24-Pet-A1 Principles of Stratigraphy and Sedimentation · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 17-Pet-A1 Principles of Stratigraphy & Sedimentation, 2019-May. 3 hours duration; closed book, approved Sharp/Casio 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 6 marks each, for a Part B total of 36 marks – an 86-mark maximum (50 for Part A + 36 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, porosity); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial/deltaic/deep-marine systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, alluvial fans, deltas, deep-marine systems); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (basin analysis, well-log correlation, seismic/acoustic impedance); 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.
Total versus effective porosity. Total porosity is the percentage of the total rock volume occupied by void space, whether or not that space is connected to any other pore. Effective porosity is the percentage of the total rock volume occupied by interconnected, fluid-conducting void space only, excluding isolated (dead-end) pores; effective porosity is therefore always ≤ total porosity. The distinction matters because reservoir performance (fluid storage that can actually be produced, and flow capacity) depends on effective, not total, porosity, whereas total porosity is what a density or neutron log more directly measures.
| Type | Fabric-selective? | Origin |
|---|---|---|
| Interparticle (between grains) | Yes | Depositional – primary, present at deposition |
| Intraparticle (within a grain, e.g. inside a foram test) | Yes | Depositional – primary |
| Fenestral (e.g. "birdseye") | Yes | Depositional – primary voids from gas escape/desiccation |
| Intercrystalline (between crystals, e.g. dolomite rhombs) | Yes | Diagenetic – created during dolomitization/recrystallization |
| Moldic (dissolution of a grain, leaving its mold) | Yes | Diagenetic – selective dissolution of an unstable grain (e.g. aragonite) |
| Vuggy (irregular voids cross-cutting fabric) | No | Diagenetic – dissolution not confined to a single fabric element |
| Fracture | No | Diagenetic/tectonic – brittle failure post-lithification |
| Channel/cavern (karst) | No | Diagenetic – large-scale meteoric dissolution |
Depositional (primary) porosity types – interparticle, intraparticle, fenestral – form at or soon after deposition and are fabric-selective (they occupy a specific, identifiable fabric element). Diagenetic (secondary) porosity types – moldic, intercrystalline, vuggy, fracture, cavern – form later, via dissolution, dolomitization or fracturing, and are progressively less predictable and less fabric-selective; this is why carbonate reservoir porosity/permeability is notoriously harder to predict from depositional facies alone than clastic (sandstone) reservoir quality, which is dominated by depositional/compactional controls.
