Three major types of stratigraphic trap: 24-Pet-B4 Well Testing, Undated paper, Q9

Question 9 of 13: Three major types of stratigraphic trap

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 17-Pet-B4 (May 2019, per its page footers). 3 hours duration; closed book. This paper's own cover page reads “17-Pet-B4, Petroleum Geology” and all five sections are descriptive/interpretive petroleum geology (terminology, source rocks, thermal maturation, stratigraphic traps, structural traps), no well-test pressure-transient content anywhere. The section marks are Section 1 = 30, Section 2 = 18, Section 3 = 22, Section 4 = 15, Section 5 = 15 (sum 100), used throughout below. The exam is entirely qualitative (explain/define/sketch).

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

Section 4, Q4-1: Three major types of stratigraphic trap (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.

The three major types of stratigraphic trap are: (1) traps caused by depositional/facies change (primary stratigraphic traps – a porous, permeable facies pinches out laterally into an impermeable one); (2) traps caused by unconformities (a permeable unit is truncated updip beneath an unconformity, or a younger unit onlaps/subcrops against the unconformity surface, in either case sealed by overlying impermeable strata); and (3) traps caused by diagenesis (secondary stratigraphic traps – a porosity/permeability change created AFTER deposition, e.g. a dolomitization front or a cementation/leaching boundary).

Topic. Levorsen's three-fold classification of stratigraphic traps.

Key relations. The unifying feature of every stratigraphic trap is that the SEAL (or the reservoir/seal juxtaposition) is created by a change WITHIN the stratigraphic column itself – by facies, by an unconformity surface, or by diagenesis – rather than by post-depositional deformation (structural traps, Section 5).

Why this works. Each of the three mechanisms operates at a different stage of a rock's history – facies change is set at DEPOSITION, unconformity traps are set by an EROSIONAL/non-depositional hiatus, and diagenetic traps are set AFTER burial – so the three-way split reflects when, not just how, closure was created.

Common pitfall. Treating an unconformity trap and a facies-change (pinch-out) trap as the same thing because both involve a lateral termination of the reservoir – the distinguishing feature is whether that termination is an ORIGINAL depositional edge (facies change) or an EROSIONAL/subcrop edge cut by a later unconformity.

Source. Biddle & Wielchowsky, “Trap Types in Petroleum Basins,” AAPG Memoir 60, ch.12; Selley & Sonnenberg, ch.9.