Undersaturated oil

Solution gas drive (or depletion drive) Gas cap drive Water drive with a large underlying aquifer Undersaturated oil (no gas cap) Saturated oil with a gas cap Saturated or undersaturated oil... 

On the other hand, when the black oil is above its bubble-point pressure, it is said to be undersaturated. An undersaturated oil could dissolve more gas (light molecules) if the gas were present. 

Fig. 2. Comparison of closed system gas-composition predictions (bold and dashed lines) and natural fluid compositional ranges (shaded) of undersaturated oils and condensates.

Fig. 8. Simulated extent of kerogcn transformation in the kitchen area of Snorre Field. The accumulation of undersaturated oil in the reservoir.

Only one of the Joanne wells (30/7a-3) has tested petroleum from the Pre-Cretaceous interval—an oil with a GOR of 1691 scf/bbl. Unfortunately, no analyses were undertaken on this oil due to a lack of sample material. The Chalk oils from wells 30/7a-l, 30/7a-2 and 30/ 7a-3, in comparison, have uniform GORs ranging from 2131 to 2600scf/bbl (Table 2). As discussed for the Judy structure, it is highly improbable that leakage of an undersaturated oil from the Pre-Cretaceous would result in a higher GOR fluid in the Chalk. 

Magnolia Field fluids properties, summarized in Table 1, are strikingly heterogeneous. Figure 5 illustrates this by way of a cross plot of fluid saturation pressure (bubble point and dew point pressures for oil and gas-condensate tests respectively) against measured reservoir pressure for available MDT samples. In very general terms, four fluid types may be defined. Undersaturated oils (solid circles in Fig. 5) are by far the most common. Noteworthy within this family are the two samples that have saturation... 

Fig. 5. Saturation pressure against reservoir pressure for MDT samples. The diagonal line represents saturation. Fluids are classified in this, and the majority of the figures that follow, based on reservoir phase undersaturated oils ( ), saturated oils ( ), saturated gas-condensates (A, which comprise both gas caps of the saturated oils and a shallow dry saturated gas-condensate) and undersaturated gas-condensates (O). Saturation pressures have been corrected for oil-based mud contamination. The two samples marked with an asterisk display behaviour suggesting that they are very close to their critical point.

Saturated Oils A Saturated Gas-Condensates o Undersaturated Oils o Undersaturated Gas-Condensates... 

Component distributions are in effect perfectly exponential and, presuming the saturated and undersaturated oils to be derived from a uniform source facies, suggest their derivation from source rocks of different maturity (Thompson, 2002). Slope factors (SF, defined in Fig. 13 caption) of the remaining oil samples vary between the end member values of 1.07 and 1.14 and positively correlate with other maturity indicators. In contrast, the relatively high slope factors evidenced by the gas-condensate samples (1.12-1.27) do not correlate with any of the maturity-influenced indicators discussed above, suggesting that the gas-condensates are not a high maturity product. 

The isotopic character of associated and free gases from saturated and undersaturated oil and gas-condensate reservoirs are presented in Table 3 and shown in Figure 11 (discussed above in the context of maturity). Gases show no evidence of biodegradation. Evidence of early-stage effects are the preferential removal of C3+ normal alkanes, leading to both an increase in the residual and by... 

Component profiles of the saturated and undersaturated oils (e.g. Fig. 13) do not show evidence of extensive evaporative fractionation. Residual oils formed by this process may be depleted in lower molecular weight components and deviate from the expected exponential distribution (Meul-broek et al. 1998 Losh et al. 2002 Thompson... 

Fig. 20. Log plot across one of the Magnolia pay zones showing gamma ray, lithology, resistivity, and associated gas 6 Cc, from two undersaturated oil MDT samples ( ). Calculated fluid densities at reservoir pressures are 0.639 and 0,644 g/cra for the shallow and deep sample respectively. The of the few gas...

Undersaturated oil occurs in the Cretaceous strata both above Smorbukk and Smorbukk Sor, e.g. the saturation pressure in 6506/11-2 (Smorbukk) is 434 bar with a reservoir pressure of 630 bar. 

It is, in our opinion, based on the geochemistry of the undersaturated oil in the Cretaceous Lange Formation strata above Smorbukk, a likely assumption that parts of the leaked oil from a palaeo-accumulation in Garn in Smorbukk migrated vertically by fracture-induced mechanisms into the Cretaceous strata above (cf. Fig. 7). 

The undersaturated oils in the Cretaceous sands generally occur at depths ca. 600-900 m shallower than the shallowest portion of the Jurassic reservoir strata, and are found in overpressured strata. The overpressure today at the Lysing level above Smorbukk is ca. 140 bar, and above Smorbukk in well 6506/11-2 the saturation pressure is 434 bar at a reservoir pressure of 630 bar. Above Smorbukk Sor the... 

Jones (1988) reports temperature data during production testing. The data show an increase in bottomhole temperature during flow. The temperature rise from several fields was of the order of 2 to 5°C. All these reservoirs had an undersaturated oil. Figure 3.35 shows a typical downhole pressure and temperature profile during a production test. Jones suggests that the interpretation of temperature fall-off may exhibit the same features as seen on the pressure buildup. Therefore, temperature data may be a useful source of information that can supplement the pressure data. 

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