Three major groups of structural traps: 24-Pet-B4 Well Testing, December 2014, Q13

Question 13 of 18: Three major groups of structural traps

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, Q5-1: Three major groups of structural traps (3 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.

(1) Fold (compressional/anticlinal) traps – closures formed by upward flexure of reservoir and seal beds, e.g. simple anticlines, drape folds, fault-propagation and fault-bend folds. (2) Fault traps – closures formed where displacement across a fault juxtaposes reservoir against an impermeable unit (or the fault plane itself, via clay/gouge smear, acts as a seal). (3) Traps associated with diapirism/piercement structures – closures formed around a buoyant, mobile intrusive body (most commonly salt, sometimes shale or mud) piercing and deforming the overlying strata, creating a family of associated fold-, fault- and truncation-style closures around the diapir.

Topic. Structural trap classification.

Key relations. Fold traps = closure from bed curvature alone; fault traps = closure from juxtaposition/fault-plane sealing; diapiric traps = a composite family of closures generated by a mobile, low-density intrusive body.

Why this works. Each group is defined by the tectonic process creating the closure (compression/buckling, brittle offset, or ductile diapiric flow), which in turn controls what geometric and seismic signatures an explorationist looks for.

Common pitfall. Treating salt-related traps as a fourth, unrelated category rather than as a specific mechanism generating both fold-style (supra-salt) and fault-style (radial/peripheral) closures simultaneously, as elaborated in Q5-3.

Source. Selley & Sonnenberg, ch.7; Biddle & Wielchowsky, AAPG Memoir 60 ch.12.