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.
Freshly deposited sand has loosely packed grains touching only at scattered point contacts, with initial porosity typically 40–50%. As burial depth (and overburden/effective stress) increases, compaction proceeds in two stages. First, mechanical compaction dominates in the shallow subsurface: grains rotate and slide into a tighter packing under the weight of overlying sediment, point contacts progress toward long (planar) contacts, and pore fluid is expelled as pore volume shrinks – porosity falls rapidly over the first one to two kilometres of burial. Second, at greater depth and higher effective stress, chemical compaction (pressure solution) takes over: calcite or quartz dissolves preferentially at the most highly stressed grain contacts, producing concavo-convex and eventually sutured (stylolitic) contacts, which reduces porosity further even where mechanical rearrangement alone has been exhausted. The overall porosity-depth trend is a curve of diminishing returns (commonly modeled by Athy's exponential law, φ = φ₀·e−cz) – steep porosity loss at shallow depth, flattening at greater depth – though cementation can independently reduce porosity at any stage, superimposed on the compaction trend.
