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.
The answer is (c) Dolomite. Carbonaceous sedimentary rocks are, by definition, rocks whose bulk composition and origin are dominated by organic carbon (plant/algal debris or its thermally altered products): coal and anthracite are progressively coalified accumulations of terrestrial plant material (anthracite being the highest-rank, most carbon-enriched end of the coalification series); oil shale is a fine-grained mudrock whose matrix is enriched in kerogen (immature organic matter that yields oil on retorting); bitumen is a solid to semi-solid hydrocarbon residue left after the lighter fractions of petroleum have biodegraded or evaporated. Dolomite, by contrast, is CaMg(CO3)2 — a carbonate mineral/rock formed chiefly by diagenetic replacement of calcite (Question 9) or, less commonly, direct precipitation; it is a carbonate rock, not an organic-carbon-rich (carbonaceous) one, and its formation has no necessary relationship to organic matter accumulation at all. Even where a dolomite bed IS associated with organic-rich source rock in a sedimentary sequence (as commonly happens in restricted, evaporitic basin settings where both form together), the dolomite itself remains a carbonate-mineral replacement product, not the organic-carbon accumulation; the two rock types can sit side by side in the same succession while belonging to entirely separate compositional families.
