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

Question 18 of 20: Depositional Sequences and Systems Tracts

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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 18: Depositional Sequences and Systems Tracts (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.

Depositional sequence. A depositional sequence is a relatively conformable succession of genetically related strata, bounded at both its top and base by unconformities or their correlative conformities (sequence boundaries), representing the stratigraphic record of one complete cycle of relative sea-level (base-level) rise and fall.

Systems tracts. A sequence is subdivided into systems tracts – linked, contemporaneous depositional systems defined by their position on the relative sea-level curve and their internal stacking pattern:

Systems tracts and the relative sea-level curve Relative sea level LST lowstand, prograding TST transgressive, retrograding HST highstand, prograding/aggrading MFS SB SB
Systems tracts across one relative sea-level cycle: lowstand systems tract (LST, deposited near the lowstand, prograding above the sequence boundary SB), transgressive systems tract (TST, deposited during rise, retrogradational, capped by the maximum flooding surface MFS), and highstand systems tract (HST, deposited near and after the peak, progradational-to-aggradational, up to the next sequence boundary).

Lowstand systems tract (LST) – deposited during and just after relative sea-level fall/lowstand, directly above the sequence boundary; typically shows a progradational stacking pattern (e.g., incised-valley fill, basin-floor fan, lowstand prograding wedge), corresponding to the lowest position and earliest rise on the sea-level curve. Transgressive systems tract (TST) – deposited during relative sea-level rise; shows a retrogradational (backstepping) stacking pattern as accommodation outpaces sediment supply, and is capped by the maximum flooding surface (MFS), corresponding to the rising limb of the curve. Highstand systems tract (HST) – deposited as sea-level rise slows, peaks and begins to fall; shows an aggradational-to-progradational stacking pattern, corresponding to the peak and early fall of the curve. (Some four-tract models add a falling-stage systems tract, FSST, for the forced-regressive deposits laid down while sea level is actively falling, between the HST and the next LST.)