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.
Carbonate δ13C and δ18O (both reported in permil, ‰, relative to the VPDB standard) are routinely measured together on carbonate cements, fossils, and whole-rock samples, because each isotope system is sensitive to a different variable and their JOINT behaviour distinguishes diagenetic pathways that neither could resolve alone.
Oxygen isotope fractionation during carbonate precipitation is strongly temperature-dependent (heavier δ18O at lower precipitation temperature) and also tracks the isotopic composition of the precipitating fluid itself. Primary marine carbonate precipitated from normal seawater at surface temperature has a characteristic δ18O signature; any later alteration by a DIFFERENT fluid (isotopically light meteoric water, or normal seawater but at elevated BURIAL temperature) measurably shifts δ18O more negative, making it a sensitive detector of diagenetic overprinting and a tool for paleotemperature and paleoclimate reconstruction (e.g. foraminiferal δ18O as an ice-volume proxy).
Marine dissolved inorganic carbon, and the carbonate precipitated from it, sits close to 0‰. Soil-zone and organic-derived CO₂ (from oxidized organic matter in a meteoric vadose environment) is strongly depleted, typically −8 to −12‰ or lighter. Because fractional recrystallization at depth does not meaningfully change the bulk carbon source (the carbon is still recycled from the same marine carbonate), δ13C stays close to its original value through burial diagenesis, while it swings sharply negative wherever meteoric, organically-sourced carbon has been incorporated.
Plotting δ13C against δ18O separates diagenetic pathways that a single isotope cannot: meteoric diagenesis produces a strongly COVARIANT trend, both isotopes driven negative together (light soil-carbon and light meteoric-oxygen act simultaneously); burial recrystallization at elevated temperature by normal-marine-derived pore fluid depletes δ18O substantially while leaving δ13C comparatively unchanged (near-vertical trend on the cross-plot). The same joint approach identifies the fluid source of a dolomitizing brine (Question 11): seepage-reflux (evaporated seawater) dolomite tends to retain near-marine to slightly heavy isotope values, while meteoric-mixing dolomite trends toward lighter, more negative values on both axes.
