Chemical analyses were mad of bluish dark grey mudstones collected at natural gas test boring well of 60 meters deep at Ashikajima, Choshi-shi, Chiba Prefecture. The followings have been clarified by geochemical consideration of the analytical results.
1. It is presumed from the relative abundance of Ga-Rb-B, absolute abundance of B (100 ppm), Clcontent in interstitial water (maximum 6g/
l), existence of pyrite that the mudstones are marine origin.
2. It is presumed that the mudstones were deposited in the sea, not very far from the land, as they contain relatively large amount of titanic minerals.
3. This is also supported by the fact that the mudstones contain high amount of organic carbon, and by the relation between organic carbon and org. C/org. N, as well as the relation between organic C and the degree of hydrocarbonization
* of the rocks.
*degree of hydrocarbonization
=
carbon in hydrocarbons (paraffin+cycloparaffin+aromatic)/
organic C _??_
hydrocarbon×0.86/
organic carbon 4. The mudstones contain 1-2.4% of organic carbon, which is fairly high value as marine mudstones.
5. The degree of hydrocarbonization of the mudstones shows 0.009-0.04 (average 0.021), which is as high as the value of mother rocks of the Niigata oil field.
6. Oil and gas showings have been observed at this well. It is considered that the oil and gas are derived from these mudstones of the Cretaceous system.
7. Gas component (in vol. %) is; He··0.020, O
2··0.05, N
2··13.43, CH
4··85.46, CO
2··0.88, C
2H
6··0.23, C
3H
5··0.04.
8. It is presumed that meteoric water invadad at the crushed part at 30m deep of the well.
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