Well your profile says you're Petroleum Engg. student.... . Well what i'd humbly suggest is Geography. Excellent scoring subject. Physical Geography (paper 1) is a bit hard in the beginning because it seems all new but once you get the hang of it its just the same as Everyday Science. You must have studied rocks and deep earth drilling in Engg. and technically you covered the Rocks and Internal earth structure chapters already.
Human Geography (paper 2) is a general paper. Yes, you have to go through the books but not as intensively as Paper 1. You have loads of time for the 2011 papers and the course is not that much of a burden.
Paper 1 is easily covered from numerous books available and the internet. I suggest to finalize your choice talk to anybody who has passed the papers with Geography as their optionals. But if you ask me personally Geography is the way to go.

Best of luck Engineer Saib