I completed my btech as mechanical engineering but I want to do MS as petroleum engineering or Marine engineering is it possible
Yes, you can surely pursue your MS in Petroleum engineering or Marine Engineering after your B.Tech degree as a Mechanical Engineer. All the UK universities which offer this degree are used to people entering with this qualification. However, it helps if you have a basic understanding of geology, and/or have some experience in the industry. You will typically find the maths and computing easier than someone with a geology degree, but have to work harder and faster on the geology. Generally speaking, all students will find some subjects new to them and will have to work harder at these topics. However, some other students will have experience and will help if you let them (and offer your help in return for others). And the staff should be used to teaching the mixed class.
Hope this will be helpful...
Yes it is possible to do MS in petroleum or Marine Engineering for Mechanical Engineering Students.