The reservoir is found in the condition of equilibrium between the gravitational and capillary forces. Then, the individual rock and fluid characteristics of the reservoir control the distribution of various fluids in the reservoir and the height of transition yone. Generally thers is no sharp boundary line between oil and water level or between gas and water level in the reservoir. In Shin-Tainai gas field and Tainaigawa area, Kita-Kambara Plain, Niigata Prefecture, several reservoirs have deposited at Shiiya stage, Miocene. It is shown that each members encountered different gas-water contact levels. Provided the hydrodynamic conditions do not exist there, such different or tilted contacts may be the result of change in rock characteristics (permeability and porosity) which affect the capillary pressure of the reservoir and the height of the transition zone. As the porosity and permeability of reservoir become lower, the thickness of the transition zone becomes larger due to the higher capillary pressures. In fact, core-derived capillary pressure measurement data can not be available in Shin-Tainai gas field, but it was shown that water distribution as determined from electric logs and capillary pressure measurement are normally in good agreement. Approximate position of the gas-water contact can be determined from other data, then the height of transition zone can he estimated. It is obvious that amount of structural closure for water-free production must be in excess of vertical height of transition zone. At Shin-Tainai NS-4 well, II horizon (upper Shiiya formation), composed of very fine sand and shale alternation, has extremely low porosity and permeability. In spite of its relatively higher level (below sea level, -1, 700_??_-1, 750 meters) and apparent cycle skipping on sonic log, considerable amount of water produced from this horizon. According to the water saturation versus vertical heights plot, the capillary pressure is estimated to generally high and the height of transition zone is abnormally thick (about 80_??_100 meters). Although the total amount of closure of this horizon is about 200 meters, the vertical height from the spill point to the uppermost position of this reservoir of this well is only 90 meters below, water-free production can not be expected. At the same well, IV horizon (middle Shiiya formation), composed of coarse to medium sand and shale alternation, has a range of porosity from 20 to 25 percents. Water saturation versus vertical heights plot shows the lower capillary pressures and the thinner transition zone. Although this horizon is encountered at relatively lower level (below sea level, -1, 900 meters ±), it is natural that large amount of free gas column exist there, because this horizon has same amount of closure as II horizon mentioned above. Other field examples were shown for the understanding of the relation between the distribution of various fluid and capillarity in the reservior.
Considerations of upward and downward migrations of fluid in the subsurface are discussed in this paper. When shales are deposited and compacted, most compaction current expelled from them seems to go upward, because the shallower the depth, the more porous or permeable are the shales in the normal condition. However, if a permeable sandstone reservoir exists below the shales, some amount of the downward compaction current from the closest parts of shales to the permeable sandstone would occur. Although the amount of the downward compaction current in this case is relatively small, it could be more important than the upward because its efficiency for the formation of hydrocarbon accumulation seems to be larger. The thicknesses of the upward and downward fluid movement zones in this simple situation are related to the minimum permeabilities in both zones, and can be determined by using Athy's porosity-depth curve as the standard relation under compaction equilibrium condition, and Kozeny's and Archie's functions relating porosity to permeability.