| Markwest Liberty Midstream & Resources, LLC | Owner/operator | Denver, CO | 4 | 11 |
| Markwest Liberty NGL Pipeline, L.l.c. | Owner/operator | Denver, CO | | 1 |
| Markwest Michigan | Owner/operator | | | |
| Markwest Michigan Pipeline, LLC | Owner/operator | Denver, CO | | 2 |
| Markwest Oklahoma Gas Company, LLC | Owner/operator | Denver, CO | | 65 |
| Markwest PNG | Owner/operator | | | |
| Markwest PNG Utility, L.l.c. | Owner/operator | Denver, CO | | 1 |
| Markwest Panola Util Co, L.l.c. | Owner/operator | | | 12 |
| Markwest Pinnacle LP | Owner/operator | | | |
| Markwest Pinnacle PNG Utility, LLC | Owner/operator | Denver, CO | | 1 |
| Markwest Pipeline Company, L.l.c. | Owner/operator | Denver, CO | | 2 |
| Markwest Power Tex, L.l.c. | Owner/operator | Denver, CO | | 1 |
| Markwest Ranger Pipeline Company, LLC | Owner/operator | Denver, CO | | 1 |
| Markwest Texas PNG Utility, L.l.c. | Owner/operator | Denver, CO | | 1 |
| Markwest Utica Emg, L.l.c. | Owner/operator | Denver, CO | 1 | 1 |
| Marlin Midstream, LLC | Owner/operator | Dallas, TX | | 3 |
| Marshall County Gas District | Owner/operator | Guntersville, AL | | 1 |
| Marshall Municipal Utilities | Owner/operator | Marshall, MO | | 1 |
| Marshfield Utilities | Owner/operator | Marshfield, WI | | 1 |
| Martin Energy Services LLC | Owner/operator | | | 12 |
| Martin Midstream Partnership LP | Owner/operator | | | 1 |
| Martin Operating Partnership, L.p. | Owner/operator | Kilgore, TX | | 11 |
| Martinez Pipeline Company LLC | Owner/operator | Parsippany, NJ | | 1 |
| Martinez Terminal Company LLC | Owner/operator | | | 1 |
| Maruichi Leavitt Pipe and Tube | Pipe mill | | | |
| Marysville Hydrocarbons LLC | Owner/operator | | | 1 |
| Mas Energy, LLC | Owner/operator | | | |
| Mas Ranger, LLC | Owner/operator | | | |
| Mas Shreveport RNG LLC | Owner/operator | Atlanta, GA | | |
| MasTec, Inc. | Contractor | Coral Gables, FL | 1 | |
| Mascaro Construction Company, LP | Contractor | Pittsburgh, PA | 1 | |
| Mason Construction | Contractor | | 1 | |
| Massachusetts Municipal Wholesale Electric Company | Owner/operator | Ludlow, MA | | 1 |
| Massachusetts Wholesale Electric Co | Owner/operator | Ludlow, MA | | 1 |
| Masters Resources LLC | Owner/operator | | | 1 |
| Matador Production Company | Owner/operator | Dallas, TX | | 2 |
| Matador Resources Co. | Owner/operator | | 1 | |
| Material Producers Inc | Owner/operator | | | 1 |
| Matheson Tri-Gas, Inc | Owner/operator | Bayport, TX | | 1 |
| Matriks Energy LLC | Owner/operator | Ashland, KY | | 1 |
| Matrix Oil Corp | Owner/operator | La Habra Hegihts, CA | | 2 |
| Maurepas Pipeline, LLC | Owner/operator | | 2 | |
| Maverick Natural Resources | Owner/operator | Birmingham, AL | | 1 |
| Maverick Permian LLC | Owner/operator | | | 1 |
| Maverick Terminal Three Rivers, LLC | Owner/operator | San Antonio, TX | | 2 |
| Maverick Terminals Corpus, LLC | Owner/operator | San Antonio, TX | | 2 |
| Maverick Tube Conroe | Pipe mill | | | |
| Maverick Tube Hickman | Pipe mill | | | |
| Maverick-Dimmit Pipeline LTD | Owner/operator | | | |
| Max Midstream Texas, LLC | Owner/operator | Houston, TX | | 17 |