“Consumption” means the use or employment of natural gas. Consumption does not include the use or employment of natural gas to generate power for sale, the sale or purchase of natural gas for resale to end users, the sale or use of gas for enhanced oil recovery, natural gas utilized in cogeneration technology projects to produce electricity, or natural gas that is produced in California and transported on a proprietary pipeline. Consumption does not include the consumption of natural gas which this state is prohibited from taxing under the United States Constitution or the California Constitution.