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24-Pet-B4 Well Testing · December 2015

Question 3 of 18: Deep stratified lake – organic carbon and dissolved oxygen profile

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 2015-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, hydrocarbon chemistry, migration/unconventional reservoirs, carbonate traps, structural 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, carbonate systems); Tissot, B.P. & Welte, D.H., Petroleum Formation and Occurrence, 2nd ed., Springer (kerogen typing, maceral groups, catagenesis); Boggs, S. Jr., Petrology of Sedimentary Rocks, 2nd ed., Cambridge (source-rock and carbonate lithofacies); Allen, P.A. & Allen, J.R., Basin Analysis: Principles and Applications to Petroleum Play Assessment, 3rd ed., Wiley-Blackwell (structural styles, unconventional systems); Biddle, K.T. & Wielchowsky, C.C., “Trap Types in Petroleum Basins,” AAPG Memoir 60, ch.12 (structural trap classification); James, N.P. & Jones, B., Origin of Carbonate Sedimentary Rocks, Wiley-Blackwell (carbonate platform/ramp/sabkha facies models); Law, B.E. & Curtis, J.B., “Introduction to unconventional petroleum systems,” AAPG Bulletin 86 (basin-centred gas); Mossop, G.D. & Shetsen, I. (eds.), Geological Atlas of the Western Canada Sedimentary Basin, CSPG/Alberta Research Council, 1994 (WCSB stratigraphy and Canadian basin geography).

Section 1, Q1-3: Deep stratified lake – organic carbon and dissolved oxygen profile (7 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.

700 m stratified lake: dissolved O₂ and preserved TOC vs. depth Lake column 0–100 m: oxic epilimnion 100–500 m: dysoxic thermocline zone 500–700 m: anoxic hypolimnion org-rich, laminated (unbioturbated) mud 0 m 100 m 500 m 700 m Dissolved O₂ (mg/L) → Depth ↓ ≈8 mg/L ≈0 mg/L Dissolved O₂ Preserved TOC (wt%) → <0.5% 3–6% Preserved TOC
A 700 m stratified lake: an oxic epilimnion (0–100 m, DO ≈ 8 mg/L, TOC preserved <0.5%), a dysoxic transitional thermocline zone (100–500 m, DO falling toward zero, TOC rising through ≈0.5–2%), and an anoxic hypolimnion (500–700 m, DO ≈ 0 mg/L, TOC preserved 3–6% in laminated, unbioturbated mud).

Dissolved oxygen is highest near the surface, where wind mixing and photosynthesis keep the epilimnion saturated (≈8–10 mg/L, typical of cold, well-mixed surface water), and falls progressively through the thermocline as bacterial respiration consumes oxygen faster than density stratification allows it to be replenished from above, reaching effectively zero (anoxic) below about 500 m. Preserved total organic carbon (TOC) shows the mirror-image trend: in the oxic epilimnion and upper water column, settling organic matter is almost completely oxidized by aerobic bacteria and grazers before burial, so sediment TOC is low (well under 1%); through the dysoxic transition it rises as oxidation becomes progressively less complete; and below the anoxic boundary (≈500 m) organic matter reaching the lake floor is preserved essentially unconsumed, with no bioturbation to further degrade or homogenize it, giving finely laminated, organic-rich (typically 3–6% TOC, locally higher) source-rock-quality mud.