Stratigraphic analysis of sparse seismic data in 1981 indicated the presence of a lower Miocene carbonate platform complex over the western part of the Dongsha Massif, 220 km (137 mi) southeast of Hong Kong. Two bank-edge trends were recognized, and a large structural closure within the carbonate platform was mapped along the axis of the uplift. The study also predicted the presence of a widespread, thick basal sandstone below the carbonate complex that could serve as a conduit for long distance migration of hydrocarbons into the closure. Although the prospect was a high risk play in deep water 38 km (24 mi) south of the edge of a Paleogene basin with possible oil source rocks, it was intriguing because of its size and similarity to the giant Bombay High oil field offshor India.

After drilling by other companies demonstrated that oil had migrated well out of the source basin (Huizhou sag), Amoco negotiated for Contract Area 29/04 and was awarded the area in November 1985. The first Amoco well, LH 11-1-1A, tested 2240 bbl/day of 21 degree API oil from a 75-m (246-ft) pay section in a carbonate platform sequence topped at 1197 m (3927 ft). A 401-m (1316-ft) lower carbonate section and a 149-m (489-ft) sandstone unit resting on basement were water bearing. Additional drilling has confirmed a giant in-place oil accumulation, but commerciality is yet to be determined. The carbonate complex includes a leached limestone platform facies with a reefal southwestern margin, and has excellent porosity and permeability. The oil is less waxy and heavier than other Huizhou ag-sourced oils because of biodegradation after migration.