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24-Pet-B4 Well Testing · Undated paper

Question 6 of 13: Three main stages of organic maturation

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 17-Pet-B4 (May 2019, per its page footers). 3 hours duration; closed book. This paper's own cover page reads “17-Pet-B4, Petroleum Geology” and all five sections are descriptive/interpretive petroleum geology (terminology, source rocks, thermal maturation, stratigraphic traps, structural traps), no well-test pressure-transient content anywhere. The section marks are Section 1 = 30, Section 2 = 18, Section 3 = 22, Section 4 = 15, Section 5 = 15 (sum 100), used throughout below. The exam is entirely qualitative (explain/define/sketch).

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

Section 3, Q3-1: Three main stages of organic maturation (10 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.

Stages of organic maturation with depth/temperature and vitrinite reflectance Diagenesis Catagenesis (oil window) Metagenesis (dry gas) 0°C ~50°C ~100°C ~150°C >200°C Temperature / burial depth → Ro≈0% Ro≈0.5% Ro≈1.3% Ro≈2.0% Ro≈4.0+% Ro oil window
The three stages of organic maturation with approximate temperature boundaries and the corresponding vitrinite reflectance (Ro) scale used to calibrate them in a real well.

1. Diagenesis – from deposition to shallow burial, roughly T < 50 °C. Dominated by biological (bacterial) and low-temperature chemical processes: loss of water, CO2 and functional groups (decarboxylation) as biopolymers condense into kerogen. Any hydrocarbon generated at this stage is BIOGENIC METHANE only (from bacterial methanogenesis) – there is no significant thermogenic oil or wet gas yet.

2. Catagenesis – the main hydrocarbon-generation stage, roughly T ≈ 50–150 °C (peak oil generation near 100–120 °C). Thermal (thermogenic) cracking of kerogen breaks C–C bonds, first generating LIQUID OIL, then, with increasing depth/temperature, progressively lighter oil, condensate, and WET GAS (C2–C5) as both the remaining kerogen and the earlier-generated oil are progressively cracked. This is the principal, largest-volume petroleum-generation stage and is commonly called the “oil window.”

3. Metagenesis – the late, high-temperature stage, roughly T > 150–200 °C. Any kerogen remaining after catagenesis, and any oil generated earlier that is still present, is thermally cracked all the way to DRY GAS (essentially pure methane), leaving behind a solid, hydrogen-poor residue (pyrobitumen). Beyond metagenesis, at still higher temperature/pressure, the organic matter is destroyed/graphitized (low-grade metamorphism) and all further generative potential is lost.

Across the three stages, the RELATIVE amount and character of product therefore shifts systematically: essentially none (diagenesis, apart from minor biogenic gas) → mostly liquid oil, increasingly mixed with wet gas (catagenesis) → entirely dry gas (metagenesis) – and the relative split between oil and gas generated during catagenesis is further controlled by kerogen TYPE (Q2-2): a Type I-rich rock yields mostly oil through catagenesis, a Type III-rich rock yields mostly wet gas even within the same temperature window.