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.
Hydrolysis is a chemical weathering reaction in which water (acting as a source of H+, its acidity commonly enhanced by dissolved CO2 forming carbonic acid) attacks a silicate mineral's crystal structure, exchanging cations for H+ and breaking Si–O–Al framework bonds; the mineral is converted into a new, more stable clay mineral, releasing the displaced cations and silica into solution.
Acting on potassium feldspar (orthoclase/microcline, KAlSi3O8), hydrolysis proceeds (in its typical, incongruent, kaolinite-forming form) as:
$$2\,\text{KAlSi}_3\text{O}_8 + 2\,\text{H}^+ + 9\,\text{H}_2\text{O} \;\longrightarrow\; \text{Al}_2\text{Si}_2\text{O}_5(\text{OH})_4 + 4\,\text{H}_4\text{SiO}_4 + 2\,\text{K}^+$$
i.e., K-feldspar plus hydrogen ions and water yields kaolinite (a 1:1 clay mineral) plus dissolved silicic acid plus potassium ions in solution.
Products. (1) Kaolinite – the residual clay mineral, which may accumulate as regolith/laterite in place or be eroded and transported as detrital clay. (2) Dissolved K+ – released to soil/groundwater and ultimately river/ocean water, where it may be taken up by clay minerals (cation exchange), by organisms, or concentrated into evaporite minerals. (3) Dissolved silica (H4SiO4) – carried in solution, later reprecipitated as chert/opal-CT or taken up biologically (e.g., by diatoms) to form biogenic silica. Under more prolonged, intense (typically tropical, humid) weathering, hydrolysis can proceed further still, stripping out the remaining silica from kaolinite to leave gibbsite (Al(OH)3), the aluminum hydroxide that is the principal ore mineral of bauxite – but kaolinite is the characteristic first-order product from feldspar hydrolysis under most temperate weathering regimes.
