NivaarExam PrepOfficial exam papers ↗

24-Pet-B4 Well Testing · December 2014

Question 15 of 18: Salt dome cross section and associated trap types

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 2014-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, generation, migration, traps, Canadian basin geography) – 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. The paper is almost entirely qualitative (draw/describe/define/list), with one true numeric calculation (Q3-2, capillary seal-breach column height).

Reference texts: Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, migration, traps); Tissot, B.P. & Welte, D.H., Petroleum Formation and Occurrence, 2nd ed., Springer (kerogen typing, oil/gas windows, primary migration); Boggs, S. Jr., Petrology of Sedimentary Rocks, 2nd ed., Cambridge (source-rock lithofacies); Allen, P.A. & Allen, J.R., Basin Analysis: Principles and Applications to Petroleum Play Assessment, 3rd ed., Wiley-Blackwell (extensional basins, structural styles); Biddle, K.T. & Wielchowsky, C.C., “Trap Types in Petroleum Basins,” AAPG Memoir 60, ch.12 (stratigraphic/structural/salt trap classification); Mossop, G.D. & Shetsen, I. (eds.), Geological Atlas of the Western Canada Sedimentary Basin, CSPG/Alberta Research Council, 1994 (WCSB stratigraphy and play types).

Section 5, Q5-3: Salt dome cross section and associated trap types (8 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.

Salt dome and associated trap types SALT STOCK 1 crestal (supra-salt) dome 2 flank (piercement) trap 3 radial/peripheral fault trap 4 turtle-back / overhang trap 5 sub-salt (deep) trap ≈ 1 km
A piercement salt dome (stock) with a bulbous, mushroom-shaped cap, and five associated trap types: (1) a crestal (supra-salt) domal fold trap directly above the salt; (2) a flank (piercement) trap where uplifted, upturned beds abut the salt stock's steep flank; (3) traps in the radial and peripheral fault system generated by the salt's rise (grabens over the crest, ring faults around the flanks); (4) a turtle-back/overhang trap in beds arched beneath a bulbous salt overhang; (5) a deep sub-salt trap in beds beneath the salt canopy, sealed and structurally complicated by the salt above. Gulf Coast Basin salt-dome province (e.g. Spindletop, Texas, discovered 1901) is the classic dominant example; a Canadian analogue is the more subdued salt-withdrawal/dissolution structuring in the Devonian Elk Point Basin evaporites of Alberta and Saskatchewan.

A rising salt diapir deforms and offsets the surrounding strata as it pierces upward, creating simultaneously several distinct trap styles around a single structure: (1) crestal/supra-salt domal traps directly above the salt, formed by drape folding of the overlying beds; (2) flank (piercement) traps where beds are truncated and upturned against the steep salt/sediment contact; (3) radial and peripheral fault traps, generated by extensional collapse over the salt's crest (radial grabens) and by the salt's lateral withdrawal from the surrounding rim-syncline (peripheral faults); (4) turtle-back/overhang traps, where beds are arched beneath a bulbous, overhanging salt cap; and (5) deep sub-salt traps, in beds structurally complicated but also protected/sealed by the salt canopy above. The Gulf Coast salt-dome province (Texas/Louisiana, e.g. Spindletop, the discovery that launched the modern Texas oil industry in 1901) is the classic salt-dome-dominated oil-producing region.