With around 4,000 offshore structures and 15,000 workers "camping out" offshore each night, there are plenty of aircraft needed.
A guess would be 1,000+ pilots in the GOM with PHI the largest followed by AirLog at just under 300 pilots, and ERA making the big three. There are a couple handfull of other operators: Omni, Houston, Republic, Evergreen that provide serice to the oil & gas companies and a few oil companies operate their own fleets of varying size from 3 or 4 to 10+: Taylor, Exon, Chevron, El Paso, etc.
There are bases from the SW Texan coastline through to the eastern end of Louisiana with several companies operating out of Galveston, TX, Sabine,LA Cameron, LA, Morgan City, LA,m Intracoastal City and Venice, LA. Most companies are headquartered in LA.
That should get you started and there's plenty of GOM people here who can answer more specific questions.
Im not a Gomer yet, but hoping to head that way next month.
Regards,
James