24-Pet-A1 Principles of Stratigraphy and Sedimentation · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2016-May. 3 hours duration; closed book, no calculator permitted. The paper has two parts: Part 1 (Questions 1–12, Sedimentology and Sedimentary Processes – 10 marks each; answer any eight of the twelve, 80 marks total) and Part 2 (Questions 13–20, Stratigraphy and Sedimentary Basin Analysis – 10 marks each; answer any five of the eight, 50 marks total) – a 130-mark maximum (80 for Part 1 + 50 for Part 2).
Reference texts: Boggs, S. Jr., Principles of Sedimentology and Stratigraphy, 5th ed., Pearson (grain texture, sediment transport, bedforms, carbonate/evaporite systems, sequence stratigraphy, unconformities, stratigraphic principles); Tucker, M.E., Sedimentary Petrology, 3rd ed., Blackwell (carbonate classification, diagenesis, dolomitization); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, trace fossils, coastal processes); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, basin classification, reservoir quality); 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.
(a) Requirements for marine evaporite precipitation. Four conditions must coincide: (1) an arid to semi-arid climate in which evaporation greatly exceeds the combined freshwater and marine inflow, driving the brine toward and past saturation; (2) a restricted basin geometry – a barrier such as a sill, reef trend or fault-bounded high – that limits free exchange with the open ocean so the brine is not continually diluted; (3) periodic or continuous seawater replenishment across the barrier to resupply dissolved salts as evaporation concentrates the brine (without any inflow the basin simply dries up rather than precipitating a thick evaporite pile); and (4) sufficient subsidence to create accommodation space for the evaporite thickness that accumulates. As seawater evaporates, minerals precipitate in a fixed order of increasing solubility (Usiglio's sequence): carbonates (calcite, dolomite) first at roughly 50% volume reduction, then gypsum/anhydrite (CaSO⋅4) at about 80–90% reduction, then halite (NaCl) beyond about 90%, and finally the most soluble potassium–magnesium salts (sylvite, carnallite) only at extreme (>95%) concentration.
(b) Structural groups of anhydrite/gypsum rocks. Nodular (chicken-wire) anhydrite – irregular, rounded to polygonal nodules of anhydrite separated by thin stringers of host carbonate or dolomite mud, resembling chicken-wire mesh in cross-section; forms by displacive and replacive growth of anhydrite crystals within a supratidal sabkha mud, pushing the host sediment into thin partitions. Laminated (bedded) anhydrite/gypsum – millimetre- to centimetre-scale alternating laminae of anhydrite/gypsum and carbonate or organic-rich mud, precipitated subaqueously from a density-stratified brine pool in a barred, deep evaporitic basin (varve-like couplets record seasonal or event-scale brine concentration cycles). Massive/bedded anhydrite – thick, structureless to crudely bedded anhydrite formed by sustained subaqueous precipitation once the basin brine is persistently supersaturated. Enterolithic (contorted) anhydrite – tightly folded, convoluted laminae produced by the volume increase that accompanies gypsum-to-anhydrite (or anhydrite-to-gypsum) conversion, which displaces and crumples the enclosing sediment.
