1. A method for fracturing a formation in a reservoir using a fracturing fluid mixture comprising natural gas and a base fluid, the method comprising: (a) determining certain reservoir conditions of the reservoir comprising the formation; (b) selecting a desired phase behavior of the fracturing fluid mixture during a fracturing operation in the formation under the determined reservoir conditions, (c) determining properties of the fracturing fluid mixture required to achieve the desired phase behavior, the determined properties including a composition of base fluid in the mixture and a mole fraction of natural gas in the mixture; (d) preparing the fracturing fluid mixture having the determined properties; and (e) injecting the fracturing fluid mixture into the reservoir under the determined reservoir conditions, such that the mixture exhibits the desired phase behavior during the fracturing operation.