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.
Q6-1 — Carbonate ramp: plan view, cross-section, environments, reservoir potential.
A carbonate ramp is a gently, uniformly dipping (typically <1°) depositional profile from shoreline to basin with no break in slope or shelf-margin edge (distinguishing it from a rimmed shelf/platform, which has an abrupt margin and steep fore-reef slope). Energy decreases smoothly and continuously basinward, so the environments and their facies grade into one another rather than being separated by a sharp physiographic break.
| Environment | Character | Reservoir potential |
|---|---|---|
| Inner ramp (peritidal flats, restricted lagoon) | Shoreward of fair-weather wave base; low-energy, often restricted circulation; muddy carbonate, evaporites, tidal-flat cycles, local ooid/skeletal tidal-bar sands | Generally poor (mud-dominated, low primary porosity) unless dolomitized, or where high-energy tidal-bar/shoal sands occur locally |
| Mid ramp (between fair-weather and storm wave base — high-energy shoal belt) | Highest-energy zone on the ramp; well-sorted ooid, skeletal and peloidal grainstone shoals, patch reefs/bioherms | Best reservoir facies — high, well-connected primary interparticle porosity, further enhanced by early marine or meteoric (subaerial-exposure) diagenesis |
| Outer ramp (below storm wave base, above basin) | Low-energy; fine skeletal wacke-/packstone, deposited from suspension and occasional distal storm layers | Poor reservoir (fine-grained, low permeability), but can be a good regional top seal, and if organic-rich can itself be a source rock |
| Basin | Deepest, lowest-energy setting; very fine pelagic/hemipelagic carbonate mud, commonly organic-rich | Not a reservoir; primary source-rock and regional-seal potential |
The reservoir "sweet spot" on any carbonate ramp is therefore the mid-ramp shoal belt, the one zone where wave/current energy is high enough to winnow away lime mud and leave clean, well-sorted grain-supported carbonate sand — the carbonate analogue of a high-energy siliciclastic shoreface (Section 4), but produced in situ by biological/chemical precipitation rather than by transport of terrigenous grains.
