Hello.
according to the basic eligibility criteria to take admission in Bsc geology a candidate need to pass his/her class 12th with minimum aggregate 55%  with necessary subjects physics, chemistry and  biology/ or mathematics but this is not the only eligibility criteria as you didn't mention the name of university or college where you are going to take admission so it is quite difficult to give you all the details because majority of University take admission on the basis of entrance exam and some University take admission on the basis of your class 12th score merit also this eligibility percentage in 12th may vary according to the university.
Best of luck.