Three major types of structural trap: 24-Pet-B4 Well Testing, Undated paper, Q12

Question 12 of 13: Three major types of structural 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 5, Q5-1: Three major types of structural 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 structural trap are: (1) fold traps (closure created by folding of the reservoir/seal pair into an anticline or similar upward flexure); (2) fault traps (closure created by faulting, juxtaposing the reservoir against an impermeable unit across the fault plane, or creating a fault-bound anticline); and (3) traps related to salt (or shale) diapirism (closure created by the flow/piercement of a mobile, low-density evaporite – or over-pressured shale – body through the overlying section).

Topic. Three-fold classification of structural traps by deformation mechanism.

Key relations. Unlike stratigraphic traps (Section 4), every structural trap involves POST-DEPOSITIONAL deformation of an already-continuous reservoir/seal pair – by folding, by faulting, or by the flow of a mobile evaporite/shale body.

Why this works. Folds, faults, and salt/shale diapirism are the three fundamentally different physical processes that can deform strata into a closed structural high – ductile flexure, brittle offset, and buoyancy-driven mobile-layer flow – and most real structures (Q5-2) combine two of the three (e.g. a fault-propagation fold, or a salt-cored anticline).

Common pitfall. Listing “anticline” and “fold” as two separate major types – an anticline IS a fold trap; the three MAJOR categories are fold, fault, and salt/shale-related, each of which then has multiple sub-types (Q5-2).

Source. Biddle & Wielchowsky, AAPG Memoir 60, ch.12; Allen & Allen, Basin Analysis, 3rd ed., ch.5.