24-Pet-A1 Principles of Stratigraphy and Sedimentation · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2013-May. 3 hours duration; closed book, no calculator permitted. Candidates answer any 10 of the 15 questions (10 marks each, 100 marks total) and are asked to illustrate answers with drawings wherever possible.
Reference texts: Boggs, S. Jr., Principles of Sedimentology and Stratigraphy, 5th ed., Pearson (texture classification, evaporites, clay minerals, sediment gravity flows, storm/shelf processes, bedforms, stable isotopes, geological time scale); Tucker, M.E., Sedimentary Petrology, 3rd ed., Blackwell (carbonate fabric, dolomitization, reef facies); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (depositional systems, transgression/regression, sequence stratigraphy); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rock maturation, petroleum systems); 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.
A transgression is a landward migration of the shoreline (relative sea-level rise at the shore); a regression is a seaward migration (relative sea-level fall or, more commonly, sediment supply simply outpacing accommodation). Because "relative" sea level is what matters, three independent factors control which one occurs at any point in time and space:
These three combine into a single governing variable, accommodation space (the room available for sediment to accumulate, set by relative sea level) versus sediment supply: transgression occurs when the rate of accommodation increase exceeds sediment supply; regression occurs when supply exceeds (or keeps pace with) accommodation increase.
Differentiating them in the stratigraphic record relies on Walther's Law (vertically stacked facies were once laterally adjacent environments): a transgressive succession is retrogradational – landward-stepping, fining- and deepening-upward, typically capped by an erosional ravinement surface cut by shoreface wave action as the shoreline sweeps landward over its own older deposits, then overlain sharply by deeper-water facies (onlap). A regressive succession is progradational – seaward-stepping, coarsening- and shoaling-upward, with shallower facies advancing basinward over deeper ones (offlap), and no ravinement surface. A special case, forced regression, occurs when relative sea level itself falls: the shoreline is driven rapidly seaward independent of sediment supply, often incising valleys into the previously exposed shelf – distinguishing it from a normal (supply-driven) regression is a key sequence-stratigraphic skill.
