Shelf Margin Deltas and Associated Deep-Water Deposits: Implications on Reservoir Distribution and Hydrocarbon Entrapment, Block VI-1, Ulleung Basin, East Sea, South Korea

Abstract

Block VI-1 covers 13,000 km 2 (3.2 million acres) in the southwestern part of the Ulleung Basin in water depths from 30 to 1,700 meters (100 to 5,500 feet). Approximately 20 wells have been drilled on the block since 1973. The bl ock has 12,865 linear km (8,000 miles) of 2D seismic data and 1,500 Km 2 (580 miles 2 ) of 3D seismic data. The block has three discoveries in Miocene Aged sediments. One of these discoveries, in a shelf edge delta, is currently under development. The majority of Miocene to Recent sediments in Block VI -1 are shelf edge deltas and their associated deep water /slope deposits. Progradation of the shelf margin began in a northeastern direction and migrated to a northward direction as tectonic activity increase d in the basin (Chough et al., 1997). Three distinct periods in the overall progradation relate to different tectonic periods: back-arc extension (16-12 Ma) associated with rapid basin subsidence, back -arc closing (12-6.5 Ma) associated with uplift in th e disturbed zone and subsidence in the undisturbed zone, and continued back-arc closing (6.5 Ma – Present) characterized by slow subsidence in the basin. The reservoirs and hydrocarbon accumulations in the Ulleung Basin occur in shelf edge deltas and deep water sandstones-conglomerates. The shelf edge delta sandstone reservoirs are generally coarser grained and lower in porosity than age -equivalent deep water sandstones. Shelf edge delta traps form by a combination of recent regional warping and stratig raphic pinch out against shale-filled canyons. Deep water sandstones tend to be finer grained than the shelf edge sandstones, but coarse grained deep water sediments are observed. Porosity of the deep water sandstones is generally higher than the shallow er water facies, but permeability is often lower. Observed hydrocarbon accumulations in the deep water facies occur in structural closures or truncation traps in areas of extreme structural deformation.