CO2 sequestration in deep sedimentary formations of the southwestern margin of the Ulleung Basin, offshore, east sea, Korea

摘要

A theoretical CO2 storage capacity of the southwestern margin (ca. 13,000 km²) of Ulleung Basin, offshore Korea is estimated using the relatively large amount of exploratory data including 2D (ca. 8,500 L-km) / 3D (ca. 2,400 km²) multi-channel reflection seismic data integrated with twenty-three (23) well bore data that had been acquired in the area through exploration activities over the two decades since the late 1980s. The study area is located between the Korea Peninsular to the west and the Japanese Arc to the east, and it is divided into two distinct regions: one is a tectonically deformed area (Dolgorae) and the other is a relatively less influenced area by the plate motions (Gorae) during the Tertiary. The KNOC (Korea National Oil Corporation) has discovered several discoveries in the areas one of which is a commercially producing gas field (Dong-Hae 1). The study area is believed to be evolved as a rifted basin margin during the Oligocene followed by a rapid thermal subsidence until the early Miocene, accumulating alluvial/fluvio-deltaic sediments across the entire area. During the middle Miocene, the subsidence was ceased by a tectonically induced compressional stress from both the south by the Philippine plate and the east by the Pacific plate, producing faulted and folded structural features which are the main targets for CO2 sequestration as well as hydrocarbon exploration. The uplifted areas provided relatively long-lasting sediment sources contributing to the formation of a continental-shelf-to-slope system that reaches a maximum thickness of 10 km. The thick sediment filling is the focus of the study. The prestack time-migrated 2D/ 3D seismic data were interpreted using a conventional seismic interpretation technique to identify sequences bounded by regional unconformities above and below throughout the Gorae area within a limited depth of 2,200 m, considering both sealing capability above 800 m and tightness of the - ormation below 3,000 m. The seismic interpretation results in five different seismic units, although ten sequences are recognized, in the time domain that are converted into the depth domain using an average velocity from checkshot surveys conducted in the area. To estimate possibly effective CO2 storage volumes, the depth-converted seismic sequences are limited in area of the shelf margins prior to incorporation of the other required parameters that include porosity and permeability from the borehole data, and theoretically inferred CO2 density with depth. In addition, an efficient factor defined by a depositional environment (i.e., continental shelf) which is, for the first time in Korea, used for the greenhouse-gas storage capacity calculation. The factor was evaluated by comparing levels P10, P50, and P90, representing 2.2%, 5.5%, and 13.7%, respectively, and the corresponding accumulated possible storage volumes in the study are 2,053 Mton, 5,131 Mton, and 12,782 Mton. The theoretically computed potential volumes will be effectively revised not only because the entire area is investigated in the next phase but also because more variables will be considered. The approaches made in this study will be applied to the other continental shelves (i.e., Yellow Sea and northern part of East China Sea) around the Korean Peninsula.