24-Pet-A1 Principles of Stratigraphy and Sedimentation · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2016-May. 3 hours duration; closed book, no calculator permitted. The paper has two parts: Part 1 (Questions 1–12, Sedimentology and Sedimentary Processes – 10 marks each; answer any eight of the twelve, 80 marks total) and Part 2 (Questions 13–20, Stratigraphy and Sedimentary Basin Analysis – 10 marks each; answer any five of the eight, 50 marks total) – a 130-mark maximum (80 for Part 1 + 50 for Part 2).
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 (carbonate classification, diagenesis, dolomitization); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, trace fossils, coastal processes); 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.
(a) Distinguishing criterion. The primary criterion is detrital (clay/silt) matrix content: sandstones with less than 15% matrix are classified as arenites (clean, texturally mature sandstones, typically better sorted with grain-to-grain or cement-bound framework), while sandstones with more than 15% matrix are classified as wackes (colloquially "greywackes" – poorly sorted, texturally immature sandstones in which detrital mud fills much of the intergranular space).
(b) Types of arenites and wackes. Crossing the matrix criterion with the QFL (quartz-feldspar-lithic) compositional triangle produces parallel families of names:
| Composition | Arenite (<15% matrix) | Wacke (>15% matrix) |
|---|---|---|
| Quartz-rich (Q > 90%) | Quartz arenite – texturally and mineralogically mature, well-sorted, rounded quartz grains, minimal matrix | Quartz wacke – quartz-dominated but poorly sorted with abundant detrital mud matrix |
| Feldspar-rich (F > 25%, F > L) | Arkose – feldspar-rich, near a granitic/gneissic source, clean framework | Feldspathic wacke – feldspar-rich but matrix-rich, rapidly deposited near source |
| Lithic-rich (L > 25%, L > F) | Litharenite – rich in unstable rock fragments, clean framework | Lithic wacke (greywacke) – rock-fragment-rich, matrix-rich, classic turbidite/deep-marine sandstone |
