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24-Pet-B4 Well Testing · December 2016

Question 20 of 22: Prograding wave-dominated delta – profile and plan-view map

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 2016-Dec. 3 hours duration; closed book. This sitting's own cover page reads “98-Pet-B4, Petroleum Geology” and every question is descriptive/interpretive petroleum geology (source rocks, hydrocarbon chemistry, oil sands and tight-oil development, shale gas and coalbed methane, carbonate traps, siliciclastic/deltaic traps) – no well-test pressure-transient content anywhere. Five (5) of the paper's six 20-mark sections are marked (NOTES item 5); all six are solved in full below so this set also serves as a complete study reference. This sitting's Section 6 is a Siliciclastic Traps section (wave-dominated delta, grain-size–permeability–porosity relations, sandstone diagenesis). The paper is entirely qualitative (draw/describe/define/list), with no numeric given data anywhere.

Reference texts: Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, migration, traps, carbonate systems, oil sands); Tissot, B.P. & Welte, D.H., Petroleum Formation and Occurrence, 2nd ed., Springer (kerogen typing, maceral groups, catagenesis); Boggs, S. Jr., Petrology of Sedimentary Rocks, 2nd ed., Cambridge (source-rock, deltaic and carbonate lithofacies; sandstone diagenesis); James, N.P. & Jones, B., Origin of Carbonate Sedimentary Rocks, Wiley-Blackwell (carbonate platform/ramp/sabkha facies models); Butler, R.M., Thermal Recovery of Oil and Gas, Prentice Hall, 1991 (SAGD, CSS, oil sands thermal recovery); Green, D.W. & Willhite, G.P., Enhanced Oil Recovery, SPE Textbook Series Vol. 6 (thermal EOR mechanisms); Law, B.E. & Curtis, J.B., “Introduction to Unconventional Petroleum Systems,” AAPG Bulletin 86, 2002 (shale gas, tight gas, coalbed methane); Lee, W.J. & Wattenbarger, R.A., Gas Reservoir Engineering, SPE Textbook Vol. 5 (unconventional gas reservoir characterization).

Section 6, Q6-1: Prograding wave-dominated delta – profile and plan-view map (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.

Prograding wave-dominated delta: profile and plan view Plan view fluvial channel arcuate, smoothed delta-front shoreline (beach/barrier sand) delta plain (fine sand/mud, marsh) reworked shoreface sand sheet wave approach ≈ 10 km Depositional profile (proximal → distal) delta plain fine sand/mud, coal, marsh delta front well-sorted, coarser sand (reworked) prodelta clay/silt, thin distal sand FINING ← grain size → COARSENING (seaward) ≈ 2 km (vert. exagg.)
Plan view (left): a wave-dominated delta forms an arcuate, wave-smoothed shoreline (unlike a river-dominated delta's bird’s-foot lobes), with delta-front/shoreface sand reworked and redistributed alongshore by wave action. Profile (right): the classic seaward-coarsening/-thickening progradational sequence, from fine-grained delta-plain deposits, through well-sorted delta-front/shoreface sand (coarsest, most wave-winnowed), to fine prodelta clay/silt at the toe.

A wave-dominated delta is distinguished from a river- or tide-dominated delta by the redistribution of fluvially supplied sediment along the shoreline by wave energy, producing a smooth, gently arcuate (rather than protruding, bird's-foot or estuarine-funnel) shoreline in plan view, with beach-ridge/barrier sand plastered symmetrically around the river mouth. In profile, normal progradation builds the classic seaward-coarsening, seaward-thickening succession: delta plain deposits (fine sand, mud, coal/marsh, distributary-channel sand) at the landward, topset position; delta front deposits – the coarsest, best-sorted sand in the system, because wave winnowing continuously reworks and sorts sediment at the shoreline – forming the foreset; and prodelta deposits (fine clay and silt, with only thin, distal storm-sand interbeds) at the toe of the bottomset, grading basinward into open-marine/lacustrine mud. Because progradation is normal (steady sediment supply, no relative sea-level rise interruption), each new delta-front sand sheet stacks directly seaward of and slightly above the previous one, producing a laterally extensive, vertically stacked series of well-sorted sand bodies that make excellent reservoirs once buried and charged.