Abstract: Hydrocarbon gas, oils, and condensates from offshore and onshore concessions or business units are often transported via shared pipelines to shared main treatment or storage facilities. The concessions are often owned by, or licensed to a number of oil companies. At the main treatment facilities, hydrocarbon fluids such as gas and condensate are processed to meet sales and fiscal specifications. The gas is then sold to shippers in terms of standardized volumes (standard cubic-meters) or combustion energy (joules), and the gas condensate is usually sold in terms of standard volume (standard cubic meters) or mass (kilograms or tonnes). All the hydrocarbon gas and condensate sold at these main treatment facilities and associated monies should be allocated back to the individual concessions and then ultimately traceable back to the individual reservoirs or wells that generated them!When hydrocarbon gas from two or more entry sources (e.g. two or more different entities) is commingled and processed in a common pipeline and terminal or processing system and the sources have different ownership and/or operate under different tax regimes, then a gas allocation system is needed.