Hydrocarbon compound formulas and structures: 24-Pet-B4 Well Testing, December 2014, Q4

Question 4 of 18: Hydrocarbon compound formulas and structures

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 2, Q2-1: Hydrocarbon compound formulas and structures (8 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) Butane C₄H₁₀ – a saturated, straight-chain alkane (paraffin). (b) Octane C₈H₁₈ – a saturated, straight-chain alkane, one of the components used to define gasoline octane rating. (c) Benzene C₆H₆ – an aromatic hydrocarbon, a planar six-carbon ring with delocalized π-bonding (alternating/resonant double bonds). (d) Cyclohexane C₆H₁₂ – a saturated cyclic alkane (naphthene), a six-carbon ring with only single bonds.

Skeletal (line) structural formulas: butane and octane are open-chain saturated alkanes (each vertex/end = a carbon with implicit H's); benzene is an aromatic ring with a delocalized π-system (drawn as an inscribed circle) alternating with the ring's σ-bonds; cyclohexane is the fully saturated analogue of benzene, a plain hexagon with no ring unsaturation.

Topic. Hydrocarbon compound classes: alkanes, aromatics, naphthenes.

Key relations. Saturated acyclic alkane: C_nH_2n+2 (butane, octane); saturated cyclic (naphthene): C_nH_2n (cyclohexane); aromatic (one ring, three degrees of unsaturation beyond the ring itself): C_nH_2n-6 (benzene).

Why this works. Each ring closure or double bond removes two hydrogens relative to the fully saturated open-chain formula (a "degree of unsaturation"); benzene's ring plus three formal double bonds accounts for its H-deficit relative to cyclohexane, which has the same six-carbon ring but zero double bonds.

Common pitfall. Confusing "aromatic" with "having a double bond" generically – benzene's resonance-stabilized ring behaves chemically very differently from a simple alkene (e.g. it resists addition reactions that alkenes undergo readily), which is why petroleum geochemists track BTEX aromatics separately from olefins.

Source. Tissot & Welte, ch.1 (hydrocarbon compound classes); Selley & Sonnenberg, ch.5.