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KOREA (SOUTH): SEDIMENTARY BASINS (source: Korea National Oil Corporation)

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Ulleung Basin

Located in the East Sea between Korea and Japan, the Ulleung Basin has 10 kilometer thick clastic sediments in its southwestern margin where the Donghae-1 gas field was discovered. The basin consists of two exploration blocks, 6-1 in the south and 8 in the north, which are 12,918 square kilometers wide and 8,481 square kilometers wide, respectively.

Block 6-1 was first explored by Royal Dutch Shell Oil in 1971. The company acquired 5,193 kilometers of 2D seismic data before drilling its first exploratory well. The well, although not tested, encountered a number of gas shows. KNOC has paid attention to Block 6-1 after 1983. Since then, it has conducted numerous seismic surveys and twenty two drillings. These efforts resulted in twelve minor discoveries of gas and one major discovery of an economically valuable gas reserve in 1998 as described above (see Donghae-1 section below for further discussion).The Donghae-1 gas field contains the late Miocene sandstone reservoirs which are believed to be deformed shallow marine sands sealed with silty shales.

After 2003, exploration reached deepwater areas, including the continental slope of the Ulleung Basin. Woodside Energy Limited and KNOC agreed to explore deepwater blocks, Block 6-1 North and Block 8, in February 2007 and Woodside signed a joint operating agreement with Korea’s Ministry of Trade, Industry & Energy. Under the agreement, KNOC and Woodside hold a 50% working interest in each of the two deepwater blocks. The project began with the acquisition of 5,107 L-kilometers and drilled an exploratory well named Jujak-1 in 2012. In May 2014, the Company and Woodside conducted 504 square kilometers of 3D marine seismic survey. In September 2015, the Company drilled one exploratory well named Hongge-1 with Woodside.



Yellow Sea Basin

The Yellow Sea Basin has three exploration blocks, 1, 2, and 3 and includes a number of subbasins that are less explored compared to the Ulleung Basin. Through exploration activities, including drilling of six wells, the company acquired 35,827 line kilometers of 2D seismic data and 298 square kilometers of 3D seismic data.

Block 1 has an area of 36,460 square kilometers. KNOC and Texaco acquired 8,520 kilometers of 2D seismic data and drilled one exploratory well (Haema-1). Nine prospects have been proposed in the block so far.Block 2 has an area of 39,433 square kilometers. Through collaboration with Gulf Oil and Marathon Oil, KNOC obtained 35,468 square kilometers of 2D seismic data and drilled four exploratory wells. KNOC and China’s CNOOC have recently agreed to exchange existing seismic data and interpretations to help each other better understand the history and the hydrocarbon potential of the basin.

Block 3 has an area of 41,427 square kilometers. KNOC and Shell carried out the initial exploration from 1970 to 1997 without drilling a well.

In spite of no successful results, the Yellow Sea Basin is still considered to have highly potential areas according to a recent KNOC study. The study integrated newly acquired 2D and 3D data with the results of remote sensing analysis of satellite images, and adopted new processing techniques for new and existing seismic data. Besides, 2D seismic interpretation coupled with remote sensing analysis focused on offshore oil slicks allowed KNOC to discover new prospects in the area and do more accurate basin analysis.



Jeju Basin

The Jeju Basin is a tertiary basin located between Korea, Japan, and China. Block 4, 5, and 6-2 to the northeast span the basin. The young sedimentary area is defined by a series of NE-SW trending half-grabens with the highs separating the Socotra subbasin from the Domi subbasin. The Socotra subbasin spans the southern part of Block 4 and the Domi basin lies in Block 6-2. The southern boundary of the Jeju Basin meets the northern boundary of the Joint Development Zone (JDZ) between Korea and Japan.

Western oil companies, including Gulf, Texaco, and Shell began exploration in blocks 4, 5, and 6-2 in the 1960s. They obtained 12,781 kilometers of 2D seismic data and drilled one exploratory well in Block 4. In Block 5, 2D data of 11,995 kilometers was acquired and four wells were drilled, and in Block 6-2, 2D data of 12,786 kilometers was gathered and three wells were drilled. Through these activities, they discovered several oil and gas shows. In 2009, additional drilling was performed in Block 6-2.

The JDZ has been less explored compared to other blocks. In 2002, KNOC obtained 19,571 kilometers of 2D seismic data and 563 square kilometers of 3D seismic data and drilled seven boreholes. Three of them discovered oil and gas shows.

KNOC and several Japanese oil companies conducted a joint study on petroleum potentials throughout the JDZ area from 2004 to 2009. The study used the existing data integrated with satellite-derived oil seepage slicks. Recent discoveries near the Chinese coastline raised the chances of the hydrocarbon potential in Jeju Basin.