Minami-Aga Oil Field, Japan, as Viewed from the Crude Oil Properties (Part 2) (1970)

This paper is a sequel of our previous paper "Part 1" that appeared in the Journal of the Japanese Association of Petroleum Technologists, Vol. 35, No. 4 (1970). In this paper we discuss (1) abundance ratios of some isomers of lighter hydrocarbons that would characterize the Minami-Aga crude oil, and (2) n-heptane-2 methylhexane-3 methylhexane relationship as viewed from the crude oil evolution.At first we deal with the subject (1). In general, the abundance ratios of hydrocarbon isomers, which could characterize the crude oil from a reservoir and may have been kept almost constant during the time of secondary migration and accumulation of the crude oil, must satisfy at least the three following conditions.a) The boiling points of the hydrocarbon isomers are nearly equal.b) An abundance ratio has no correlation with the specific gravity of the crude oil and various reservoir geological parameters such as depth, thickness, porosity, water saturation of the reservoir, etc.c) An abundance ratio of the crude oils from all the producing wells of a reservoir shows nearly equal values of narrow scattering.From such a viewpoint benzene and gaseous hydros arbons should be excluded. We adopted the specific gravity of the crude and the depth, thickness, water saturation, porosity and temperature of the reservoir as the parameters against the various hydrocarbon ratios (Figs. 12, 14, 16, 17, 18, 19 and 20). Figures 13, 15 and 21 show the cases of the Kubiki (Katamachi Ie) crude (Kawai and Totani, 1970) for comparison. From these figures we can find out that such abundance ratios as (cyclopentane+2.3dimethylbutane+2methylpentane)/3methylpentane, (2.3dimethylpentane+cyclohexane)/n-heptane (Silverman, 1965) and C7 branched paraffins/n-heptane (Figs. 12, 16 and 18) are rather satisfactory. But even such abundance ratios are ill suited for the Kubiki crude. This is probably because the reservoir and geohistorical conditions of the Kubiki field are much more complicated than those of Minami-Aga field, thereby secondary alteration of crude oil has taken place remarkably in the Kubiki field. The abundance ratio of 2.4dimethylpentane/2.2dimethylpentane (Fig. 14) is see- mingly a good ratio, but by the use of this ratio we cannot draw a clear distinction between the Minami-Aga crude and Kubiki crude (Fig. 15). Some of the abundance ratios show correlations with the specific garvity of oil and some reservoir parameters. And some of the abundance ratios of the crude from the B block are more or less different from those of the A block crude and C block crude, whereas the latter two crudes show often similar tendencies in the abundance ratios. This is probably due to rather different reservoir conditions of the B block from those of the A and C blocks.Next, n-heptane-2methylhexane-3methylhexane relationship is discussed. Figure 22 is the triangular diagram. The crude oils from the 24 producing wells of Minami-Aga field are concentrated in a small part within the diagram, but the Kubiki crude occupies a different and larger part near the bottom of the triangle. The crude oils from the Ponca City field (Texas, Ordovician; Martin et al., 1963) and Khafji field (Middle East, Cretaceous) are situated near the corner of n-heptane. It is considered that the point in the diagram moves from the bottom part towards the top concurrently with the evolution of crude oils, and that the Minami-Aga crude is derived from one origin.