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.
Answer: (a) Rift basin, (b) Passive margin basin, (d) Aulacogen. These three all form directly by lithospheric stretching (extension). A rift basin forms by active crustal extension and thinning along normal faults, typically the first stage of continental breakup. A passive margin basin forms after rifting proceeds to full continental separation and seafloor spreading begins; the once-active rift margin subsides thermally and becomes a passive, extension-derived continental-margin basin. An aulacogen is a failed arm of a three-armed (triple-junction) rift system that stops extending once seafloor spreading is established on the other two arms, leaving a fault-bounded trough that continues to subside and fill with sediment – it is a direct, if failed, product of the same extensional rifting event.
By contrast, foreland basin (c), retro-arc basin (e) and fore-arc basin (g) are all products of a convergent (compressional) tectonic regime – foreland basins form by lithospheric flexure under the load of an adjacent thrust belt, while fore-arc and retro-arc (back-arc) basins form on either side of a magmatic arc above a subduction zone. Intracratonic basin (f) is generally classified as a separate, sag-type basin – a broad, slowly subsiding interior-craton depression driven mainly by long-term thermal or lithospheric-loading subsidence rather than by active extensional faulting at the time of its main subsidence, and so is not grouped with the actively extensional basin types even though some intracratonic sags initiate above an earlier, now-inactive rift.
