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

Question 19 of 20: Parasequence Stacking Patterns (S versus A)

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 19: Parasequence Stacking Patterns (S versus A) (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 parasequence is a relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces (and their correlative surfaces), each individual parasequence recording a single progradational, shallowing-upward depositional cycle. When many parasequences stack vertically, the stacking pattern of the set is controlled by the ratio of sediment supply rate (S) to the rate at which new accommodation space is generated (A):

Parasequence stacking patterns Progradational (S > A) shoreline steps basinward Aggradational (S = A) shoreline stationary Retrogradational (S < A) shoreline steps landward
Parasequence stacking patterns: progradational (S > A, successive parasequences build seaward of the last, shoreline advances basinward), aggradational (S = A, parasequences stack directly on top of one another, shoreline roughly stationary), and retrogradational (S < A, successive parasequences step landward and thin, shoreline transgresses/retreats, sediment-starved).

S > A (progradational stacking). Sediment supply outpaces the rate of new accommodation creation, so each successive parasequence is deposited basinward of the one before it; the shoreline advances (progrades) basinward over time, and successive parasequences may partly overstep the top of the one below.

S = A (aggradational stacking). Sediment supply just balances accommodation creation, so successive parasequences stack essentially vertically on top of one another with little lateral shift; the shoreline position stays approximately fixed through time.

S < A (retrogradational stacking). Accommodation is created faster than sediment can fill it, so each successive parasequence is deposited landward of, and typically thinner/more distal than, the one before it; the shoreline steps landward (transgresses, "backsteps") over time, and the parasequence set becomes progressively more sediment-starved upward.