Abstract

The Aga-Oki field, located in the Japan Sea, was discovered in 1972 and began to produce oil and gas in 1976.

The field is thought to be one of the exceptionally large fields in Japan. The discovery of this large scale offshore field is very significant for Japan, which is poor in energy sources and is surrounded by seas.

Air gun seismic surveys conducted at three different times, clarified that the field consists of two parallel anticlines buried below a dome situated on the southern end of a meridionally extended basement uplift. Six exploratory and delineation wells confirmed the presence of seven pay zones; four in the lowest Pliocene and three in the uppermost Miocene. Of these seven zones, only one produces oil and the others produce gas and condensate. All the reservoirs are sandstones, which are so thinly alternating with shale that most log interpretation efforts are strongly affected by shaliness. Therefore, formation evaluation was very complicated.

In actual well log analysis of development wells, satisfactory results have been obtained by using special values for m (cementation factor) and n (saturation exponent value).

Originally these parametric values were empirically calculated to compensate for the difference between the results derived from log interpretation and results from core analysis and production tests. These values have also been supported by a method of studying logical parameters using log-derived values only — the method is newly developed in this field.

On the other hand, it has been known that the product of ϕ (porosity) and sw (water saturation), indicates a constant value in certain reservoir conditions.

The new interpretative results, obtained in this field by using the special values of m and n, described above, have also indicated a clear relation of Φ.Sw = C, in irreducible water saturation conditions.

Therefore, water free hydrocarbon productive intervals in development wells, have been successfully detected by Φ vs Sw plots which are based on the fundamental identification patterns established through the results of log analysis and production tests of wildcat wells.