If you have a place where you stay at the oil field, then you are not regarded as being a traveller once you get there, rather you have to fast in your city and in your workplace, and you are a traveller only for the distance between them when you are coming and going. 

In this case the ruling is like that on one who has two wives in different cities; he is known as saahib al-iqaamatayn (one who has two places of residence). So he has to offer prayers in full and fast in both places, and he may break his fast and shorten his prayers when travelling between them. 

If you do not have a place of residence at work at the oil field, rather it is only work, then you are a traveller and it is permissible for you not to fast when you are at work, until you come back, but you still have to pray. 

And Allaah knows best.