24-Pet-A1 Principles of Stratigraphy and Sedimentation · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 17-Pet-A1 Principles of Stratigraphy & Sedimentation, 2018-May. 3 hours duration; closed book, no calculator permitted. The paper has two parts: Part A (Questions 1–10, Sedimentology and Sedimentary Processes) – Questions 1 and 2 are mandatory (10 marks each, 20 marks), plus any five of the remaining eight (3–10) at 6 marks each (30 marks), for a Part A total of 50 marks; and Part B (Questions 11–19, Stratigraphy and Sedimentary Basin Analysis) – answer any six of the nine at 5 marks each, for a Part B total of 30 marks – an 80-mark maximum (50 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 (sandstone/carbonate classification, diagenesis, dolomitization); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial/deltaic systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, deep-marine and shelf systems); 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) Excess in coarse-grained particles. In the Folk & Ward (1957) graphic-statistics scheme, grain-size skewness (SK) is computed from the phi (φ) percentiles of the cumulative grain-size distribution, and the sign convention is such that a negatively-skewed curve has its "tail" extended toward the coarse end of the distribution – i.e., an excess (or a relative surplus) of coarse material beyond what a symmetric (normal) distribution would show. Conversely, a positively-skewed curve has a tail toward the fine end (an excess of fines). Physically, negative (coarse) skew is typical of high-energy, winnowed environments – a beach or wave-agitated shoreface where fine material is preferentially removed by wave/current action and carried offshore or suspended away, leaving the coarse fraction enriched relative to a normal distribution – while positive (fine) skew is typical of lower-energy environments (e.g., some fluvial or tidal-flat settings) that retain a fine "tail" the current could not fully remove. Options (b), (c) and (d) describe positive skew, sorting (a separate statistical moment), and bimodality, respectively, none of which is what negative skewness measures.
