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24-Pet-B4 Well Testing · December 2014

Question 12 of 18: Six stratigraphic trap examples

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, Q4-3: Six stratigraphic trap examples (12 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.

Six stratigraphic trap examples (2 pinchout, 2 unconformity, 2 diagenetic) oil in sand shale seal 1. Fluvial/deltaic channel-sand pinchout reef carbonate (oil) basinal shale seal 2. Reef (carbonate buildup) pinchout oil below unconformity overlying shale seal 3. Sub-unconformity truncation trap onlap sand, oil shale seal onlapping high 4. Onlap / buried-hill trap dolomitized zone (oil) tight limestone (regional seal) 5. Dolomitization sweet-spot trap porous sand (oil) cemented (tight) updip 6. Updip cementation (permeability) trap
Two pinchout traps (fluvial/deltaic channel sand thinning into shale; carbonate reef buildup passing laterally into basinal shale), two unconformity-bound traps (truncation of tilted reservoir beds sealed by an overlying unconformity; onlap of reservoir sand against a buried structural/erosional high, sealed above), and two diagenetic traps (a dolomitized "sweet spot" in otherwise tight limestone; updip diagenetic cementation reducing permeability to create a permeability-pinchout seal).

Pinchout traps. (1) A fluvial or deltaic channel/bar sandstone deposited as a laterally discontinuous body within a marine or lacustrine shale; the sand thins and grades into shale away from the channel/bar axis, trapping oil generated in the enclosing shale updip of the pinchout (classic example: East Texas Field, Woodbine Fm channel-sand pinchout). (2) A carbonate reef or bioherm that built up as a discrete, porous mound and then was buried and surrounded by low-permeability basinal shale or lime mudstone as the reef drowned; hydrocarbons accumulate in the reef's own porous framework and flanking talus, sealed laterally and above by the enclosing basinal facies (classic example: the Devonian Leduc reef trend, WCSB, Alberta).

Unconformity-bound traps. (3) A truncation trap, where tilted or folded reservoir beds are bevelled off at an erosional unconformity and then sealed by low-permeability beds deposited directly on top of the unconformity surface – hydrocarbons accumulate immediately below the unconformity in the truncated reservoir (classic example: the East Texas–Gulf Coast sub-Cretaceous unconformity plays; Prudhoe Bay's own truncation component). (4) An onlap (buried-hill) trap, where younger reservoir beds onlap and pinch out against a buried topographic high (itself often capped by low-permeability weathered/paleosol material) and are then sealed above by younger shale draping the whole feature – hydrocarbons accumulate in the onlapping reservoir against the flank of the buried high (classic example: buried karst-hill fields of the Ordovician in the Tarim Basin, China; analogous buried-hill plays in the Bohai Bay Basin).

Diagenetic traps. (5) A dolomitization sweet spot, where a body of limestone has been selectively dolomitized (often along a fault/fracture-controlled fluid pathway), increasing porosity and permeability locally relative to the surrounding tight, undolomitized limestone that then acts as the seal – hydrocarbons accumulate within the higher-quality dolomite body itself (classic example: the Devonian Nisku and Swan Hills dolomite reservoirs, WCSB). (6) An updip diagenetic cementation (permeability-pinchout) trap, where a laterally continuous, initially porous sandstone loses reservoir quality updip due to progressive quartz, calcite, or clay cementation (often related to a facies change increasing detrital clay content, or to a diagenetic front such as a paleo-oil/water contact), creating an internal permeability barrier that behaves as a seal even though no lithologic or stratigraphic boundary is obviously present (classic example: many tight-updip clastic traps in the Alberta Basin's Cardium and Viking sandstones).