Section 2: Hydrocarbon Chemistry and Generation: 04-Geol-B6, May 2018, Q2

Section 2: Hydrocarbon Chemistry and Generation (20 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.

Q2-1 — API gravity.

Given. Oil density is conventionally reported as API gravity (American Petroleum Institute), a scale defined relative to the specific gravity (SG) of the oil at 60°F relative to water.

Find. The API formula, and the typical API ranges that define light versus heavy oil.

Approach. Apply the API definition directly and its inverse to check a representative light-oil density.

API formula. $$\text{API} = \frac{141.5}{SG} - 131.5$$ where $SG$ is the oil's specific gravity at 60°F relative to fresh water ($SG=1.000$). Water plots at exactly API 10, and API increases as density decreases (lighter oil = higher API).
Worked check. A representative light oil of $SG=0.850$: $$\text{API} = \frac{141.5}{0.850} - 131.5 = 166.5 - 131.5 = 35.0^{\circ}\text{API}$$
Standard classification bands (industry/API convention): $$\boxed{\begin{gathered}\text{Light oil: API} > 31.1^{\circ}\ (SG<0.870)\\ \text{Medium: } 22.3^{\circ}\text{-}31.1^{\circ}\\ \text{Heavy oil: API} < 22.3^{\circ}\ (SG>0.920)\end{gathered}}$$ (extra-heavy/bitumen — e.g. Alberta oil sands — is API < 10°, denser than water.)

Result	Value
API formula	$\text{API}=141.5/SG-131.5$
Light oil	API > 31.1° (SG < 0.870)
Heavy oil	API < 22.3° (SG > 0.920)

Q2-2 — Typical viscosities. Viscosity, unlike API, is not a simple linear function of density — it is exquisitely sensitive to composition (wax, asphaltene and resin content) and, in the reservoir, to temperature. Representative reservoir-condition ranges:

Fluid	API (approx.)	Viscosity (reservoir cond.)
Light oil	>31.1°	<10 cP (often 1-5 cP)
Heavy oil	10-22.3°	≈100-10,000 cP
Bitumen	<10°	>10,000 cP (Athabasca bitumen is ≈10⁵-10⁶ cP at surface temperature, falling to the low thousands at typical SAGD reservoir temperature)

The progression light → heavy → bitumen reflects loss of light ends (evaporation, water-washing, biodegradation) and enrichment in high-molecular-weight asphaltenes/resins, which raise viscosity far more steeply than they raise density.

Q2-3 — Definitions.

(a) Kerogen is the fraction of sedimentary organic matter that is insoluble in common organic solvents (distinguishing it from bitumen). It is a complex, high-molecular-weight macromolecular solid dispersed through the rock's mineral matrix, and is the direct precursor of oil and gas: as burial temperature rises through the oil and gas windows, kerogen thermally cracks (catagenesis/metagenesis) to release progressively lighter hydrocarbon products.

(b) Bitumen, in the source-rock geochemical sense, is the organic matter that is soluble in organic solvents (chloroform, dichloromethane) — it forms as an intermediate product when kerogen first begins to break down thermally, before further cracking converts it into free, mobile oil and gas. In the separate economic/reservoir sense used for the Alberta oil sands, "bitumen" refers to the extremely viscous, biodegraded extra-heavy oil (API < 10°) trapped in unconsolidated Cretaceous sand at shallow depth, having lost its light ends to biodegradation and water-washing after migration.

Q2-4 — Oil and gas window temperatures. These are universal temperature thresholds tied to kerogen thermal maturity (vitrinite-reflectance-calibrated catagenesis/metagenesis scheme of Tissot & Welte), which must be converted to a basin-specific depth through that basin's own geothermal gradient:

Window	Temperature range
Oil window	≈60-150°C (peak generation ≈100°C)
Gas window	≈150-225°C (wet gas/condensate 150-175°C grading to dry thermogenic gas to ≈225°C; above this the kerogen is overmature/graphitized and generation ceases)

Q2-5 — Structural formulas.

Structural formulas: (a) octane, a saturated straight-chain alkane; (b) cyclopentane, a saturated 5-carbon ring; (c) benzene, an aromatic 6-carbon ring with a delocalized π system (inner circle); (d) ethyne, a 2-carbon alkyne with one carbon-carbon triple bond.

Octane (C₈H₁₈) is a saturated, straight-chain alkane; cyclopentane (C₅H₁₀) is a saturated cyclic alkane; benzene (C₆H₆) is the archetypal aromatic hydrocarbon, its six π electrons delocalized around the ring; and ethyne/acetylene (C₂H₂) is the simplest alkyne, containing one carbon-carbon triple bond.

Topic. Oil density (API) and viscosity classification, kerogen/bitumen terminology, thermal maturation windows, and basic hydrocarbon structural chemistry.

Key relations. $\text{API}=141.5/SG-131.5$; oil window ≈60-150°C, gas window ≈150-225°C (temperature-, not depth-, defined).

Why this works. API, viscosity and the oil/gas windows are three different lenses on the same underlying process: as kerogen matures thermally, molecules crack into progressively shorter, lighter, less viscous chains — API and viscosity both record how far that cracking has progressed, while the "window" concept simply names the temperature at which enough cracking has occurred to generate the corresponding product class. Structural formulas (Q2-5) are the molecular-scale picture of exactly what those "light" (short, saturated, e.g. octane) versus more complex ring/aromatic products actually look like.

Common pitfall. Treating API and viscosity as interchangeable proxies for one another — two oils of similar API can have very different viscosity if their wax/asphaltene content differs; and quoting oil/gas window boundaries as fixed depths rather than fixed temperatures that must be converted through the local geothermal gradient.

Source. Tissot & Welte, Petroleum Formation and Occurrence, 2nd ed., ch.III (maturation, oil/gas windows); Selley & Sonnenberg, Elements of Petroleum Geology, 3rd ed., ch.5 (API/viscosity classification, hydrocarbon chemistry).

Question 2 of 6: Section 2: Hydrocarbon Chemistry and Generation

Section 2: Hydrocarbon Chemistry and Generation (20 Marks)