24-Pet-A1 Principles of Stratigraphy and Sedimentation · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 17-Pet-A1 Principles of Stratigraphy & Sedimentation, 2018-May. 3 hours duration; closed book, no calculator permitted. The paper has two parts: Part A (Questions 1–10, Sedimentology and Sedimentary Processes) – Questions 1 and 2 are mandatory (10 marks each, 20 marks), plus any five of the remaining eight (3–10) at 6 marks each (30 marks), for a Part A total of 50 marks; and Part B (Questions 11–19, Stratigraphy and Sedimentary Basin Analysis) – answer any six of the nine at 5 marks each, for a Part B total of 30 marks – an 80-mark maximum (50 for Part A + 30 for Part B).
Reference texts: Boggs, S. Jr., Principles of Sedimentology and Stratigraphy, 5th ed., Pearson (grain texture, sediment transport, bedforms, carbonate/evaporite systems, sequence stratigraphy, unconformities, stratigraphic principles); Tucker, M.E., Sedimentary Petrology, 3rd ed., Blackwell (sandstone/carbonate classification, diagenesis, dolomitization); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial/deltaic systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, deep-marine and shelf systems); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, basin classification, reservoir quality); International Commission on Stratigraphy, International Chronostratigraphic Chart (geological time scale).
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.
Diagenesis modifies the porosity inherited at deposition through processes that either reduce or increase pore space, often acting on the same rock at different burial stages:
| Porosity-destroying | Porosity-enhancing |
|---|---|
| Mechanical compaction – grain rearrangement into tighter packing under overburden stress, expelling pore fluid | Dissolution – undersaturated pore fluids dissolve unstable grains, allochems or early cement, creating secondary (moldic/vuggy) porosity |
| Chemical compaction (pressure solution) – dissolution at highly-stressed grain contacts, producing concavo-convex/sutured (stylolitic) contacts | Dolomitization – replacement of calcite by dolomite carries a ~13% mole-for-mole volume reduction, commonly creating intercrystalline porosity |
| Cementation – precipitation of calcite, quartz overgrowths or authigenic clay in pore space from circulating fluids | Fracturing – brittle failure (tectonic stress, overpressure) creates open fracture porosity and permeability, especially valuable in low-matrix-porosity rocks |
| Authigenic clay growth – pore-lining/pore-filling clay minerals (illite, chlorite, kaolinite) occlude pore throats and reduce permeability even where bulk porosity looks preserved | Dedolomitization / selective leaching – later dissolution of an earlier diagenetic phase (e.g., dolomite or anhydrite nodules) can re-open porosity that an earlier cementation event had destroyed |
