Bombay High refers to

Concept: An oil field is classified primarily by two criteria: its physical location — an offshore field sits under the seabed and is worked from platforms in open water, while an onshore field is drilled from rigs standing on land — and its dominant hydrocarbon output — an oil field's primary product is crude oil, even when some natural gas is produced alongside it as associated gas, whereas a gas field's development target is free natural gas.

Application: Bombay High is a hydrocarbon structure on the continental shelf of the Arabian Sea, about 176 km off the coast of Mumbai. ONGC has developed and operated it as a subsea field since the mid-1970s, with production drawn from platforms anchored offshore rather than from any land-based rig. Its wells target crude oil as the primary output; the natural gas recovered alongside it is associated gas released as a by-product of oil extraction, not the structure's development objective. Both criteria — location and dominant output — identify it as an offshore oil field.

Cross-check: Checking this against the other classifications offered:

• an onshore oil field fails on location: Bombay High is not drilled from land; it is a subsea structure worked from platforms out at sea.

• an offshore gas field fails on dominant output: its wells are developed for crude oil, with gas only an associated by-product, not the primary target.

• an onshore gas field fails on both counts: it is neither land-based nor developed primarily for gas.

Only the classification that matches both the location and the output criteria together — offshore and oil — correctly describes Bombay High.