24-Pet-A1 Principles of Stratigraphy and Sedimentation · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2013-May. 3 hours duration; closed book, no calculator permitted. Candidates answer any 10 of the 15 questions (10 marks each, 100 marks total) and are asked to illustrate answers with drawings wherever possible.
Reference texts: Boggs, S. Jr., Principles of Sedimentology and Stratigraphy, 5th ed., Pearson (texture classification, evaporites, clay minerals, sediment gravity flows, storm/shelf processes, bedforms, stable isotopes, geological time scale); Tucker, M.E., Sedimentary Petrology, 3rd ed., Blackwell (carbonate fabric, dolomitization, reef facies); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (depositional systems, transgression/regression, sequence stratigraphy); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rock maturation, petroleum systems); 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 reef is a wave-resistant carbonate build-up constructed by frame-building organisms in place (Dunham's boundstone, Question 1). The standard facies model organizes the build-up into a belt of zones arranged symmetrically about the reef crest, each with a distinct energy level, biota, and Dunham texture – producing one of the most reliable, laterally predictable facies successions in sedimentary geology.
| Zone | Energy / character | Typical texture |
|---|---|---|
| Fore-reef | Steep, seaward-dipping slope; accumulates reef-derived talus shed from the crest, deepening and fining basinward into inter-tonguing basinal shale/lime mudstone | Breccia, floatstone/rudstone (Question 1) |
| Reef core / crest | Highest wave energy; the framework itself dissipates wave energy, hosting the highest biotic diversity (corals, coralline algae, rudists, or other frame-builders depending on geological age) | Boundstone (framestone/bindstone) |
| Back-reef / reef flat | Winnowed, agitated shallow platform immediately behind the crest; high-energy but not framework-building | Grainstone (skeletal/oolitic sand shoals) |
| Lagoon | Low-energy, restricted circulation behind the reef flat; may host scattered patch reefs, and in an arid climate can itself evaporate (Question 2) if circulation is restricted enough | Wackestone/mudstone |
Petroleum relevance. The lateral facies change from porous reef-core/reef-flat carbonate into basinal shale (a potential source rock) sealed updip by lagoonal mudstone or evaporite makes reef trends outstanding stratigraphic traps – the Devonian Leduc reef trend of the Western Canada Sedimentary Basin (which made Leduc No.1 Alberta's discovery well in 1947) is the classic Canadian example of exactly this facies architecture as a petroleum play.
