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.
All three commodities concentrate in terrestrial (non-marine) systems, but each is trapped by a different physical or chemical mechanism, so each targets a different part of the same broad fluvial landscape.
Target fluvial (braided or meandering) and aeolian sandstone sitting unconformably on crystalline or reduced basement, specifically at the redox interface where oxidized, uranium-bearing groundwater moving through permeable sandstone meets a reducing agent (organic matter, sulphides, or reduced basement rock). Uranium is soluble as the oxidized uranyl ion (UO₂²⁺) but precipitates as insoluble uraninite (UO₂) when reduced, so ore forms exactly at that redox front – either as a tabular roll-front deposit that migrates down-dip through the sandstone over geological time, or as a high-grade unconformity-related deposit where basinal brines pooled directly against reduced basement, the style that makes the Athabasca Basin of northern Saskatchewan the world's richest uranium province.
Target the backswamp / floodplain mire of a fluvial or deltaic system – a poorly-drained, vegetated low subject to slow, continuous subsidence that keeps pace with peat accumulation (autochthonous, in-place organic build-up) while staying protected from clastic (sand/silt) influx that would dilute or bury the peat prematurely. A humid climate sustaining continuous plant growth is essential; the resulting peat, buried and compacted under later sediment, becomes coal.
Target high-energy channel-lag deposits and point bars of a braided or meandering gravel-bed river, particularly bedrock riffles, bar heads, and the inside of channel bends, where a sudden drop in flow-velocity/turbulence at a hydraulic discontinuity lets the dense (specific gravity ≈19) gold grains settle out while lighter quartz and lithic sand is carried further downstream – the same hydraulic-sorting principle that concentrated the placer gravels of BC's Cariboo and Yukon's Klondike fields.
