04-Geol-B6 · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-B6-1 Petroleum Deposits, 2018-May. Closed book; Casio/Sharp approved calculator only; 3 hours. The exam is organized as six equal-value (20-mark) sections and instructs that "the first five sections as they appear in the answer book will be marked". Per the exam's own instructions (Note 8), oilfield-unit questions are answered in oilfield units; all other questions are answered in the units given.
Reference texts: Selley & Sonnenberg, Elements of Petroleum Geology, 3rd ed. (source rocks, generation, migration, traps ch.3-9); Tissot & Welte, Petroleum Formation and Occurrence, 2nd ed. (kerogen types, thermal maturation, oil/gas windows ch.II-IV); Allen & Allen, Basin Analysis, 3rd ed. (migration, petroleum systems ch.9-10); Bjørlykke, Petroleum Geoscience, 2nd ed. (diagenesis, siliciclastic & carbonate reservoirs ch.8-14); Tearpock & Bischke, Applied Subsurface Geological Mapping, 2nd ed. (structural trap geometry ch.10-13); Nichols, Sedimentology and Stratigraphy, 2nd ed. (shorefaces, carbonate ramps ch.15-17).
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.
Q1-1 — Depositional conditions for organic-matter accumulation and preservation. Preservation of organic matter (OM) is governed by the productivity-preservation-dilution balance: (1) anoxic to dysoxic bottom water is the single most important control — without free oxygen, aerobic bacteria cannot oxidize OM as fast as it is buried, so a much larger fraction survives; (2) high primary productivity in the overlying water column (upwelling zones, nutrient-rich lakes) supplies abundant OM in the first place, and can itself help drive bottom water anoxic by consuming oxygen during decay of the sinking flux; (3) fine-grained, low-energy, quiet-water settings below storm wave base avoid winnowing/reworking that would re-expose buried OM to oxidation, and avoid diluting TOC with coarse clastic input; (4) a moderate sedimentation rate that buries OM quickly enough to remove it from the oxidizing zone, without diluting TOC below an economic threshold. Water-column density/thermal/salinity stratification that suppresses vertical mixing and oxygen resupply is the physical mechanism that most often produces conditions (1)-(3) together.
Q1-2 — Kerogen types and their hydrocarbon products. Kerogen is classified by its van Krevelen H/C and O/C ratios, which record the organic precursor:
| Type | Precursor / origin | H/C | Dominant product on maturation |
|---|---|---|---|
| Type I | Lacustrine algal (e.g. Botryococcus) & amorphous liptinite | high (>1.4) | Waxy, paraffinic light oil (best oil-prone) |
| Type II | Marine planktonic algae & zooplankton, some bacterial reworking | moderate (≈1.2-1.4) | Oil, with associated gas |
| Type II-S | Marine, sulphur-rich (anoxic/euxinic, e.g. carbonate source rocks) | moderate | Oil generated at lower maturity than ordinary Type II (weak C-S bonds crack early) |
| Type III | Terrestrial higher-plant material (woody, lignin/cellulose-rich) | low (≈0.7-1.0) | Gas, minor waxy oil; coal-forming |
| Type IV | Recycled/oxidized OM (inertinite, charcoal) | very low (<0.6) | Inert – negligible hydrocarbon yield |
The controlling variable is the H/C ratio inherited from the precursor biomass: algal/lipid-rich Type I/II material is hydrogen-rich and cracks preferentially to liquid hydrocarbons, while lignin- and cellulose-rich terrestrial Type III material is hydrogen-poor and yields mostly gas; Type IV is already too oxidized/aromatized to generate anything of consequence.
Q1-3 — Anoxia and suboxia. Anoxia is the condition of a water mass or sediment pore water containing effectively zero dissolved oxygen (conventionally <0.1 mL O2/L), under which only anaerobic (sulphate-reducing, methanogenic) organisms and bacteria can survive and macrofaunal bioturbation is absent, so organic matter and fine laminae are preserved undisturbed. Suboxia (dysoxia) is the intermediate condition with a small but non-zero oxygen concentration (≈0.1-1.0 mL O2/L) — too little to support a diverse, fully bioturbating benthic community, but enough for a low-diversity, oxygen-tolerant infauna to exist, so preservation is enhanced relative to fully oxic settings but is not as complete as under true anoxia.
Q1-4 — Three settings that accumulate large volumes of organic matter (source-rock precursors).
(a) Lacustrine rift lake. Deep, thermally or chemically stratified lakes (common in early-rift half-graben basins) develop a permanent or seasonal chemocline; below it the hypolimnion is cut off from atmospheric oxygen resupply and becomes permanently anoxic, so the abundant algal productivity in the sunlit epilimnion above rains down and is preserved almost entirely undegraded — the classic source of Type I kerogen.
(b) Deep marine setting beneath an oxygen-minimum zone (OMZ). On a distal shelf-slope-basin profile, an OMZ (a mid-water oxygen minimum sustained by high surface productivity consuming O2 as it sinks, combined with sluggish deep circulation) can impinge directly on the slope or basin floor. Sedimentation there is slow and clastic-starved, so OM is not diluted, and bottom water is oxygen-depleted, so it is preserved — producing marine Type II source rocks.
(c) Deltaic / prodelta setting. Here preservation is achieved less by anoxia and more by burial speed: the high sedimentation rates typical of a delta front bury terrestrial (Type III) organic matter delivered by the river so rapidly that oxidizing bacteria run out of time before it passes below the oxidized surface layer, even though prodelta bottom water itself is often only weakly dysoxic.
