Columbia subject headings:
Diversification in industry--United States--Case studies.
Petroleum industry and trade--Mergers--United States.
Harvard subject headings:
Diversification in industry--United States--Case studies.
Petroleum industry and trade--Mergers--United States.
Petroleum industry and trade--United States--Consolidation.
Princeton subject headings:
Diversification in industry--United States--Case studies.
Petroleum industry and trade--United States--Consolidation.