The lexicon of the top500 ( The Encyclopedia for Classification Codes ) explains the codes and definitions in the world of economical classifications.
NACE Code 11.1 Extraction of crude petroleum and natural gas
The NACE Code 11.1 is described as
Extraction of crude petroleum and natural gas
It belongs to the code group
NACE Code 11 Extraction of crude petroleum and natural gas; service activities incidental to oil and gas extraction, excluding surveying
To this Code "11.1" belongs the sub codes
NACE Code 11.10: Extraction of crude petroleum and natural gas