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.
Marine evaporites precipitate in strict solubility order as seawater is concentrated by evaporation in a restricted basin where evaporation exceeds inflow – a barred lagoon, a sabkha, or a large silled basin cut off from the open ocean. As concentration rises, minerals precipitate in the order dictated by their solubility product, so a single evaporating basin builds an outward-younging, concentric mineral zonation.
| Mineral | Formula | Concentration factor to saturate |
|---|---|---|
| Calcite / dolomite (minor) | CaCO₃ / CaMg(CO₃)₂ | ≈2× seawater – precipitates first, forms the barrier facies |
| Gypsum (dehydrating to anhydrite with burial) | CaSO₄·2H₂O / CaSO₄ | ≈3.5× |
| Halite | NaCl | ≈10–12× |
| Sylvite, carnallite, polyhalite ("potash salts") | KCl; KMgCl₃·6H₂O; K₂Ca₂Mg(SO₄)₄·2H₂O | ≈60–100× – the last, most soluble salts, confined to the deepest basin centre |
Two basin geometries produce this zonation in practice: a deep-water, silled basin (a barred marine embayment periodically topped up over a sill, e.g. the classic Zechstein and Michigan/Elk Point basin models), and a shallow-water sabkha, a supratidal flat where evaporation draws seawater and Mg-rich brine up through the sediment by capillary action, precipitating gypsum/anhydrite nodules displacively within an otherwise normal tidal-flat carbonate sequence.
Economic importance. Potash salts (sylvite, carnallite) are the world's dominant potassium-fertilizer feedstock; the Middle Devonian Prairie Evaporite of the Elk Point Basin beneath Saskatchewan hosts the largest known potash reserves on Earth and supplies roughly a third of global production. Halite is mined directly for de-icing salt and the chlor-alkali industry, and bedded/domal salt is excavated for underground gas-storage caverns. Gypsum is the feedstock for wallboard and cement retarder. In petroleum systems specifically, evaporites are outstanding top seals (very low permeability, ductile, self-healing under stress) and, where thick enough to flow, drive salt tectonics – diapirism and associated rollover/turtle-structure trapping geometries that create some of the world's largest hydrocarbon accumulations (Gulf of Mexico, North Sea Zechstein, offshore West Africa).
