Abstract
The petroleum system in the deep water of the Toyama Trough to the southwest offshore of the Sado Island in the Japan Sea was re-evaluated based on the results of newly-conducted basin modeling simulation, fluid inclusion analyses of METI Sado Nanseioki wells, and interpretation of METI Sado Seiho 3D seismic survey.
METI Sado Nanseioki wells demonstrated the distribution of thick Neogene sediments in the deepwater region and confirmed a 15 meter-thick oil column in the lower part of the Shiiya Formation at METI Sado Nanseioki S. The middle Miocene Lower Teradomari Formation is thought to be a source rock of the oil. This source rock is considered to contain a mixture of Type II and III kerogens and to have good source rock potential (0.90-2.98%TOC). The bitumens include both marine and terrestrial organic matter, notably characterized by very abundant oleanane.
The GCMS analysis indicates that hydrocarbon inclusions in the Lower Teradomari Formation have the same characteristics as the oil recovered from Shiiya Formation. It implies that the hydrocarbon was temporally trapped in the Lower Teradomari Formation, and then migrated to the overlying Shiiya Formation reservoirs through faults in a very short space of time.
The basin modeling simulation results indicate the following interpretations ;
· Hydrocarbon generation has started from the late Miocene in the depression to the southwest of AWABI structure.
· Hydrocarbon is currently being generated in the depression around the AWABI and BURI structures.
· Hydrocarbon has migrated through the D location of METI Sado Nanseioki since the Late Miocene.
The re-evaluation results provide deeper understanding of the petroleum system in the area and suggest that structural traps at the Lower Teradomari Formation level located in the surrounding area would be favorable for efficient accumulation of hydrocarbons at the present day.
