ABSTRACT

Compositional changes of phenanthrene (P) and methylphenanthrenes (MP) in mudstones received burial diagenesis and contact thermal alteration were examined to test their usefulness as thermal indicators. Mudstone samples were obtained from the MITI Takada-heiya well (Neogene Takada basin, Niigata, Japan) and from outcrops of the Kusanagi formation (Neogene, Yamagata, Japan), where the thermal alteration by dolerite intrusion occurred in Miocene time. Concentrations of MP increased in the sediments deeper than the peak of oil generation in both the MITI Takada-heiya well and the thermally altered Kusanagi shales. The main formation zone of MP seems to correspond to the relatively higher maturation stage (mean vitrinite reflectance: %Rm > 1.0; JNOC, 1988) and the wet gas generation zone. Relative abundances of 2- and 3-MP to P and/or other MP isomers (MPI₁ by Radke et al., 1982a; MPI₃ by Angelin et al., 1983) increased with increasing maturity in the oil and wet gas generation zone in both the MITI Takada-heiya well and the thermally altered shales. However, the MPI vs. %Rm relationships in burial diagenesis (the MITI Takada-heiya well) and in contact thermal alteration (Kusanagi shales) were different from each other. Markedly different heating rate and temperature may be the major reasons for the difference. In the overly matured zone from 2.0%Rm to 3.0%, MPI₃ may indicate the apparent equilibrium for methyl-shift of MP.