Abstract
Investigating the organic constituents of mudstone over 700 samples, the writer ascertained that the characteristics in the organic matter of rock is closely related to the oil producing capacity of formations.
To determine the characters of mudstone in organic matter, the measurements used in this study are follows: Amount of extracts obtained by thermal extraction; hydrocarbon and non-hydrocarbon contents of extracts are determined by chromatography; calculated hydrocarbon contents of rocks (HyR) and ratio of carbon in hydrocarbons to total organic carbon (Ch/Co). Since the latter two coefficients, HyR and Ch/Co, are suitable to represent the characteristics of organic matter in rock, the writer tentatively adopted them as the indices of the oil generation capacity of rocks.
The values of HyR and Ch/Co of mudstone samples from the productive formations (Niigati) are 15 ppm and 0.0015 in the minimum, 1, 500 ppm and 0.20 in the maximum, averaging 208 ppm and 0.0208. In the productive formations, the mudstones occuring adjacent, both stratigraphically and horizontally, to oil reservoirs, show higher values of HyR and Ch/Co than those of the corresponding horizons apart from reservoirs. The mean values in non-productive (Kanto and Miyazaki) formations measure 36 and 30 ppm in HyR, 0.0041 and 0.0043 in Ch/Co. Thus in productive formations, these values are strikingly higher than in non-productive formations. This relation may have been resulted from the difference in depositional environments of the formations. The mudstones of non-productive 'formations were deposited in the sea where the bottom current was prominent and the oxidation was active due to the condition open to the ocean, whereas those of productive formations were deposited under the reductive environments without prominent bottom current and flanked by emerging zones.
