Cross-sections of the three stratigraphic trap types: 24-Pet-B4 Well Testing, Undated paper, Q10

Question 10 of 13: Cross-sections of the three stratigraphic trap types

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, Q4-2: Cross-sections of the three stratigraphic trap types (9 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.

(a) A facies-change (pinch-out) trap: a reef or lens-shaped reservoir body is encased laterally and vertically by impermeable shale deposited around it – the seal is the ORIGINAL depositional facies boundary. (b) An unconformity trap: reservoir strata are truncated updip beneath an angular unconformity, and younger, impermeable strata onlap the unconformity surface, sealing the truncated edge from above. (c) A diagenetic trap: a dolomitization front creates secondary porosity/permeability within an otherwise tight limestone – the seal is the diagenetic boundary itself, not a primary depositional or erosional surface.

Topic. Cross-sectional geometry that distinguishes the three stratigraphic trap types.

Key relations. In every panel, closure is provided by a boundary INTERNAL to the stratigraphy (a facies edge, an unconformity surface, or a diagenetic front) rather than by faulting or folding of the beds themselves.

Why this works. Sketching the reservoir/seal boundary explicitly as one of these three surface TYPES (rather than just drawing a generic pinch-out) is what makes the trapping mechanism – and therefore the exploration risk (facies prediction vs. subcrop mapping vs. predicting diagenetic fluid pathways) – identifiable from the cross-section alone.

Common pitfall. Drawing an unconformity trap without also showing the ONLAPPING seal above it – truncation alone creates a permeable edge, not a trap; the trap requires an impermeable unit resting on the unconformity surface to close it from above.

Source. Biddle & Wielchowsky, AAPG Memoir 60, ch.12; Boggs, Petrology of Sedimentary Rocks, 2nd ed., ch.9–10 (reef and carbonate diagenetic facies).