The recent growth of hydraulic fracking has made compressed natural gas (CNG) a viable option for fueling the United States transportation sector in a dual fuel scenario with ultra low sulfur diesel (ULSD). To clarify literature trends, this effort investigates the influence of CNG speciation (methane, ethane, propane, and isobutane) by employing energy substitution rates (ESR) of 7%, 18%, and 40% (approximately) to prevent significant changes to ignition delay. Results demonstrate that maintaining hydrocarbon (HC) constituents within typical global levels has no noticeable bearing on the findings; however, added CNG does noticeably change the peak rate of heat release. Overall, nitrogen oxides (NOx) and particulate matter emissions remained lower than ULSD values except for the highest load and ESR rate when NOx increased due to a significant growth of the premixed combustion phase. At all ESR values, methane and nonmethane hydrocarbons increased subsequently leading to a decrease in combustion efficiency.