Stratigraphic pinchout trap cross section: 24-Pet-B4 Well Testing, December 2014, Q11

Question 11 of 18: Stratigraphic pinchout trap cross section

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 4, Q4-2: Stratigraphic pinchout trap cross section (4 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 sandstone reservoir wedge deposited within, and thinning/pinching out updip into, an enclosing shale that forms both the top seal and the updip (lateral) seal. Gas caps the structure where present, oil occupies the mid-section down to the oil–water contact, and water fills the downdip, still-porous but non-hydrocarbon-bearing base of the sand – the trap is created purely by the loss of reservoir facies/porosity updip, with no structural closure required.

The trap forms where a permeable reservoir facies (here, sandstone) grades laterally, updip, into an impermeable facies (shale) of the same or a genetically related unit – the updip seal is therefore stratigraphic (a facies change), not structural. Buoyancy segregates the fluids within the remaining, still-porous part of the sand in the standard order (gas above oil above water), with the updip pinchout itself acting as the trap's effective "spill point."

Topic. Stratigraphic (facies-pinchout) trap.

Key relations. Trap closure = updip loss of reservoir porosity/permeability (facies change), not folding or faulting; fluid stacking (gas/oil/water) still follows buoyancy segregation within whatever reservoir volume remains.

Why this works. A trap only needs a permeability contrast around a permeable body's boundary; a facies pinchout provides exactly that updip boundary without any tectonic deformation, which is why pure stratigraphic traps can occur even in structurally simple, flat-lying basins.

Common pitfall. Drawing the pinchout as a sharp vertical cutoff – real facies transitions (and the seal they create) are gradational over some lateral distance, and the effective trap boundary is wherever permeability first drops below the threshold needed to hold the hydrocarbon column (a capillary, not geometric, cutoff).

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