Abstract
METI “Sado Nanseioki” wells were drilled in the deep water southwest offshore of the Sado Island in the Japan Sea to explore the Awabi Structure after the MITI “Sadooki Nansei” seismic survey.
Along with the presence of Direct Hydrocarbon Indicators around the crest of the structure, methane hydrate and oil seepage were recovered during the site survey prior to the drilling, which indicate an existence of the active petroleum system.
METI “Sado Nanseioki” wells proved a distribution of the thick Neogene sediments in the deep water and discovered a 15 meter-thick oil column in the lower part of the Shiiya Formation. Detailed paleontological, geochemical and fluid inclusion analyses were carried out, and resulted in a reliable correlation to the Niigata standard stratigraphy and understanding of hydrocarbon history of the structure.
A severe truncation surface revealed by the seismic survey was identified within the upper part of the Lower Teradomari Formation at about 8.5 Ma. Structural configuration of the Awabi Structure varies greatly between above and below the truncation surface. Above it, the structure is an asymmetrical anticlinal structure accompanied by a reverse fault on the western flank. Below it, the structure is an N-S trending nose structure. Structural growth commenced in the southern part during the time of the Shiiya Formation, and culminated after 1.3 Ma with growth center shifting north- and northeast-ward.
A close genetic relationship was proved between oil, hydrocarbon shows, methane hydrate and oil seepage at sea bottom. Hydrocarbon generation in the Lower Teradomari or Nanatani Formation, temporal trap in the Lower Teradomari Formation, migration into the Shiiya Formation and leakage to the sea bottom through many small normal faults at the crest was clearly interpreted.
The MITI seismic survey and the METI wells provided important insight about the petroleum system of the deep-water part of the Japan Sea.
