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24-Pet-A1 Principles of Stratigraphy and Sedimentation · May 2016

Question 6 of 20: The Deltaic Depositional System and Its Prograding Sequence

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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 6: The Deltaic Depositional System and Its Prograding Sequence (10 marks)

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) Delta. A delta is a discrete shoreline protuberance formed where a river enters a standing body of water (a lake or sea) and deposits its sediment load faster than waves, tides and currents can redistribute it, building a wedge of sediment that progrades basinward through time.

(b) Deltaic subenvironments. Three zones prograde basinward, in order: the delta plain (subaerial, landward) – distributary channels (fining-upward sand/gravel), natural levees (thin-bedded, ripple-laminated fine sand/silt), crevasse splays (thin sandy lobes from a levee breach), and interdistributary bays/marshes (organic-rich mud and peat); the delta front (subaqueous, high-energy) – distributary-mouth bars, a seaward-thinning, coarsening-upward wedge of well-sorted sand deposited where channel flow expands and decelerates on entering open water; and the prodelta (subaqueous, most distal) – a seaward-thinning apron of suspended clay and silt settling beyond the reach of active channel and wave processes.

Deltaic depositional system (cross-section) sea level Delta plain channel, levee, crevasse splay, bay mud Delta front mouth-bar sand, coarsening upward Prodelta suspension mud, fining basinward
Idealized deltaic cross-section: delta plain (channel/levee/bay deposits) progrades over coarsening-upward delta-front mouth-bar sand, which progrades over prodelta mud.

(c) Prograding (regressive) vertical sequence. As the delta advances, each zone deposits its facies directly on top of the facies that previously lay basinward of it, producing a single, predictable coarsening-upward vertical log: prodelta mud (base) grading up into delta-front sand (coarsening upward) capped by delta-plain channel/levee/bay deposits (top) – Walther's Law recorded in one drill hole.

Prograding deltaic log Prodelta mud (base) Delta-front sand (coarsening up) Delta-plain channel/bay (top) grain size increases
Prograding (regressive) deltaic sequence: coarsening-upward from prodelta mud through delta-front sand to delta-plain deposits, recording seaward advance of the delta through time.