Should I use "Most of which" or "Some of them"?

Afghanistan is estimated to have 73 million tons of coal reserves, ... are located in the northern part of the country.

The use of “Most of which” or “Some of them” depends on the question of How many?

You are talking about the 73 million tons of coal reserves. If less than 31.5 million tons are in the northern part of the country then “Most of which” is not correct. Then you would say “Some of them”. You can understand these by thinking of 100%, 99% 51% as All (100%), nearly all or most(99%), and most (51%). Then you are left with some (49% to 1%) and less than 1% as few or nearly none.