Identify this citrus?

This citrus tree. What is it? Tell me, please!

These are small round hard oranges, perfectly round, 5 cm diameter (2 inches). The peel is enormously aromatic. A mix of orange and vanilla.

(Grand Marnier beware, if I get this tree to fruit..)

Yellow leaves, lacking fertilizer.

Thin branches, with razor sharp thorns all over! Not pee-ing near this one..