noun
offshore oil field
Refers to an oil field located under the seabed, typically exploited by offshore drilling platforms.
この海域には大規模な海底油田がある。
There is a large offshore oil field in this sea area.
油田 is a general 'oil field', while 海底油田 specifically refers to one under the sea.
Compound of 海底 (seabed) and 油田 (oil field).