18-Geol-A3 Sedimentation and Stratigraphy · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A3, Sedimentation & Stratigraphy, 2018-Dec. Closed book, no calculator, 3 hours. Part 1: Sedimentology (Questions 1–12, 5 marks each) instructs "Answer questions 1 and 2, and any other 6 questions from the remaining 10 questions (i.e., questions 3–12)" for 40 marks total. Part 2: Stratigraphy and Sedimentary Basin Analysis (Questions 13–19, 5 marks each) instructs "Answer five of the following seven questions" for 25 marks total (65/65 maximum).
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, carbonate platforms, sequence stratigraphy, biostratigraphy and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, weathering, diagenesis, grain-size analysis); Bjorlykke, Petroleum Geoscience: From Sedimentary Environments to Rock Physics, 2nd ed. (basin classification, reservoir facies).
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 is the sum of all physical, chemical and biological processes that alter a sediment after deposition and before the onset of metamorphism — spanning eodiagenesis (shallow burial, near-surface pore fluids), mesodiagenesis (deep burial) and telodiagenesis (uplift/meteoric overprint).
Diagenetic processes affect reservoir quality in both directions. Porosity/permeability-reducing processes: mechanical compaction (overburden load reduces pore volume and reorients/breaks grains), cementation (precipitation of quartz overgrowths, calcite, or clay cement occludes pore space and throats), and chemical (pressure) compaction (stylolitisation, pressure dissolution at grain contacts). Porosity/permeability-enhancing processes: dissolution (meteoric or organic-acid-driven leaching of unstable framework grains such as feldspar, or of carbonate cement/allochems, creates secondary porosity), dolomitization (the mole-volume reduction converting calcite to dolomite can generate intercrystalline porosity, though it can also occlude porosity if pervasive), and fracturing (though strictly a diagenetic/tectonic overprint rather than a chemical process) which adds permeability without adding matrix porosity. The net reservoir quality of a rock is therefore not fixed at deposition but is the cumulative outcome of which of these competing diagenetic processes dominated its specific burial history.
