gas gravity, Oil & Gas Glossary

Gas gravity, also called gas specific gravity, is the density of a hydrocarbon gas expressed as the dimensionless ratio of the molecular weight of the gas to the molecular weight of air, taken at the same temperature and pressure. Because air has an average molecular weight of about 28.96 g/mol, a gas with an apparent molecular weight of 18 g/mol has a gas gravity near 0.62, and the value scales directly with how heavy the gas mixture is: dry, methane-rich gas sits around 0.55 to 0.60, while rich or wet gas carrying ethane, propane, butane, and heavier condensate fractions climbs to 0.7, 0.9, or higher, and gases contaminated with carbon dioxide or hydrogen sulphide rise further still because both are heavier than methane. Gas gravity is prized as a single-number descriptor because it is trivially measured in the field with a recording gravitometer and it feeds nearly every practical gas calculation. Its most important role is as the input to pseudo-critical property correlations: the Standing, Sutton, and related equations estimate a gas mixture's pseudo-critical temperature and pressure directly from gravity, and those pseudo-critical values in turn yield the pseudo-reduced temperature and pressure needed to read the compressibility or z-factor from the Standing-Katz chart. The z-factor then propagates into gas formation volume factor, gas density at reservoir conditions, gas in place, and material-balance reserves, so an error in gravity ripples through the entire volumetric and deliverability workflow. Gas gravity also governs flow: the Cullender and Smith method for computing flowing bottomhole pressure in a gas well, the Weymouth and Panhandle pipeline equations, and orifice-meter gas measurement all take gravity as a core term, because a heavier gas exerts more hydrostatic head in the wellbore and behaves differently through a choke or meter run. In the Western Canadian Sedimentary Basin the parameter is central to distinguishing dry from wet gas plays. Deep Montney and Duvernay wells in the Alberta and British Columbia liquids-rich fairway commonly produce raw gas gravities well above 0.7 because of high condensate and NGL content, and that richness, quantified partly through gravity, drives the economics that make those wells drillable at prevailing AECO gas prices. Regulators and reserve evaluators expect gravity, alongside composition, to underpin the gas properties reported under AER and BC Energy Regulator requirements, and it must be corrected for any inert content before it is fed into hydrocarbon-only pseudo-critical correlations. Gravity is thus both a quick field diagnostic and a rigorous engineering input that connects wellhead measurement to booked reserves.

Key Takeaways

Ratio to air defines it: Gas gravity equals the apparent molecular weight of the gas divided by 28.96, the molecular weight of air, making it dimensionless. A gas gravity of 0.65 corresponds to an apparent molecular weight near 18.8 g/mol, and because methane is 16 g/mol, any value above about 0.55 signals the presence of heavier hydrocarbons or inert gases in the stream.
Dry versus wet indicator: Dry sweet gas runs 0.55 to 0.60, wet gas rich in ethane through pentanes reaches 0.65 to 0.90, and CO2 or H2S contamination pushes gravity higher still since both exceed methane in molecular weight. In the WCSB, Montney and Duvernay liquids-rich gas frequently exceeds 0.70, flagging valuable condensate and NGL yield.
Input to pseudo-critical correlations: Standing and Sutton correlations convert gravity to pseudo-critical temperature and pressure, for example Sutton gives Tpc = 169.2 + 349.5(SG) - 74.0(SG)^2 in degrees Rankine, valid from SG 0.57 to 1.68. These feed the pseudo-reduced conditions used to read the z-factor, so gravity underpins every gas volumetric calculation.
Drives flow and measurement: The Cullender and Smith bottomhole-pressure method, the Weymouth and Panhandle pipeline equations, and orifice metering all use gas gravity, because a heavier gas column carries more hydrostatic head and passes differently through chokes and meter runs. Accurate gravity is essential for deliverability and custody-transfer accuracy.
Correct for inerts first: Non-hydrocarbon components such as CO2, H2S, and nitrogen raise measured gravity and distort pseudo-critical estimates, so the Wichert-Aziz correction is applied to sour WCSB gas before the gravity-based correlation is trusted. Skipping this step yields z-factors and gas-in-place figures that are materially wrong for sour Nisku or Leduc gas.

Gravity to Reserves: The Volumetric Chain

Gas gravity is the first link in a chain that ends at booked reserves. A field gravitometer reading of 0.68 on a Montney well feeds a Sutton correlation to give pseudo-critical temperature and pressure, which combine with reservoir pressure and temperature to yield pseudo-reduced conditions and a z-factor near 0.85 to 0.95 at depth. That z-factor sets the gas formation volume factor and therefore the reservoir volume of each standard cubic metre, so gas in place and the recoverable reserve that an AER-filed evaluation reports both trace back to the single gravity number. A 5 percent gravity error can shift the z-factor and the volumetric estimate by a few percent, enough to matter on a multi-billion e3m3 pool.

Rich Gas Economics in the Liquids Window

In the WCSB liquids-rich fairway, high gas gravity is a revenue signal, not just a property. A raw gas gravity of 0.80 on a Kakwa-area Montney well implies substantial pentanes-plus and condensate that sell at oil-linked prices far above the AECO gas value, so two wells with identical raw gas rates but gravities of 0.60 and 0.80 have very different netbacks. Evaluators pair gravity with a full extended composition to compute condensate-gas ratio and NGL yield, which for a rich Montney well can add several thousand CAD per day of liquids revenue and often determines whether the well clears its drilling and completion cost of roughly 8 to 12 million CAD.

Fast Facts

The reference value for air, 28.96 g/mol, is itself a weighted average of a mixture that is roughly 78 percent nitrogen and 21 percent oxygen, which is why a gas made almost purely of methane at 16 g/mol still has a gravity of about 0.55 rather than the 0.55 to 1.0 range most people expect from everyday gases. Because the denominator never changes, gas gravity is one of the few reservoir parameters that requires no unit conversion between the metric and field systems: the dimensionless ratio reads the same whether the underlying molecular weights are quoted in g/mol or lb/lb-mol.

Gas gravity is the primary input to the z-factor, the compressibility correction that turns ideal-gas volumes into real reservoir volumes, and it parallels API gravity, the analogous density scale used for liquid crude rather than gas. It is a specific application of specific gravity, the general ratio of a substance's density to a reference, and it characterizes the natural gas stream whose richness in heavier hydrocarbons both raises the gravity and drives the condensate economics of a liquids-rich play.

Real-World WCSB Scenario: Sour Nisku Gas Correction

An engineer evaluating a sour Nisku gas pool near Brazeau, Alberta, measures a raw gas gravity of 0.82, but the stream carries 12 percent H2S and 8 percent CO2. Feeding that gravity straight into a Standing correlation would overstate pseudo-critical temperature and hand back an unreliable z-factor, so the engineer applies the Wichert-Aziz correction for the acid-gas content before computing pseudo-reduced properties, landing a defensible z-factor near 0.88 at the 24,000 kPa reservoir pressure.

With the corrected z-factor the gas-in-place estimate for the pool falls in line with material-balance history, and the AER-filed reserve evaluation withstands technical review. Had the raw gravity been used uncorrected, the volumetric would have been overstated by several percent, risking an overbooked reserve on a pool worth tens of millions of CAD.