Atmospheric emissions from oil and gas production operations are composed of multiple hydrocarbons and can have large variations in composition. Accurate estimates of emission compositions are needed to estimate the fate and impacts of emissions and to attribute emissions to sources. This software has a database, constructed with public domain data empirical data and thermodynamic models, that can be queried to estimate hydrocarbon compositions from emission sources present at oil and gas production sites. The database can be searched for matches using between 2 and 7 well parameters as query inputs (Gas-to-oil ratio, API gravity, separator pressure, separator temperature, methane molar fraction in produced gas, ethane molar fraction of produced gas and propane molar fraction in produced gas). The database created in this software combines data on hundreds of wellsite process streams (available in the public domain) and uses a thermodynamic model to generate approximately one million scenarios of emission compositions. These one million scenarios represent a much broader range of well site conditions than the publicly available data. Users can employ the software's algorithm and commonly available data on well sites to identify best estimates of emission compositions.