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.
Bedded chert forms primarily (syngenetically) as a direct biogenic sediment: it accumulates where surface-water productivity of silica-secreting organisms (diatoms, radiolarians, siliceous sponges) is high enough – typically in deep-marine, often upwelling-related settings – that opaline (biogenic silica) tests reach the sea floor faster than they can dissolve, building a laterally continuous, well-bedded siliceous ooze that lithifies (opal-A → opal-CT → quartz) into continuous, parallel-bedded chert layers mirroring the primary depositional bedding.
Nodular chert forms secondarily (diagenetically, epigenetically) by silica replacement of a pre-existing carbonate (typically limestone) host rock: silica-rich pore fluids, often derived from dissolution of biogenic opal or volcanic ash elsewhere in the section, migrate through the limestone and preferentially replace it along permeable pathways (bedding planes, burrows, fossil molds), producing discrete, irregular, discontinuous nodules and lenses arranged along bedding rather than forming a continuous primary layer; the host limestone's original texture is often still faintly visible ("relict") within the chert.
