坑井における古流系の研究

抄録

It may be said that occurrence of sandstones should be controled by paleocurrents as the sandstones are sediments deposited from currents which carried them. But it is not so easy to clarify paleocurrents in case of subsurface as in case of cropping areas.
Then, the writers intend to use data from densely calculated continuous dipmeter for paleocurrent analysis under ground.
The method for paleocurrent analysis which they use is based on an assumption that dips in mudstones show the geologic structure. The procedure is as follows: 1) decide the structure dip by averaging dip records in mudstones, 2) restore dip records in sandstones to the initial condition, that is the condition of the structure dip being horizontal, by using Wulff's stereo-net, 3) consider the azimuth of initial dips in sandstones shows the direction of paleocurrent which deposited the sandstones.
The writers applied the method on the Shiiya formation in Shintainai and Hirakida gas fields of the Kitakambara plain, Niigata, and got conclusions as follows; 1) there were northward and southward currents in the Shintainai area, the former transported much more sands than the latter and became stronger as time passed, 2) there was a depressional embayment in the Hirakida area, and current, which had much sands and flowed south to north in open sea west of the embayment, passed through a break of submarine ridge between the depression and open sea and flowed into the depression, where sands were deposited as fan like shape.