24-Pet-B1 Natural Gas Engineering · December 2016

Question 7 of 11: (φ_N−φ_D) vs. gamma-ray crossplot — zone identification

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exams — December 2016. 98-Pet-B1, Well Logging and Formation Evaluation (every question is log-interpretation content, not gas-engineering material). 3-hour closed-book exam, 11 questions, all marked, calculators and attached graphs/formula sheet permitted.

Reference texts: Bassiouni, Theory, Measurement, and Interpretation of Well Logs (SPE Textbook Series Vol. 4); Asquith & Krygowski, Basic Well Log Analysis, 2nd ed.; Ellis & Singer, Well Logging for Earth Scientists, 2nd ed.; Schlumberger, Log Interpretation Charts.

Check: Q3, Q7, Q8, Q9(b), Q10 and Q11 are built on the paper's printed logs and attached charts. Values printed as annotations on the logs (Q11's SSP, PSP and GR labels) are used exactly as printed. Values read off a curve or a chart (Q9(b) and the Q8 chart check, Q10's track readings) are read from the printed figure and flagged inline with their precision. All arithmetic that follows is exact.

Question 7: (φ_N−φ_D) vs. gamma-ray crossplot — zone identification (5 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: Fig. Q7 — The printed crossplot redrawn with each ellipse labelled. The straight line is the liquid-filled shaliness trend from clean rock (about 0) up to shale (about +28). The "Gas Effect" arrow points down, because gas makes $\phi_N-\phi_D$ more negative. See the official exam paper.]

Two independent effects place the ellipses. Gamma ray (x-axis) measures clay content, from clean on the left to shaly on the right. Neutron–density separation $\phi_N-\phi_D$ (y-axis) responds to both clay and gas, and in opposite directions:

Shale drives $\phi_N-\phi_D$ positive. Clay-bound water raises the neutron's hydrogen count ($\phi_N$ high), while the dense clay framework keeps $\phi_D$ moderate. Liquid-filled rock therefore climbs the diagonal trend line as GR rises, from $\approx0$ in clean rock to the $+25$ to $+30$ separation of pure shale.
Gas drives $\phi_N-\phi_D$ negative. Gas has a very low hydrogen index, so $\phi_N$ falls. Its low density makes $\rho_b$ drop, so $\phi_D$ rises. A gas-bearing zone is pushed below the liquid trend at the same GR. This is the downward "Gas Effect" arrow printed on the figure.

Small ellipse at the top right (GR ≈ 120–150 API, separation ≈ +24 to +30): 4, Shale. It has the highest GR and the largest positive separation, at the upper end of the trend line.
Long, narrow ellipse lying along the trend line (GR ≈ 50–120, separation ≈ +5 to +22): 3, Shaly liquid-bearing formation. These are intermediate clay contents with liquid in the pores. The points stay on the liquid trend and simply move up it as shale volume increases.
Large ellipse below and to the right of the trend (GR ≈ 60–130, separation ≈ −3 to +17): 1, Shaly gas-bearing formation. It covers the same GR range as zone 3 but sits displaced downward from the trend. The clay raises the separation, and the gas pulls it back down.
Small ellipse at the origin (GR ≈ 20–45, separation ≈ 0): 5, Clean liquid-bearing formation. Low GR and $\phi_N\approx\phi_D$ mark the clean, liquid-filled end of the trend.
Lowest ellipse (GR ≈ 15–60, separation ≈ −2 to −10): 2, Clean gas-bearing formation. It has low GR and is the only zone with a clearly negative separation (the classic neutron–density crossover), because gas acts with no clay to offset it.

Q7 — Ellipse-to-zone assignment
Ellipse on the printed crossplot	Zone
Top right, highest GR and separation	4 — Shale
Elongated, along the diagonal trend line	3 — Shaly liquid-bearing
Large, below the trend at mid-to-high GR	1 — Shaly gas-bearing
Small, at the origin (low GR, ≈0 separation)	5 — Clean liquid-bearing
Bottom, low GR, negative separation	2 — Clean gas-bearing

Topic: Neutron–density separation vs. gamma-ray crossplot for shale and gas identification.

Key relations: Liquid-filled rock: $\phi_N-\phi_D\approx V_{sh}(\phi_{N,sh}-\phi_{D,sh})$, which grows with GR. Gas: $\phi_N$ falls and $\phi_D$ rises, so $\phi_N-\phi_D$ decreases, reaching a negative "crossover" in clean gas sands.

Why this works: GR moves a point sideways according to clay content only. The separation moves it up with clay and down with gas. Every liquid-filled zone therefore falls on one shaliness trend, and gas zones plot below that trend at their own GR.

Common pitfall: Reading gas as a large positive separation. Gas lowers $\phi_N$ and raises $\phi_D$, so it pushes $\phi_N-\phi_D$ down (the printed arrow). The large positive separations belong to shale. In a shaly gas sand, clay partly masks the crossover, so the zone plots below the liquid trend rather than below zero.

Source: Bassiouni, Theory, Measurement, and Interpretation of Well Logs (neutron–density shale and gas effects); Schlumberger Log Interpretation Principles (porosity-log crossplots).

← Question 6 Next: Question 8 →