Is there an estimation how much organic matter there is still in the ground?
There are estimates produced my the major coal, gas and oil producing nations, but there are large error bars for some of them.

Wikipedia has oil, gas and coal split down by country - well worth a read of this or the CIA World Factbook, especially on the description of the various categories of oil reserves (Proven and Unproven and the various estimation methods used to work out how much is actually likely to be retrievable) but in summary:


World Coal Reserves - 909,064,000,000 tonnes
World gas reserves -  175,400,000,000,000 cubic metres
World oil reserves -  1,324,000,000,000 barrels


Now these numbers may not mean very much unless aligned with production and usage rates.