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.
Procedure. Sandstones are classified from their detrital framework-grain composition and their textural maturity. In thin section, at least 300 framework (sand-sized) grains are point-counted under a petrographic microscope and sorted into three end-member categories: quartz (Q) – monocrystalline and polycrystalline quartz and chert; feldspar (F) – K-feldspar and plagioclase; and lithic/rock fragments (L) – fine-grained volcanic, sedimentary or metamorphic clasts. The three counts are recalculated to 100% and plotted on a QFL ternary diagram, whose position fixes the compositional class. Independently, the volume percentage of interstitial detrital clay/silt matrix is measured: less than 15% matrix gives a well-washed arenite, 15% or more gives a muddy wacke (greywacke) – the textural suffix that pairs with the QFL-derived name.
| Class | Composition | Description |
|---|---|---|
| Quartz arenite | >90% Q | Almost pure quartz sand; reflects prolonged transport/recycling and intense chemical weathering that destroys the less-stable feldspar and lithic grains; typically well sorted and well rounded – a mature, texturally and compositionally "clean" sandstone (e.g., many shallow-marine/aeolian blanket sands). |
| Subarkose | 75–90% Q, F>L | Quartz-dominated with a modest, but recognizable, feldspar content; transitional between quartz arenite and arkose, indicating a granitic/gneissic source with moderate weathering or transport. |
| Sublitharenite | 75–90% Q, L>F | Quartz-dominated with a modest lithic-fragment content; transitional between quartz arenite and litharenite, indicating a mixed sedimentary/volcanic source terrane. |
| Arkose | <75% Q, F>L, F>25% | Feldspar-rich (typically >25% feldspar); derived from rapid erosion of granitic or gneissic basement under arid or cool climate (little chemical weathering) with short transport – feldspar is mechanically durable but chemically unstable, so it survives only where weathering time is short. |
| Litharenite | <75% Q, L>F, L>25% | Rock-fragment-rich; derived from erosion of fine-grained sedimentary, volcanic or low-grade metamorphic source rocks, again implying short transport/rapid burial, since lithic fragments are mechanically the least durable framework grains and disintegrate quickly with abrasion. |
| Quartz wacke / feldspathic & lithic greywacke | Same QFL fields, ≥15% matrix | The muddy (immature) textural equivalents of the arenites above; the abundant detrital clay/silt matrix indicates rapid deposition (commonly by turbidity currents) that did not allow winnowing of fines – poor sorting and angular grains are typical. |
