Question 1 of 13: Terminology – five sets of related terms

Section 1, Q1: Terminology – five sets of related terms (30 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) Petroleum, Crude Oil, Natural Gas. Petroleum is the umbrella term for the whole family of naturally occurring hydrocarbon substances found in the subsurface – solid, liquid and gaseous. Crude oil is the naturally occurring LIQUID hydrocarbon mixture recovered from a reservoir (with dissolved gas at reservoir conditions), and natural gas is the naturally occurring GASEOUS hydrocarbon mixture (predominantly methane, C₁, with lesser ethane–butane and non-hydrocarbon gases such as CO₂/N₂/H₂S). The difference is one of scope and phase: petroleum is the genetic/compositional umbrella, while crude oil and natural gas are its two principal surface-condition phases, distinguished by volatility (crude oil is dominated by C₁₅₊ components that remain liquid at surface, natural gas by C₁–C₄ components that remain gaseous).

(b) Heavy Oil, Light Oil, Bitumen. These three are points along one continuum of API gravity and viscosity. Light oil has API > 31.1° and viscosity typically <10 cP, and flows readily to a wellbore under natural reservoir energy. Heavy oil has API ≈ 10–22.3° and viscosity roughly 100–10,000 cP, and generally needs artificial lift and/or thermal or chemical assistance to flow economically. Bitumen has API < 10° and viscosity >10,000 cP – for practical purposes immobile at reservoir temperature without added heat or dilution, and is instead mined or produced by thermal methods (e.g. SAGD). Heavy oil and bitumen are typically NOT primary (as-generated) products – they are usually a lighter oil that has subsequently been biodegraded and/or water-washed near surface (part e), which strips light ends and raises viscosity.

(c) Oil Sands, Oil Shale, Coalbed Methane. All three are “unconventional” because a conventional well cannot produce them without a non-standard technique, but each for a different underlying reason. Oil sands (tar sands) are unconsolidated to poorly consolidated sandstone (or carbonate) reservoirs saturated with bitumen – a conventional-looking clastic RESERVOIR, but the hydrocarbon itself is too viscous to flow without added heat (in-situ thermal recovery) or without mining. Oil shale is a fine-grained, organic (kerogen)-rich, generally thermally IMMATURE sedimentary rock – the kerogen has never been converted to oil in the subsurface, so the rock must be retorted (pyrolyzed) at surface, or heated in situ, to crack the kerogen into a synthetic crude; this is distinct from “shale oil”/tight oil, which is already-generated conventional-quality oil produced from a mature, low-permeability shale/siltstone by horizontal drilling and hydraulic fracturing. Coalbed methane (CBM) is natural gas (largely methane) generated within, and held ADSORBED on the internal surface area of, a coal seam's organic matrix, rather than trapped as a free fluid beneath a conventional structural or stratigraphic seal; it is produced by dewatering (depressurizing) the coal to desorb the gas.

(d) Alkanes, Naphthenes, Aromatics. These are the three principal hydrocarbon compound families found in crude oil, distinguished by saturation and ring structure. Alkanes (paraffins) are saturated, open-chain (straight or branched) hydrocarbons, C_nH_2n+2, with only single C–C bonds. Naphthenes (cycloalkanes) are saturated hydrocarbons in which some carbons close into a ring, C_nH_2n for a single ring (e.g. cyclohexane). Aromatics are UNsaturated cyclic hydrocarbons built around one or more benzene rings (delocalized π-bonding), e.g. benzene, toluene, and the BTEX series. Crude oils are informally classified by which family dominates – paraffinic-base, naphthenic-base, or aromatic (asphaltic)-base – and this compositional split strongly influences refining behaviour and (for aromatics) toxicity/water solubility.

(e) Water Washing, Biodegradation, Thermal Cracking. All three ALTER an oil accumulation after it has been emplaced in a trap, but by different mechanisms and in different depth/temperature regimes. Water washing is the selective removal of the most water-soluble components (light aromatics such as benzene/toluene, and to a lesser extent light paraffins) from reservoired oil by contact with actively flowing formation or meteoric water. Biodegradation is microbial (bacterial) consumption of hydrocarbons, occurring preferentially at shallow depth/low temperature (below the “palaeopasteurization” threshold, roughly 80 °C, above which reservoir bacteria are killed) – bacteria attack n-alkanes first, then isoprenoids, then naphthenes, progressively stripping the least resistant compounds and enriching the residual oil in resins/asphaltenes; this is the principal mechanism that converts a normal oil into heavy oil or bitumen (part b). Thermal cracking is the breakdown of large hydrocarbon molecules into smaller ones by elevated temperature at depth (deep burial into catagenesis/metagenesis), converting oil into progressively lighter oil, condensate, wet gas, and ultimately dry (methane) gas. The key contrast: water washing and biodegradation are shallow, low-temperature, aqueous/biological processes that make oil HEAVIER (lower API), while thermal cracking is a deep, high-temperature, purely chemical (abiotic) process that makes oil LIGHTER (higher API), ultimately to gas.