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.
Primary (direct) precipitation of dolomite, CaMg(CO₃)₂, from normal seawater is kinetically inhibited – the tightly bound water molecules surrounding the Mg²⁺ ion in solution and the strict cation ordering dolomite's crystal structure requires make direct nucleation extremely slow at low temperature – so almost all dolostone in the rock record instead forms by replacement (secondary) dolomitization of a precursor limestone, driven by a Mg-rich fluid passing through the rock in large volume. Because dolomitization is essentially mole-for-mole (roughly one Mg²⁺ must be delivered, and one Ca²⁺ removed, for every mole converted) and seawater's Mg is dilute, an enormous water/rock ratio and a sustained fluid-flow "pump" are required – which is why several distinct hydrogeological models are proposed, each supplying that flow differently.
| Model | Mg²⁺ source and fluid driver |
|---|---|
| Evaporative / seepage-reflux | Dense, Mg-enriched brine forms by evaporation in a restricted lagoon/sabkha; being denser than normal seawater, it sinks and refluxes downward and outward through the underlying platform limestone (Question 2's evaporite setting). |
| Mixing-zone ("Dorag") | Meteoric freshwater mixes with seawater in a coastal aquifer; the mixture is undersaturated with respect to calcite but supersaturated with respect to dolomite over a range of mixing proportions. Once the dominant explanation for ancient dolostones, it is now considered a less general/less proven mechanism, applicable to only some occurrences. |
| Burial / compactional | Mg-rich fluids expelled from compacting shales (and released by the smectite→illite transformation, Question 8) migrate through adjacent limestone during burial, driven by compaction-expulsion or basin-scale thermal convection. |
| Hydrothermal | Hot, Mg-rich fluids migrate along faults/fractures, driven by thermal convection; produces coarsely crystalline, often saddle-textured dolomite associated with base-metal (Mississippi-Valley-Type) mineralization. |
All four models share the same underlying requirement – sustained Mg²⁺ supply, a means of removing the displaced Ca²⁺, and enough time/temperature to overcome dolomite's kinetic ordering barrier – and geologists distinguish which one operated in a given case using petrographic texture (fine mimetic vs. coarse/saddle crystals), stratigraphic association (adjacent evaporites favour reflux; regional unconformities favour mixing-zone), and stable-isotope signatures (Question 14).
