24-Pet-A1 Principles of Stratigraphy and Sedimentation · December 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2014-Dec. 3 hours duration; closed book, no calculator permitted. The paper has three parts: Part A1 (Questions 1–7, sedimentary processes, 5 marks each, answer any 4 of 7 for 20 marks), Part A2 (Questions 8–15, brief/multiple-choice, 3 marks each, answer any 5 of 8 for 15 marks), and Part B (Questions 16–24, stratigraphy, 5 marks each, answer any 6 of 9 for 30 marks) – a 65-mark maximum.
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); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, 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.
The Bouma Sequence is the idealized, five-part vertical succession of sedimentary structures deposited by a single, waning turbidity current – a dense, sediment-laden gravity flow that moves down a submarine slope and decelerates as it spreads across the basin floor, depositing its coarsest and most turbulent load first and its finest, quietest load last.
Each division corresponds to a stage in the flow's deceleration: Ta is deposited by rapid, en-masse suspension fallout under high-energy turbulent flow (often with a sharp, sometimes erosive/sole-marked base); Tb by traction under upper-flow-regime plane-bed conditions; Tc by lower-flow-regime current ripples (locally disturbed into convolute lamination by rapid dewatering); Td by very-low-energy upper plane-bed/parallel lamination as the flow nearly stops; and Te by slow pelagic/hemipelagic settling of mud between turbidity-current events, unrelated to the flow itself. A complete Ta–Te bed is uncommon – most turbidites are "incomplete," missing their lower divisions with increasing distance from source (a distal bed may begin at Tc or Td) or their upper divisions where amalgamated by a later flow.
Sedimentological significance. The Bouma sequence is the diagnostic signature of a deep-marine, gravity-flow (turbidite) depositional system – submarine fans, channel-levee complexes and basin-floor lobes – and, because these often stack into thick, laterally extensive sand bodies encased in impermeable basin mud, they form major deep-water petroleum reservoirs worldwide (e.g. Gulf of Mexico, offshore West Africa and Brazil turbidite plays).
