Evolution of the Deltaic Petroleum System in the Ulleung (Tsushima) Back-Arc Basin, East Sea, Korea

Ko, Jaehong ¹; Yoon, Junil ²
(1)Korea Institute of Geoscience & Mineral Resources, Daejeon, Korea, Republic of. (2) University of Science & Technology, Daejeon, Korea, Republic of.

The Ulleung basin was formed with Japan and Yamato basins as a result of extension behind the Japanese island-arc in Miocene. Its shelf basin was constructed by stacked deltaic sequences of more than 9 km in thickness. The basin is platformal in the northwest, and folded and faulted by thrusts in the southeast. Deformation occurred around 12.5 Ma, the timing of which is related to collision of the Izu-Bonin arc against the Japan Island. The deformed and uplifted blocks were peneplained in 6.3 Ma. Recently, N-S trending broad folds occurred in previously undeformed shelf area. Depositional sequences consist of three major transgressive-regressive-transgressive cycles, separated by unconformities of 12.5 and 6.3 Ma. Each cycle corresponds to opening and continuous subsidence, destruction of back-arc due to compression, and post-closure regional subsidence, respectively. The deltaic system consisting the depositional sequences includes coastal, delta plain, delta front, prodelta and submarine fan facies, each being distinctive in log and seismic characteristics. The coastal facies are characterized by serrated gamma log responses and discontinuous seismic reflectors suggesting fluvial formation: delta plain by association of fining-upward cycles and shale-filled distributary channels: delta front by blocky or coarsening upward sandstones: prodelta by shale dominance and low seismic impedance: and deep water by thick sandstone-shale combinations interpreted as submarine fan complexes. Only 24 wells have been drilled over the area of 12,000 km², since exploration began in early 1970’s. Numerous shows and flows were reported in most wells, but disappointedly only a small gas field of 200 Bcf reserve has been discovered and producing. Rocks having the confirmable source potential have not been found, but deep water facies and prodeltaic shale would certainly be good candidates. Sandstones of good porosity are widespread in shelfal sequences that include delta front and plain sands. In deep water facies turbiditic sandstones of reservoir quality are frequent as well. Channel-filling shale, thick prodeltaic shale, and shale intercalating provide seal. Traps are made by old NE-SW and younger N-S folds. The basin is considered to be frontier. Many structures remain untested, and numerous geophysical anomalies are observed especially in deep sections. Drilling to test deep structures would open new plays with promising reserves close to a consumer market.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.