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.
Q6-1 — Detached, rimmed carbonate platform. A detached (isolated) rimmed platform is a carbonate bank surrounded on all sides by open ocean (not attached to a continental shoreline), with a wave-resistant organic (reef) or shoal-grainstone rim that protects a lower-energy interior.
| Environment | Character | Reservoir potential |
|---|---|---|
| Open-marine basin | Normal-salinity, open circulation; fine pelagic/hemipelagic carbonate mud and allochems shed off the platform | Poor as reservoir (fine-grained, low permeability); can be an excellent SOURCE rock if anoxic |
| Fore-reef slope | Talus of reef-derived debris shed down-slope; poorly sorted, wide grain-size range | Fair to good if dolomitized/leached (breccia-hosted porosity), otherwise muddy matrix baffles permeability — variable |
| Reef crest / rim | High-energy, wave-resistant framework (coral/algal boundstone) and flanking grainstone shoals | Best reservoir facies — high primary (framework, interparticle) porosity plus secondary dissolution/dolomitization porosity, and good permeability from winnowed, mud-free grain packing |
| Lagoon | Low-energy, restricted-circulation muddy carbonate (wackestone/mudstone) interior | Generally poor reservoir (low permeability from lime mud), though local patch-reef/grainstone shoal belts within the lagoon can be locally good; often a good top SEAL / source rock if restricted enough to go anoxic |
| Tidal flat / back-reef | Peritidal, often evaporitic; fenestral fabric, early dolomitization common | Variable — frequently tight from early evaporitic cementation, but fenestral and dissolution (vuggy) porosity can locally be excellent |
The reservoir-quality pattern across the platform is controlled by depositional energy (which sorts out mud, preserving primary porosity in the rim) combined with early diagenetic overprint (dolomitization and dissolution, which most readily affects the rim/back-reef where meteoric or reflux fluids concentrate) — the rim is almost always the platform's best reservoir facies, while the lagoon more often supplies the seal and/or source.
