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24-Pet-B4 Well Testing · December 2016

Question 3 of 22: Silled, salinity-stratified basin – 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, 2016-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, oil sands and tight-oil development, shale gas and coalbed methane, carbonate traps, siliciclastic/deltaic traps) – 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. This sitting's Section 6 is a Siliciclastic Traps section (wave-dominated delta, grain-size–permeability–porosity relations, sandstone diagenesis). The paper is entirely qualitative (draw/describe/define/list), with no numeric given data anywhere.

Reference texts: Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, migration, traps, carbonate systems, oil sands); 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, deltaic and carbonate lithofacies; sandstone diagenesis); James, N.P. & Jones, B., Origin of Carbonate Sedimentary Rocks, Wiley-Blackwell (carbonate platform/ramp/sabkha facies models); Butler, R.M., Thermal Recovery of Oil and Gas, Prentice Hall, 1991 (SAGD, CSS, oil sands thermal recovery); Green, D.W. & Willhite, G.P., Enhanced Oil Recovery, SPE Textbook Series Vol. 6 (thermal EOR mechanisms); Law, B.E. & Curtis, J.B., “Introduction to Unconventional Petroleum Systems,” AAPG Bulletin 86, 2002 (shale gas, tight gas, coalbed methane); Lee, W.J. & Wattenbarger, R.A., Gas Reservoir Engineering, SPE Textbook Vol. 5 (unconventional gas reservoir characterization).

Section 1, Q1-3: Silled, salinity-stratified basin – 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.

1500 m silled, salinity-stratified basin (Black Sea analog): O₂ and preserved TOC vs. depth shallow restricting sill (e.g. Bosphorus strait) thin oxic inflow (open ocean) Basin water column 0–100 m: oxic mixed layer 100–300 m: dysoxic halocline 300–1500 m: anoxic, H₂S-rich (euxinic) org-rich, laminated (unbioturbated) mud 0 m 100 m 300 m 1500 m Dissolved O₂ (mg/L) → Depth ↓ ≈8 mg/L 0 mg/L (H₂S present) Dissolved O₂ Preserved TOC (wt%) → <0.5% 2–8% Preserved TOC
A 1500 m silled basin (Black Sea analog): a narrow, shallow sill (e.g. the Bosphorus strait) restricts deep-water exchange with the open ocean, admitting only a thin oxic surface inflow. Below it, an oxic mixed layer (0–100 m, DO ≈ 8 mg/L, TOC preserved <0.5%) grades through a dysoxic halocline (100–300 m) into a permanently anoxic, sulfidic (euxinic) deep basin (300–1500 m, DO ≈ 0 mg/L, TOC preserved 2–8% in laminated, unbioturbated mud).

Dissolved oxygen is highest in the thin surface mixed layer, where wind mixing and the sill-limited inflow of oxygenated ocean water keep it near saturation (≈8–10 mg/L); it falls steeply through the halocline as the density contrast created by the strong salinity gradient (fresher, less dense water above; saltier, denser water below, sourced from the restricted deep inflow/outflow across the sill) prevents vertical mixing, so bacterial respiration of sinking organic matter consumes oxygen faster than it can be resupplied from above. Below about 300 m the water is permanently anoxic and sulfidic (euxinic, i.e. free H2S present), essentially DO ≈ 0. Preserved total organic carbon (TOC) shows the mirror-image trend: in the oxic mixed layer, settling organic matter is almost completely oxidized by aerobic bacteria and grazers before burial, giving low sediment TOC (well under 1%); through the dysoxic halocline it rises as oxidation becomes progressively less complete; and below the anoxic/euxinic boundary (≈300 m) organic matter reaching the basin floor is preserved essentially unconsumed, with no bioturbation to further degrade or homogenize it, giving finely laminated, organic-rich (typically 2–8% TOC) source-rock-quality mud – the same mechanism responsible for the real Black Sea's modern sapropelic muds and for classic ancient silled-basin source rocks.