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.
(a) Two directional (paleocurrent) sedimentary structures. Cross-bedding (trough or planar) – the foreset laminae of a migrating bedform (dune, ripple) dip in the direction of sediment transport, so measuring the dip azimuth of the foresets (or, more reliably, the axis of a trough cross-bed set) gives the paleoflow vector directly. Flute casts (a type of sole mark) – asymmetric scours cut into a muddy substrate by a turbulent current and then infilled and preserved as casts on the base of the overlying sandstone bed; the blunt, deeper end of the cast points up-current and the tapering end points down-current, so its long axis read from blunt to tapered end gives the paleoflow direction directly. (Imbricated clasts – long axes dipping up-current – and asymmetric ripple marks, with the steep lee face pointing down-current, are equally acceptable examples.)
(b) Rose-diagram directional patterns. Plotting the azimuth of many paleocurrent readings as a circular (rose) histogram produces one of several characteristic patterns: unimodal – one dominant, narrow direction; bimodal-bipolar – two modes 180° apart (flow reverses along the same axis); bimodal-oblique – two modes that are not opposite (roughly 90–150° apart); and polymodal (multimodal, or a broad, low, roughly uniform girdle) – three or more modes, or a diffuse spread around the full circle.
(c) Depositional system per pattern. Unimodal → fluvial channel or delta-front distributary-mouth bar (a single dominant, unidirectional current). Bimodal-bipolar → tidal channel or tidal inlet, where flow reverses along the channel axis on the flood and ebb tide (the classic bimodal-bipolar structure is herringbone cross-stratification). Bimodal-oblique → a longshore/wave-dominated beach or barrier-island setting, where swash-backwash and along-shore drift combine at an angle. Polymodal → a storm-dominated shelf or an aeolian dune field, where the current or wind direction varies episodically and the bed is reworked from many directions over time.
