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24-Pet-B4 Well Testing · December 2014

Question 16 of 18: Extensional basin 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 5, Q5-4: Extensional basin 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.

Extensional (rift) basin: tilted fault blocks Precambrian basement reservoir (rotated block crest) reservoir (rotated block crest) reservoir (rotated block crest) footwall hanging wall normal fault syn-rift / post-rift fill (seal + source)
A series of rotated (tilted) fault blocks bounded by listric normal faults, typical of an extensional (rift) basin. Each block's uplifted footwall crest is truncated at the fault, while the corresponding down-dropped hanging wall forms a half-graben that fills with syn-rift sediment; potential reservoirs occur both in the rotated, often eroded/truncated crest of each footwall/tilted block (structural + truncation trap) and within syn-rift clastic wedges themselves, sourced and sealed by the organic-rich lacustrine or marine shales that typically fill the deepest part of each half-graben (the same setting as Q1-3's stratified-lake source rock).

Extensional basins form by lithospheric stretching, accommodated by arrays of normal faults that break the upper crust into rotated (tilted) fault blocks. Each fault has a footwall (the block structurally above/against the fault surface, typically uplifted and eroded at its crest) and a hanging wall (the block that has moved down relative to the footwall, forming a half-graben depocentre). Potential reservoirs occur (a) in the truncated, structurally high crest of the rotated (footwall) blocks themselves, often further enhanced by sub-unconformity truncation where the block crest was subaerially eroded before burial, and (b) within syn-rift clastic wedges deposited into the hanging-wall half-graben, which are typically both sourced and sealed by the same organic-rich lacustrine/marine shales that dominate rift-basin deposition.