Thermal maturation history of sedimentary rocks in the Niigata basin was estimated from the temperature history of sedimentary rocks which was determined from a paleotemperature and burial history of sediments, using Lopatin's method.
Age of each formation and restored depth at each stage which was constructed after correcting an effect of compaction of sediments, provided a burial history of sediments. A paleotemperatute in the Niigata basin has been discussed in the part one of this paper.
Time-temperature indices (TTI) were assessed from the temperature histories of sedimentary rocks on about one hundred wells. A following empirical equation was provided by plottings vitrinite refiectances (R0%) vs. TTI. log R0=0.07575 log TTI-0.2515
Estimated oil and gas generation areas at each stages from the Nanatani stage (15. M.Y.B.P.) to present were determined based on the results of the calculated maturation history.
The distribution of oil and gas fields is in accord with estimated present oil and gas generation areas. Timings of migration to several oil and gas fields in the Niigata basin are interpreted based on the history of maturation and tectonic movement. The results are following.
1) Main hydrocarbon migration occured during and after the Nishiyama stage (0.-4.3M.Y.B.P.).
2) Main hydrocarbon migration to oil fields occured at the oil generation stage (0.5%_??_R0⟨0.7%) of organic maturation levels.
3) Main hydrocarbon migration to gas fields occured at the gas generation stage (R0_??_0.7%) of organic maturation levels.