Primary porosity in carbonate rocks originates from primary pore-spaces between and also within particles during deposition. Secondary porosity is a modification of primary porosity influenced by subsequent physical and chemical reactions after deposition. These reactions are compaction, fracturing, solution, cementation and inversions such as dolomitization, calcitization and so on.
Oil-bearing Neogene carbonate strata of Akita, Japan, of bathyal to inner-neritic origin, are composed mainly of small skeletal debris. During syn-diagenesis, calcium, magnesium and carbon dioxide from both the sea-water and also from basin volcanic detritus altered the originally siliceous sediments to dolostone and dolomitic limestone.
The most fundamental and significant pore-space reducing reaction in these carbonates was compaction. However, this was resisted by the presence of authigenic clay minerals. The next most important reaction tending to reduce porosity was cementation. Factors effective in increasing pore-space were inversione, such as dolomitization and transformations of opal to calcite, dolomite and quartz.

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However many quantitative experiments have been studied on determination of quantities of paraffin troubled materials in crude oils, they were not enough to decide quantitatively the troubled deposits in statical conditions. The cold point precipitation method was very convenient and practical one, but might be not precise.
In our study, consequently, the cold precipitation and filtration processes were applid at each temperature for the purpose.
In these results, troubled paraffin deposits were scarcely contained in non paraffin crude oils as Sarukawa 7(I), 37(IIa), and were less than one percent even 0°C. In comparison with these, Fukubezawa 6, 23 crude oils have 3 to 8 percents at 5°C.
These deposits were analysed by infrared spectrometer. As the results, it was recognized that the saturated paraffinic hydrocarbons might be gradually increased with lowering temperature. The other way, aromatic compounds might be decreased with lower temperature.

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The exploration works for the Nishkruosawa formation, the lowest oil bearing horizon in the Akita area, are still much inferior to the above-situated formations. It is expected, however, that the Nishikurosawa formation could be the most important target for exploration in the future.
Known surface and subsurface geological evidences show the following lithological facts. The mudstone distributes dominantly in Yuri oil producing area and Uyashinai area. The tuff, mainly composed of basaltic pyroclastics abundantly distributes in the area of western side of Lake Hachieogata and the Northern Akita area.
On the other hand, the facies of fossil Foraminifera included in these strata show some differences between areas. The deep water benthonic species and warm water planktonic ones are dominant in Yuri and Uyashinai area. On the contrary, shallow water benthonic species are dominant and planktonics are rare in the western side of Lake Hachirogata area.
That is to say, Yuri oil producing area seems to be a subsiding zone forming a kind of trough, and showing a remarkable contrast compared with the Northern Akita province which seems to be a kind of uplifting zone.
A well named Miyama AK-1 was drilled at the Northern part of Yuri oil producing area in 1971. This well penetrated the Nishikurosawa formation to the extent of 1, 000 meters, and remarkable development of acidic volcanic rocks were found in the formation. As a result of Schlumberger logging, it was recognized that these acidic rocks have high porocities and will become a good hydrocarbon reservoir.
In the last decade, many big oil and gas fields were discovered in acidic volcanic reservoir of Nanatani formation, which is assigned to the corresponding horizon with Nishikurosawa formation, in Niigata oil producing area. It is abvious that the acidic rocks in this horizon are making a remarkable contribution for acumulation of hydrocarbons.
From the above mentioned facts, the writers consider that the trough-like zone where mudstone are dominant should be the important objects for tracing the reservoir of Nishikurosawa formation. Namely, Yuri oil producing area, Akita area and Noshiro area would be the interesting areas for the oil exploration in the near future.

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