Tectono-Stratigraphy of the Ulleung Basin, Offshore Korea and Its Implications for Hydrocarbon Exploration

Byeongoo Choi, Booyong Kim, Ilsoo Kim, Yongsu Lim, and Byeongwook Yoo
Korea National Oil Corporation, Anyang, South Korea

Sediments of the Ulleung basin are summarized into four tectono-stratigraphic units after evaluation of 2-D, 3-D seismic and well log data. The basin was created by back-arc rifing as a result of subduction of the Pacific plate beneath the Eurasian plate. The initial age of rifing is expected at Oligocene by analysis of the basin evolution. Petroleum system analyses imply the syn-rifting unit as a main source of the discovered gas in the basin. Subsidence was followed after the syn-rift stage during middle Miocene and mature drainage systems including open marine environment were developed. Sediments were mainly supplied from southwest, which is different from the syn-rift unit characterized as various supply directions coming from surrounded paleo-high areas. Clastic shelf sandstones and deep marine submarine fans are main reservoir rocks deposited during this stage. The closing of the basin can be subdivided into two stages: the first is characterized by predominant compression (closing I) and the next is wrenching movements during Pliocene (closing II), which might be caused by tectonic force from Eurasian plate. Compression provided regional uplift of southeastern parts of the basin and generated regional growth strata, pinch-out toward southeast and down lapping on the top of 12.5 Ma sequence boundary. The wrench movements rejuvenated the existing NE-SW thrust faults and derived several en echelon folds. Two gas discoveries are coming from the combination traps associated with the folds cutting by shale filled channels. Tectono-stratigraphic units are distinguished and useful for the hydrocarbon exploration in the basin.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005