Abstract
The Nanjo R1 exploratory well, whose main target was the water-dissolved gas in the basement rock, was successfully completed with a total depth of 2,119.49 m. Results on petroleum geology of the well are summarized as follows.
1) Sandstone members of the Tomigusuku Formation (T1 to T13), which have porosity ranging from 25 to 30%, are widely distributed in subsurface of the southern part of the Okinawa-jima on the basis of well correlation using microfossils and geophysical well logs.
2) Subsurface geologic structures inferred from dipmeter data are roughly in harmony with the published gravity and geological data.
3) The source rock analysis indicates that mudstones in the Tomigusuku Formation have poor source rock potential and are immature, although the basement rock is over-mature.
4) The occurrence of hydrocarbon gases of microbial and thermogenic origins was confirmed by the analysis of headspace gas from the drill cuttings. The thermogenic gases are mainly accumulated in the basement and the T13 member.
