04-Geol-B6 · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-B6-1 Petroleum Deposits, 2014-Dec. Closed book; Casio/Sharp approved calculator only; 3 hours.
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. (deltas, carbonate platforms 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.
Q4-1 — Diagenetic evolution of sand through burial. Diagenesis is conventionally divided into three stages — eogenesis (near-surface, early), mesogenesis (deep burial, late) and telogenesis (uplift/exposure, can be late or a "second early stage") — each with characteristic processes and porosity/permeability effects.
| # | Process | Stage / timing | Porosity | Permeability |
|---|---|---|---|---|
| 1 | Mechanical compaction (grain re-packing, rotation, ductile grain deformation) | Early, continuing throughout burial | Decreases | Decreases |
| 2 | Early (eogenetic) cementation — marine/meteoric calcite, aragonite, Fe-oxide/clay grain coats | Early (near-surface) | Decreases (usually); chlorite/grain-coat cements can locally PRESERVE porosity by blocking later quartz overgrowths | Decreases, except grain-coat cements which help preserve permeability |
| 3 | Chemical (pressure) compaction — stylolitization, grain-contact dissolution | Late, deep burial (>~2-3 km, T>70-100°C) | Decreases | Decreases |
| 4 | Quartz & carbonate cementation (overgrowths, pore-filling/pore-bridging cement) | Late, fed by silica from #3 and clay transformation | Decreases | Decreases sharply (pore-throat-lining/bridging clays disproportionately kill permeability) |
| 5 | Clay mineral authigenesis / transformation (smectite → illite, kaolinite, chlorite) | Late, burial-temperature driven | Decreases (pore-filling) or neutral | Decreases strongly (fibrous illite is especially damaging) |
| 6 | Dissolution / leaching of unstable grains & early cements (feldspar, lithics, carbonate) by organic-acid-charged fluids from an adjacent maturing source rock | Late mesogenesis, often coincides with oil-window maturity nearby | Increases (secondary porosity) | Increases, if the new pores are interconnected |
| 7 | Fracturing (tectonic / overpressure-related) | Any stage, often late/structural | Negligible change to matrix porosity | Can increase sharply along fracture network |
| 8 | Telogenetic (uplift/unconformity) meteoric dissolution & weathering | Late (post-burial-maximum, at/near an unconformity) | Increases (secondary/vuggy) | Increases, if connected |
The net effect down a typical burial path is a progressive, largely irreversible loss of porosity and permeability from mechanical then chemical compaction and cementation (#1-5), which can be partially reversed by a later dissolution event (#6, #8) if organic-acid-rich fluids or meteoric water reach the reservoir while it still has connected pore-throat pathways — which is why the BEST reservoir quality at depth is so often found not in the least-cemented sand, but in a sand that was cemented early (preserving a rigid framework against compaction) and then partially DISSOLVED late, generating a connected secondary-porosity network on top of a compaction-resistant skeleton.
