18-Geol-A3 Sedimentation and Stratigraphy · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A3, Sedimentation & Stratigraphy, 2018-Dec. Closed book, no calculator, 3 hours. Part 1: Sedimentology (Questions 1–12, 5 marks each) instructs "Answer questions 1 and 2, and any other 6 questions from the remaining 10 questions (i.e., questions 3–12)" for 40 marks total. Part 2: Stratigraphy and Sedimentary Basin Analysis (Questions 13–19, 5 marks each) instructs "Answer five of the following seven questions" for 25 marks total (65/65 maximum).
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, carbonate platforms, sequence stratigraphy, biostratigraphy and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, weathering, diagenesis, grain-size analysis); Bjorlykke, Petroleum Geoscience: From Sedimentary Environments to Rock Physics, 2nd ed. (basin classification, reservoir facies).
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 deltaic system has three principal depositional components, arranged seaward-to-landward-independent zones defined by process and grain size: the delta plain (the subaerial, landward portion — distributary channels, interdistributary bays, marshes/swamps — dominated by fluvial processes, with fining-upward channel-fill sands encased in fine-grained overbank/bay muds); the delta front (the shallow subaqueous zone at and just beyond the river mouth, where fluvial discharge meets and is reworked by basinal wave/tide/current energy — dominated by distributary-mouth-bar sands that coarsen upward as the bar progrades); and the prodelta (the distal, deepest, most basinward zone, receiving only the finest suspended mud and silt settling out beyond the reach of significant current/wave reworking).
The delta front — specifically its distributary-mouth-bar sands — contains the best conventional reservoir potential: it is the coarsest, best-sorted, most current-winnowed sand body in the system (fluvial sediment is actively reworked by basinal wave/tidal energy at the river mouth), it forms a laterally and vertically stacked, well-connected sand geometry as bars migrate and switch through avulsion, and it is typically encased updip by delta-plain muds and downdip/below by prodelta muds that can act as top and lateral seals.
