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24-Pet-B4 Well Testing · December 2014

Question 5 of 18: Temperature–depth plot – oil and gas windows

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 2014-Dec. 3 hours duration; closed book. This sitting's own cover page reads “98-Pet-B4, Petroleum Geology” and every question is descriptive/interpretive petroleum geology (source rocks, generation, migration, traps, Canadian basin geography) – no well-test pressure-transient content anywhere. Five (5) of the paper's six 20-mark sections are marked (NOTES item 5); all six are solved in full below so this set also serves as a complete study reference. The paper is almost entirely qualitative (draw/describe/define/list), with one true numeric calculation (Q3-2, capillary seal-breach column height).

Reference texts: Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, migration, traps); Tissot, B.P. & Welte, D.H., Petroleum Formation and Occurrence, 2nd ed., Springer (kerogen typing, oil/gas windows, primary migration); Boggs, S. Jr., Petrology of Sedimentary Rocks, 2nd ed., Cambridge (source-rock lithofacies); Allen, P.A. & Allen, J.R., Basin Analysis: Principles and Applications to Petroleum Play Assessment, 3rd ed., Wiley-Blackwell (extensional basins, structural styles); Biddle, K.T. & Wielchowsky, C.C., “Trap Types in Petroleum Basins,” AAPG Memoir 60, ch.12 (stratigraphic/structural/salt trap classification); Mossop, G.D. & Shetsen, I. (eds.), Geological Atlas of the Western Canada Sedimentary Basin, CSPG/Alberta Research Council, 1994 (WCSB stratigraphy and play types).

Section 2, Q2-2: Temperature–depth plot – oil and gas windows (8 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.

Check: the question text names a "pressure versus depth plot" but then asks for the oil/gas windows (defined by temperature) and supplies a geothermal gradient in °C/km, not a pressure gradient – this is treated as the standard oil-industry "temperature–depth" (geothermal) window chart, the only construction the given data (surface T, geothermal gradient) actually supports.

Given. Surface temperature T0 = 0 °C; geothermal gradient = 30 °C/km (linear, T(z) = T0 + 30z with z in km).

Approach. Convert the standard, widely cited temperature ranges for the oil and gas windows into depths using the given linear gradient, then plot maturity (relative hydrocarbon-generation volume) against depth on the same profile.

ZoneTemperature rangeDepth range (this well)Product
Immature< 60 °C0–2.0 kmnone / biogenic gas only
Oil window60–150 °C2.0–5.0 kmoil (peak ≈ 100 °C, 3.3 km)
Wet gas / condensate150–180 °C5.0–6.0 kmwet gas, condensate
Gas window (dry gas)180–225 °C6.0–7.5 kmdry (thermogenic) gas
Post-mature> 225 °C> 7.5 kmnone (kerogen exhausted)
Temperature–depth profile: oil and gas windows Temperature (°C) → Depth (km) ↓ 060120180240 02468 geotherm, 30​°C/km Immature OIL WINDOW 60–150°C, peak ≈100°C wet gas / condensate GAS WINDOW 180–225°C, dry gas post-mature
Temperature–depth (geothermal) profile at 30 °C/km from a 0 °C surface, with the oil window (60–150 °C, 2.0–5.0 km) and gas window (180–225 °C, 6.0–7.5 km) shaded; the intervening wet-gas/condensate zone (150–180 °C) marks declining oil and rising gas generation. Relative hydrocarbon volume generated rises through the oil window to a peak near 100 °C, falls off through the wet-gas zone as remaining kerogen and retained oil crack to gas, and gas generation itself declines beyond ≈225 °C once the kerogen's generative potential is exhausted.