Great question Tusker:
MY UNDERSTANDING:
1P is Proven Reserves - 90% probability of being in place.
Other categories are 2P and 3P
Official Business Definition of 2P: "Quantity of energy sources estimated with reasonable certainty, from the analysis of geologic and engineering data, to be recoverable from well established or known reservoirs with the existing equipment and under the existing operating conditions."
These reserve categories are totalled up by the measures 1P, 2P, and 3P, which are inclusive, so include the previous safer measures as:
"1P reserves" = proven reserves (both proved developed reserves + proved undeveloped reserves).
"2P reserves" = 1P (proven reserves) + probable reserves, hence "proved AND probable.
"3P reserves" = the sum of 2P (proven reserves + probable reserves) + possible reserves, all 3Ps "proven AND probable AND possible
2C Contingent Resources are not yet Reserves.
2C is in general equivalent to 2P.
TAKE A LOOK AT THE ATTACHED PRESENTATION PACK FROM GAFFNEY CLINE & ASSOCIATES
It's the best summary I've come across.
Perhaps Petrengr1 can summarize it in less than the 40+ slides it took GCA????