INFILL HISTORY OF THE ULLEUNG BASIN, EAST SEA (SEA OF JAPAN) AND IMPLICATIONS ON SOURCE ROCKS AND HYDROCARBONS

Abstract

The Ulleung Basin in the East Sea (Sea of Japan) has experienced back-arc opening in the Miocene and partial closure since the middle Miocene. Uplift along the southern basin margin, caused by the back-arc closure, resulted in a series of anticlines and thrusts, providing large volumes of sediments into the basin. Mass-transport deposits dominated in the southern basin while turbidite and hemipelagic sedimentation prevailed in the northern basin. One-dimensional basin modeling shows that late postrift, early late-Miocene marginal-marine and prodeltaic sediments are buried too shallow to generate hydrocarbons. The main phase of oil generation in the synrift lacustrine and early-postrift marginal-marine facies (late Oligocene – early Miocene in age) beneath the southern basin margin occurred in the late early to early middle Miocene, predating the tectonic uplift. This significantly reduces the chances of large oil accumulations in the southern basin margin. The dominance of massive, non-uniform sediments in the southern basin, together with the lack of preferred avenues such as faults, are not favorable for the lateral migration of hydrocarbons, sourced possibly from basinal muds in the north, to the traps in the southern basin margin. Source rocks for the recently discovered gas in the southwestern shelf are probably the lacustrine and marginal-marine sediments that have been expelling gas at least since the late middle Miocene.