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.
Skeletal (CaCO3) hard parts formed by organisms in the shallow, warm, carbonate-saturated photic zone are progressively altered as they settle or are exported to greater water depth, because seawater's capacity to dissolve calcium carbonate increases with depth (CO2 solubility rises and temperature falls with depth, both lowering the carbonate saturation state). Near the surface, aragonite and high-magnesium calcite – the most soluble carbonate polymorphs, common in corals, molluscs and some algae – are the first to show dissolution effects, while low-magnesium calcite (foraminifera, most bivalves) is more resistant and persists deeper. At the lysocline, the rate of calcite dissolution increases sharply, and carbonate-ooze sediments below it become progressively less pure with depth. Below the carbonate compensation depth (CCD), the rate of calcite supply from the surface is exceeded by the rate of dissolution, so essentially no calcite of any polymorph survives to accumulate – abyssal-plain sediments below the CCD are carbonate-free, dominated instead by insoluble residue (red clay) or, where productivity is high, biogenic silica ooze. The net depth-ordered sequence: unaltered aragonitic/high-Mg-calcite hard parts (shallow) → selective dissolution of the more soluble polymorphs and increasingly impure calcite ooze (near/below the lysocline) → complete carbonate dissolution, non-carbonate sediment only (below the CCD).
