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.
Dunham (1962) classified carbonate rocks not by grain type (as Folk's earlier scheme did) but by the depositional texture preserved at deposition – specifically, whether the rock's original components were mud-supported or grain-supported, and whether the grains were bound together organically at the time of deposition. Because texture records depositional energy directly, the classification doubles as an environmental indicator.
| Class | Fabric | Typical setting |
|---|---|---|
| Mudstone | Mud-supported, <10% grains >20 µm | Quiet, low-energy lagoon or restricted subtidal – below effective wave base, little winnowing |
| Wackestone | Mud-supported, ≥10% grains | Slightly higher-energy restricted platform interior; enough current to deliver skeletal debris but not to winnow the mud away |
| Packstone | Grain-supported, with mud filling the pores | Moderate-energy shoal margin; grains touch and support the framework but intermittent quiet periods still allow mud to settle between them |
| Grainstone | Grain-supported, no mud (open, cement-filled pores) | High-energy washed shoal, beach, or tidal-bar crest – persistent agitation winnows every trace of mud away |
| Boundstone | Original components organically bound during deposition (by corals, algae, or other frame-builders) | Reef core / build-up – the highest-energy setting, where the organisms themselves resist wave attack rather than merely surviving it |
The five classes therefore form an ordered energy series that a field geologist reads directly off a hand sample or core: mudstone and wackestone record quiet water where mud can settle and stay; packstone marks the transition where currents are strong enough to pack grains together but not to remove all mud; grainstone marks water energetic enough to winnow every particle of mud; and boundstone records organisms rigid enough to build a wave-resistant framework in place. (A later refinement by Embry & Klovan (1971) added floatstone and rudstone for matrix- and clast-supported deposits with >10% grains >2 mm – e.g. reef-talus breccia – and split boundstone into bafflestone, bindstone and framestone by binding mechanism, which is the version most modern carbonate logs use.)
Petroleum relevance. Because texture tracks energy and, in turn, original porosity, Dunham class is a first-order reservoir-quality predictor in carbonate exploration – grainstone shoal trends (e.g. the Devonian Rainbow and Golden Spike reef-margin grainstones of the Western Canada Sedimentary Basin) are classic high-porosity reservoir facies, while mudstone/wackestone lagoonal facies more often form the low-permeability seal or non-reservoir background rock in the same system.
