There are a total of 3 large oil rigs found in Venezuela.
The first rig is found in the middle of Lake Maracaibo. This particular rig is where Blanco holds his meeting which the mercenary must intercept and capture Blanco. The second rig is found southwest of the Pirate Isles. There is also a High Value Target here. The third and final rig is found southwest of Margarita. This one is occupied by the Allied Nations and is a Chinese target to destroy.
There are also many smaller and numerous oil mining derricks set up in lakes and on land that are used by Universal Petroleum. The derricks found in the water are usually guarded by mines and UP Warhorse Patrol Boats.
*In the PS2 version, there is only one oil rig while there are three in the Xbox 360/PS3 version.