HOUSTON  - A second shipment of U.S. light sweet crude oil was headed to Nigeria on Friday, according to a trader and ship tracking data, as the Dangote refinery ramps up crude purchases.

Vessel Otis, which loaded about 850,000 barrels of WTI Midland in Houston last week, was signaling Lekki in Nigeria as its destination, according to Kpler and LSEG ship tracking. It was estimated to reach the country around mid-March.

The vessel was chartered by Vitol, according to data sources and a trader.

The first shipment of 2 million barrels of U.S. crude on the very large crude carrier Gem No. 1, also chartered by Vitol, was nearing Nigeria and is expected to discharge next week.

The refinery, Africa's largest with a nameplate capacity of 650,000 barrels per day, was built on a peninsula on the outskirts of the commercial capital Lagos by the continent's richest man, Aliko Dangote.

The refinery began buying crude in December last year and Nigeria's state-owned oil firm NNPC Ltd has been the main supplier.

The refinery had issued tenders to sell two fuel cargoes for export, a tender document showed and trading sources with knowledge of the matter told Reuters earlier this month.

(Reporting by Arathy Somasekhar in Houston and Ahmad Ghaddar in London, editing by Deepa Babington)