Sequence stratigraphic framework of the coal basins in north Japan

摘要

Some recent developments in the use of well logs to recognize the earth’s orbital cycles within the Milankovitch band are described. Milankovitch orbital cycles are revealed by time series analysis of conventional well logs. Time series analysis reveals cyclic events whilst episodic events are not revealed. Cyclostratigraphy, therefore represents a method to recognize the results in the sedimentary record of orbital climate forcing sedimentary processes which may be further used as a near synchronous correlation tool across sedimentary basins. By contrast, Exxon’s third order depositional sequences are not necessarily periodic and although they are considered to represent the results of rhythmicity in sea levels in the range of the half a million to upwards of 5 million years, their use could make correlation problematic and non-synchronous. The spectral analysis of the gamma ray log over the Miocene section of the MITI Sanriku-Oki borehole offshore Hachinohe City, northeast Japan reveals well defined Milankovitch cycles in the log response of the sedimentary succession. Two spectral attributes can be generated from the power spectrum (a spectral change attribute called PEFA in this work and a spectral trend attribute -INPEFA) and used to generate near synchronous basin wide correlation. These observations and further work may represent powerful tools for prospect generation activity as the curves derived better relate geophysical well logs recorded in depth to the seismic record.