Question 2 of 7: Section 2: Hydrocarbon Chemistry and Generation

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.

1. 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 ($SG=1.000$). The scale is built so that water plots at exactly API 10, and API increases as density decreases (lighter oil = higher API).
2. Worked check. A representative light oil of $SG=0.850$: $$\text{API} = \frac{141.5}{0.850} - 131.5 = 166.5 - 131.5 = 35.0^{\circ}\text{API}$$
3. Standard classification bands (industry/API convention): $$\boxed{\text{Light oil: API} > 31.1^{\circ}\ (SG<0.870)\ \ |\ \ \text{Medium: } 22.3^{\circ}\text{-}31.1^{\circ}\ \ |\ \ \text{Heavy oil: API} < 22.3^{\circ}\ (SG>0.920)}$$ (extra-heavy/bitumen, as produced from the Alberta oil sands, is API < 10°, i.e. denser than water).

Q2-2 — 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.

Q2-3 — Geothermal gradient and the oil/gas windows.

Given. Average surface temperature $T_0=0^{\circ}\text{C}$; geothermal gradient $30^{\circ}\text{C km}^{-1}$ (linear, constant with depth).

Find. The temperature-depth profile, plus the depth range spanned by the oil window and the gas window.

Approach. The oil and gas windows are defined by temperature thresholds (kerogen thermal maturity), not directly by depth or pressure — so the linear geotherm is used to convert the standard temperature thresholds into depths for this particular basin.

1. Linear geotherm. $$T(z) = T_0 + (\text{gradient})\times z = 0 + 30\,z\quad [z\text{ in km, }T\text{ in }^{\circ}\text{C}]$$
2. Standard maturity thresholds (vitrinite-reflectance-calibrated, Tissot & Welte catagenesis/metagenesis scheme): immature (biogenic gas only) below 60°C; oil window 60-150°C (peak oil generation ≈100°C); wet-gas/condensate to dry-gas window 150-225°C; overmature (graphitized, no further generation) above 225°C.
3. Convert to depth, $z=T/30$: $$z_{\text{oil top}}=\frac{60}{30}=2.0\text{ km}\qquad z_{\text{oil base}}=\frac{150}{30}=5.0\text{ km}\qquad z_{\text{gas base}}=\frac{225}{30}=7.5\text{ km}$$

The relative volume of hydrocarbon product changes systematically down the profile: oil generation rate peaks near the middle of the oil window (≈3.5 km, ≈100°C) and then declines as remaining kerogen is exhausted and previously-generated oil itself begins to crack to gas; gas volume (both primary kerogen-cracking gas and secondary oil-cracking gas) increases steadily through the base of the oil window and dominates entirely through the gas window.

Q2-4 — Primary migration mechanisms. Primary migration is the expulsion of newly generated hydrocarbons out of a low-permeability, organic-rich source rock and into an adjacent, more permeable carrier bed. Three mechanisms enable it:

(1) Compaction-driven expulsion. As the source rock is progressively buried, mechanical compaction reduces its pore volume; the expelled pore water (and any generated hydrocarbon it carries or that has begun to form a separate phase) is squeezed toward more permeable, less-compacted carrier beds, especially early in burial before significant hydrocarbon generation has occurred.

(2) Micro-fracturing driven by generation overpressure. Hydrocarbon generation converts solid kerogen into liquid and gas products of much larger molar volume, and this occurs in a rock whose permeability is by then too low to dissipate the fluid quickly; pore fluid pressure rises until it exceeds the rock's tensile strength plus the minimum confining stress, hydrofracturing the source rock and releasing hydrocarbons along the resulting micro-fracture network in short, self-sealing pulses.

(3) Continuous oil-phase (diffusive/wetting-film) flow. Once enough oil has been generated to form a continuous, interconnected oil-wet film along kerogen surfaces and grain contacts (exceeding the critical oil saturation), the oil itself becomes mobile as a separate phase and migrates along this connected network under its own pressure and buoyancy, without needing further fracturing.