Columbia subject headings:
Gas fields--Congresses.
Natural gas--Geology--Congresses.
Oil fields--Congresses.
Petroleum--Geology--Congresses.
Harvard subject headings:
Gas fields--Congresses.
Natural gas--Geology--Congresses
Oil fields--Congresses.
Petroleum--Geology--Congresses
Princeton subject headings:
Natural gas--Congresses.
Oil fields--Congresses.