18-Geol-A3 Sedimentation and Stratigraphy · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2015-May. Closed book, 3 hours.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate classification, stratigraphic principles and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, diagenesis, evaporites, phosphorites); Selley & Sonnenberg, Elements of Petroleum Geology (reservoir quality, subsurface wireline-log interpretation).
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 is the sum of all physical, chemical and biological changes that affect a sediment or sedimentary rock after deposition and during burial, short of the temperature/pressure regime of metamorphism — it spans everything from shallow, near-surface processes (bioturbation, early cementation) through progressive burial (compaction, pressure solution, mineral replacement) and includes uplift-related processes such as meteoric-water dissolution.
| Effect | Process | Mechanism |
|---|---|---|
| Enhances porosity/permeability | Dissolution (leaching) | Meteoric or organic-acid-charged pore water dissolves unstable grains or early cement, creating secondary (moldic/vuggy) porosity |
| Dolomitization | Calcite → dolomite is a mole-for-mole volume-reducing reaction (~13% solid-volume loss), commonly generating new intercrystalline porosity | |
| Reduces porosity/permeability | Mechanical compaction | Overburden load reorganizes and deforms grains (ductile grain flattening, ductile lithic collapse), reducing pore volume with burial depth |
| Cementation | Precipitation of calcite, quartz overgrowths or authigenic clay directly occludes remaining pore space and pore throats |
These four processes commonly compete through a rock's burial history rather than acting in isolation — a sandstone can be compacted and partly cemented during early burial, then have a later meteoric or organic-acid flushing event dissolve some of that cement or unstable grains to restore secondary porosity, so the reservoir quality actually delivered to a driller is the NET result of this competing sequence, not any single process read in isolation. A petrophysicist reading the order of these events from thin-section relationships (which cement pre-dates or post-dates compaction, which grains show dissolution embayments cutting an earlier cement rim) is effectively reconstructing the rock's burial and fluid-flow history in the sequence it actually happened, which predicts reservoir quality at depth far better than the original depositional facies alone.
