Product

Associated gas (also called associated petroleum gas or APG) is natural gas that occurs together with crude oil in a petroleum reservoir, produced as a co-product or by-product of oil extraction.

Occurrence in the Reservoir

Within the reservoir, associated gas exists in two distinct forms:​

• Free gas (gas cap): Gas that exists separately above the oil as a pressurized cap in the reservoir formation
• Dissolved gas: Gas dissolved directly into the crude oil under high reservoir pressure, which is released when oil is brought to the surface (similar to carbonation escaping from a pressurized bottle)​

Composition

Associated gas is primarily a mixture of alkane hydrocarbons. Its typical composition includes:

• Methane (CH₄) as the dominant component
• Heavier hydrocarbons: ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), and pentanes
• Impurities such as water vapor, hydrogen sulfide (H₂S), carbon dioxide (CO₂), and nitrogen

Because it often contains light liquid hydrocarbons like propane and butane, associated gas is sometimes referred to as "wet gas".​

Properties

Based on the typical APG composition reported in the literature, the following values can be derived:​

Molecular Weight

The average molecular weight of APG is calculated as the mole-fraction-weighted sum of component MWs. Using the typical composition (81% methane, 6.6% propane, 5.5% ethane, 4% butane, 1.4% pentane, 1% N₂, 0.17% CO₂): ~ 21.3 g/mol

This is notably higher than pure methane (16.04 g/mol) due to the heavier C₂–C₅ hydrocarbon content, and lower than air (28.97 g/mol), confirming APG is lighter than air.

Specific Gravity (Relative to Air)

Gas-phase specific gravity is defined relative to air at the same conditions: 21.3 / 28.97 ~ 0.73

This value reflects a rich/wet gas character. Leaner APG streams (higher methane content) would have SG values closer to 0.55–0.65, while richer streams with more C₃⁺ content can push SG above 0.80.​

In practice, APG composition — and therefore its MW and SG — varies significantly by field and reservoir. Fields with high liquid content (condensate-rich) will show higher MW and SG, while gas-cap gas dominated by methane behaves closer to dry natural gas (SG ≈ 0.55–0.65).

Distinction from Non-Associated Gas

Associated gas comes from oil wells, whereas non-associated gas is produced from dedicated gas or condensate wells where little or no crude oil is present. The U.S. EIA uses a gas-oil ratio (GOR) threshold of 6,000 cubic feet per barrel to classify a well as an oil well — any gas produced from such a well is classified as associated gas.

Utilization and Disposal

After separation from the crude oil at the wellhead, associated gas can be handled several ways:​

• Sold into natural gas distribution networks (after processing)
• Reinjected into the reservoir for pressure maintenance and enhanced oil recovery (EOR)
• Used on-site for power generation (engines or turbines)
• Processed into LNG or petrochemical feedstocks
• Flared or vented — historically the most common fate in regions lacking gas infrastructure, though this is increasingly regulated​

Associated gas is generally considered an undesirable by-product of oil production, as field development costs are primarily justified by the crude oil revenue rather than the gas.