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 depositional sequence is a relatively conformable succession of genetically related strata bounded, at top and base, by unconformities or their correlative conformities (the surfaces those unconformities pass into, basinward, where the equivalent time interval is represented by continuous deposition rather than erosion) – the fundamental building block of sequence stratigraphy, deposited during one full cycle of relative sea-level change.
System tracts. A depositional sequence is subdivided by stacking pattern into three main system tracts. The lowstand systems tract (LST), deposited during and just after a relative sea-level fall/lowstand when accommodation is minimal, forms basin-floor fans and incised-valley fill, showing initially aggradational-to-weakly-progradational stacking; it is bounded below by the sequence boundary. The transgressive systems tract (TST), deposited during accelerating relative sea-level rise when new accommodation outpaces sediment supply, shows retrogradational stacking (each facies belt shifts landward relative to the one below), producing an overall deepening-upward package, and is bounded above by the maximum flooding surface (MFS). The highstand systems tract (HST), deposited as sea-level rise decelerates and stalls near its highstand, when sediment supply again outpaces the shrinking rate of accommodation creation, shows progradational stacking, producing a shallowing-upward package capped by the next sequence boundary.
Relation to sea level. The whole framework is organized around the relative sea-level (accommodation) curve: the LST corresponds to the rising limb just after the lowstand inflection, the TST to the steeply rising limb where the rate of accommodation creation exceeds sediment supply, and the HST to the flattening, highstand portion where sediment supply again exceeds accommodation creation; the sequence boundary forms near the point of maximum sea-level fall (subaerial exposure of the shelf). Each system tract's stacking pattern is therefore a direct proxy for whether accommodation was being created faster or slower than sediment was arriving to fill it.
