Columbia subject headings:
Natural gas--Geology--Congresses.
Natural gas--Prospecting--Congresses.
Petroleum--Geology--Congresses.
Petroleum--Prospecting--Congresses.
Harvard subject headings:
Princeton subject headings:
Natural gas--Geology--Congresses.
Natural gas--Prospecting--Congresses.
Petroleum--Geology--Congresses.
Petroleum--Prospecting--Congresses.
Prospecting--Congresses.