24-Pet-A1 Principles of Stratigraphy and Sedimentation · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2015-Dec. 3 hours duration; closed book, no calculator permitted. The paper has two parts: Part A (Questions 1–11, sediments/sedimentary rocks/sedimentary processes – Questions 1–4 at 10 marks each and Questions 5–11 at 5 marks each; answer any combination of questions totaling 45 marks) and Part B (Questions 12–18, stratigraphy – Questions 12–14 at 10 marks each and Questions 15–18 at 5 marks each; answer any combination totaling 30 marks) – a 75-mark maximum (45 for Part A + 30 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 (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, 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.
Facies. A body of rock possessing a distinctive set of characteristics – lithology, sedimentary structures, texture, bedding geometry, fossil content – that together distinguish it from adjacent rock bodies and allow a specific depositional process or environment to be inferred. A facies is a descriptive, mappable unit defined by what is observed in the rock (e.g. "cross-bedded sandstone facies," "bioturbated mudstone facies").
Walther's Law (Law of Succession/Correlation of Facies) states that, in a conformable (unbroken, no significant hiatus or erosion surface) vertical succession of strata, the sequence of facies observed vertically at one location must correspond to the sequence of facies that existed laterally, side by side, across the depositional surface at any one time – only facies that were once depositionally adjacent to one another laterally can be superimposed on one another vertically without a break.
Applied practically, Walther's Law lets a geologist reconstruct the lateral (paleogeographic) arrangement of environments from a single vertical section: a coarsening-upward, conformable succession from offshore mud to shoreface sand to beach sand records a shoreline that was progressively prograding (moving seaward) past that location, because the vertical order of facies must mirror the lateral order of environments that existed side by side at the time of deposition. The law applies only where the succession is truly conformable; any unconformity or abrupt facies break severs the vertical-lateral correspondence, because non-adjacent environments can then be juxtaposed by erosion or non-deposition rather than by continuous lateral migration.
