The purposes of this study are to find out relationship between geology and reservoir conditions, and crude oil properties, if any, and to reveal the direction of secondary migration of the crude oil by grasping the crude oil characteristics. Thus, our study is concerned with reservoir geology as well as petroleum geochemistry.
Minami-Aga oil field is situated in the Niigata plain, to the 250km north of Tokyo (Figure 1). The field was discovered in 1964 and is one of the largest oil fields in Japan at present. The geology and case history of the field in detail were described and presented by the Japanese Delegation to the 4th ECAFE Petroleum Symposium, held at Canberra, Austraria, in 1969. The only oil reservoir is in the upper part of the Shiiya Formation. The Shiiya Formation is composed of dark grey mudstone intercalating tuffaceous sandstone and regarded as uppermost Miocene marine sediments. The reservoir rock is poorly sorted fine to coarse tuffaceous or muddy sandstone, partly conglomeratic. The average porosity is reported to be 23%; average permeability 52-58md; average water saturation 41%.
The geologic structure consists of two anticlines, plunging to the north and bordered by the f-f' fault of north-south direction (Figure 2). The sandstone reservoir pinches out up the dip, thereby forming a combination anticlinal-pinchout trap (Figure 3). A large gas cap is found in the upper part of the reservoir, but the cap gas is not exploited. The isopachous contours of the true thickness of the reservoir are shown in Figure 4. Figure 5 shows the distribution of water saturation of the reservoir. The change in porosity is demonstrated in Figure 6. Figure 7 shows the temperature of the reservoir. From these maps we can recognize that the field is divided into three parts, i.e. A, B and C blocks. The A block is a district on the west of the f-f' fault. The C block denotes the southern wing of the east anticline, where the dip of the reservoir is very steep. The B block is a district between the A and C blocks. In the B block the dip is most gentle, and the irregularities of the various aforementioned parameters are recognized. These facts seem to have some correlation with the various crude oil properties which we will refer to afterwards.
We collected 24 degassed oil samples separately from all the producing wells. At first specific gravity of the crude oil was measured and the result is shown in Figures 8 and 9. The variation in the specific gravity of the crude from the B block is wide, whereas that of the crude from the A and C blocks has small variation and almost coincides with each other. The specific gravity of the B block crude is higher than that of other blocks, which is probably due to the remarkable change in lithology in the B block. Figure 10 is a specific gravity-absolute viscosity plot, in which a positive correlation and a wider variation of the B block are found. From the result of distillation of the crude by the Hempel method and the measurement of specific gravity of the particular cuts, we can conclude that the Minami-Aga crude is of intermediate-paraffinic base. There is the tendency that the crudes which have smaller amounts of the cut up to 300°C are from the wells situated in the anticlinal wings of high water-saturation.

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There are many "dissolved gas in water type" gas fields in Japan. Water and gas are lifted by continuous flow gas-lift in these gas fields. While the development of a field is going on, the static fluid level of gas well becomes deeper and deeper. The studies on the two phase flow incy wells usually do not make mention of submergenceand efficiency of gas-lift. Submergence and efficiency of gas lift, however, seem to be important factors on making a study of two phase flow in a gas-lift well. The purpose of this study was to determine the effects of efficiency of gas-lift and submergence on the energy-loss factor. The energy-loss factor was calculated by the method proposed by Poettmann and Carpenter. It was found in this study that the energy-loss factor could not be correlated with the submergence. The exprimental apparatus is illustrated diagrammatically in Fig. 1. The lift pipe was 2.71 meters long. The diameter was 9mm. The sand packed pipe was provided at the bottom of the vertical pipe. The use of the sand packed pipe permitted to make tests at essentially the same productivity index. Fig. 2 shows a part of the available data of the water flow rate with the air injection rate. The energy-loss factor was plotted against Dvρ in Fig. 4 and Fig. 7. From these charts it is noted that the energy-loss factor is a function of liquid flow rate and efficiency of gas-lift. Fig. 8 shows a plot of the energy-loss factor with the efficiency of gas-lift. A family of curves with liquid flow rate as parameter is drawn, as shown in Fig. 8. Fig. 9 shows the relationship between the energy-loss factor and the water flow rate. In this plot only the data that the efficiency of gas-lift are more than 10 per cent, are used. The new methods of correlation as shown in Fig. 8 and Fig 9 may be used when determining energy-loss factor of a well produced by gas-lift.

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