Timing of trap formation in the southwestern margin of the Ulleung Basin, East Sea (Japan Sea) and implications for hydrocarbon accumulations

Contractional tectonism, associated with the back-arc closure of the East Sea (Japan Sea), resulted in a series of thrust and anticline along the southern margin of the Ulleung Basin, providing numerous traps that include the Dolgorae and Gorae structures. Restoration of depth-converted seismic profiles suggests two phases of contractional deformation in the area: (1) the latest Early to late Late Miocene phase that created the Dolgorae structures and (2) the late Late Miocene to Quaternary (?) phase that created the Gorae structures. The second phase of contractional deformation was accompanied by transpressional strike-slip movements. The formation of the Dolgorae H and Gorae structures postdated the main phase of oil generation but overlapped and/or predated the main phase of gas generation. The Dolgorae I and III structures began to form during the later stage of oil generation but did not have significant relief until the beginning of the main phase of gas generation. Thus, potential for large oil accumulations in the area is probably limited. Older and deeper structures, predating the contractional deformation, may have accumulated oil if later tectonic movements had not disrupted their trap integrity.