24-Pet-A1 Principles of Stratigraphy and Sedimentation · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2014-Dec. 3 hours duration; closed book, no calculator permitted. The paper has three parts: Part A1 (Questions 1–7, sedimentary processes, 5 marks each, answer any 4 of 7 for 20 marks), Part A2 (Questions 8–15, brief/multiple-choice, 3 marks each, answer any 5 of 8 for 15 marks), and Part B (Questions 16–24, stratigraphy, 5 marks each, answer any 6 of 9 for 30 marks) – a 65-mark maximum.
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); 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.
Diagenesis encompasses all the physical, chemical and biological changes a sediment undergoes after deposition and before metamorphism – from shallow burial through deep burial and, for some rocks, into uplift/telogenetic exposure – short of the pressure-temperature regime that would define metamorphism.
The principal diagenetic processes and their effect on reservoir quality are: compaction (mechanical grain rearrangement and, at depth, pressure dissolution/stylolitization under overburden load), which reduces porosity and permeability by squeezing pore space out; cementation (precipitation of quartz, calcite, or clay cement in pore throats from pore-fluid-derived minerals), which likewise occludes porosity and permeability, often the single largest control on deep-reservoir quality loss; dissolution (undersaturated pore fluids – often organic-acid-charged during hydrocarbon maturation, or meteoric water during uplift – dissolving unstable grains or earlier cement), which can create valuable secondary porosity; and replacement/recrystallization (e.g. dolomitization of limestone), which can either enhance porosity (dolomitization's smaller unit-cell volume than calcite often creates new intercrystalline porosity) or destroy it, depending on timing and fabric.
Reservoir quality is therefore not fixed at deposition but is the net outcome of a competing sequence of porosity-destroying (compaction, cementation) and porosity-enhancing (dissolution, some dolomitization) diagenetic events through the rock's full burial history – which is why petroleum geologists construct a paragenetic sequence (the relative timing of diagenetic events from petrographic/isotopic evidence) to predict where the best reservoir quality survives at depth.
