If I do mechanical engineering, can I do master or phd in petroleum engineering after?

IN THE US, YOU CAN. HOWEVER, YOU MAY BE REQUIRED TO TAKE A FEW PREREQUISITES TO KNOW SOME FUNDAMENTALS IN PETROLEUM ENGINEERING. SUCH AS SEPARATION TECHNIQUES AND SUCH. A SMART WAY TO GO ABOUT THIS, IS TO ASK EARLY. TRY TO CALL ANY UNIVERSITY THAT OFFERS PETROLEUM ENGINEERING AND EXPLAIN YOUR SITUATION AND SEE WHAT THEY SAY. BASED ON WHAT THEY TELL YOU ABOUT WHAT CLASSES YOU NEED, YOU CAN PICK UP SOME COURSES FROM CHEMICAL ENGINEERING( USUALLY THE CLOSEST TO PETROLEUM OUT OF ALL ENGINEERING, IF YOUR UNIVERSITY HAS CHEMICAL ENG. DEPARTMENT) AS YOUR ELECTIVES AND THAT WOULD PRETTY GOOD SHORTCUT. ACTUALLY YOU MAY WANNA TRY MORE THAN ONE. EACH UNIVERSITY HAS DIFFERENT APPROACH.