearing sands. This is exactly why the crossplot is worth constructing rather than simply reading the overlay — F and G would both have been called gas from the raw separation alone.

Check: the gamma-ray and porosity values in the table are read from the printed log on page 10, which prints depth ticks only at 9400, 9500 and 9600 ft and carries no intermediate numeric grid. Each reading carries roughly $\pm 3$ porosity units and $\pm 5$ API units. The three gas verdicts (A, A₂, D) survive the full uncertainty band comfortably; the F and G verdicts rest on displacements of 1.6 and 4.7 p.u. and should be read as "no evidence of gas" rather than as a proof of complete water saturation. Note also that the shale values printed in the question, $\phi_{D,sh} = 40\ \%$ with $\phi_{N,sh} = 18\ \%$, put the shale point on the negative side of the ordinate; that is unusual for shale on a sandstone-matrix scale but it is consistent with this particular log, on which the density curve reads higher than the neutron curve throughout, and it is what makes the shale line discriminating here. The values have been used exactly as printed. At zone G the two track-1 curves separate below the caliper step, so the gamma-ray reading there could be as low as about 60 API rather than 73 API; at 60 API the shale line sits at −11.8 p.u. and G's −12.3 p.u. still lies on it, so the liquid verdict does not change.

Topic: Separating the gas effect from the shale effect with a neutron–density-difference versus gamma-ray crossplot.

Key relations: Linear shale index $I_{sh} = V_{sh} = (\gamma_{log} - \gamma_{clean})/(\gamma_{sh} - \gamma_{clean})$; shale trend $(\phi_N - \phi_D)_{line} = V_{sh}\,(\phi_{N,sh} - \phi_{D,sh})$; gas indicator $\Delta = (\phi_N - \phi_D)_{obs} - (\phi_N - \phi_D)_{line}$, with $\Delta$ strongly negative for gas.

Why this works: Shale and gas both separate the two porosity curves, so the separation alone is ambiguous — but they are not correlated in the same way with radioactivity. Shale is radioactive, and the separation it produces is proportional to how much of it there is, which the gamma ray measures. Gas is not radioactive at all, so the separation it produces is uncorrelated with the gamma ray. Plotting separation against gamma ray therefore puts every shaly liquid-bearing point on one straight line and throws gas-bearing points off it, no matter how shaly they happen to be. The construction needs only the four numbers the question supplies, which is why they are given.

Common pitfall: Calling every neutron–density crossover a gas zone. In this well that error would have promoted both F and G, and it would have ranked zone F (raw separation 15.0 p.u.) alongside the genuine gas zone D (15.4 p.u.), even though the crossplot separates the two by more than 11 porosity units. The second pitfall is mixing matrix scales: the shale anchor and the zone readings must all be taken from the same porosity presentation (here, sandstone matrix), otherwise the shale line is drawn in a different coordinate system from the points.

Source: Bassiouni, Ch. 13 (shaly-sand and gas-detection crossplots); Asquith & Krygowski, Ch. 4 and Ch. 7 (neutron–density interpretation, shale corrections); Schlumberger, Log Interpretation Principles, Ch. 6.

Question 11 of 12: Neutron–Density versus Gamma-Ray Crossplot and the Fluid Type of Zones F and G

Question 11: Neutron–Density versus Gamma-Ray Crossplot and the Fluid Type of Zones F and G (20 marks)

Fluid type of the two zones asked about