Hydrocarbon Potential in the Southwestern Margin of the Ulleung Basin, East Sea (Sea of Japan), Offshore Korea
The structural and stratigraphic framework of the southwestern continental margin of the Ulleung Basin in the East Sea (Sea of Japan), Korea is recognized from a 3D seismic survey to evaluate the remaining hydrocarbon potential. The Korean continental margin is divided mainly into the two different segments: the Dolgorae and Gorae areas. The Gorae’s seismic stratigraphic framework has been increasingly important, since a commercially viable gas reserve was discovered from an exploration, and two appraisal wells drilled in 1998 and 1999, respectively.
The structural evolution of the Gorae area is associated with the back-arc closure of the East Sea. After rapid back-arc extension in the early Oligocene, the continental margin has experienced two phases of contractional deformation. The first phase began in the middle Miocene, led to tectonic uplifting of the Dolgorae area, and was followed by late Late Miocene uplifting that caused the Gorae area to be deformed by contractional and transpressional strike-slip activity. As a result of the postrift deformation, faulted anticlinal folds are formed in the structurally younger area.
The Gorae area contains more than 6,000 m of clastic basin-fill sediments ranging from Late Paleogene to Tertiary in age. The synrift and postrift sediments in the basin margin are characterized by a transition from nonmarine to shallow marine depositional environments, and the transitional units are affected by the subsequent uplift movements. The older strata are composed of the Late Oligocene-early Miocene sediments deposited in fluvio-deltaic settings, whereas the younger strata contain the early Late Miocene-Quaternary sediments accumulated in the shoreface and inner shelf.
Seismic data interpretation integrated with available well logs suggests that hydrocarbon potential exists in the deformed sedimentary section in the Gorae area. The tectonic-induced structural traps may have been formed prior to the main phase of hydrocarbon generation and a cluster of seismic gas chimneys with amplitude anomalies indicates shallow gas sands charged by gas migrating from a deeper level. The migration of hydrocarbons to the traps may occur vertically through faulted anticlinal structures because the source rocks (probably basin muds) are assumed to be localized by non-uniform, massive mass-transport deposits. The generated hydrocarbons may also be trapped in untested, older and deeper reservoirs underneath the proven gas reserves.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009