Some Geological Controls on Oil Composition in the U.K. North Sea

Abstract

Summary The U.K. North Sea contains a wide variety of reservoired oils, ranging from the biodegraded heavy oils, via mid-gravity crudes to light oils and condensates. Statistical data from a suite of U.K. North Sea oils gives an average API gravity of 36°, an average asphaltene content of 1.4% and an average saturate/aromatic ratio of 2.02. Averages for pristane/phytane, pristane/n-C 17 and phytane/n-C 18 are 1.24, 0.63 and 0.56 respectively. It is possible to explain the ocurrence of the vast majority of the U.K. North Sea crudes in terms of generation over a range of maturities from single source rock horizons (the Kimmeridge Clay Formation hot shales), followed by alteration during migration or within the reservoir. The source rocks are shown to be immature below 175°F (80°C); early mature between 175–225°F (80–107°C); mid-mature between 225–265°F (107–130°C) and late mature above 265°F (130°C). Early mature, full maturity spectrum and late mature oils are identified using maturity-controlled sterane and triterpane ratios established from the source rock studies, and are related to bulk properties such as API gravity and saturate/aromatic ratios. Bacterial degradation in shallow cool reservoirs (less than 6000 ft; below 140°F (60°C)) is responsible for the majority of the heavy oil in the U.K. North Sea, while asphaltene enrichment is another mechanism for producing heavy oils. Though the Kimmeridge Clay Formation hot shales are fairly homogeneous over the U.K. North Sea, molecular ratios such as the pristane/phytane ratio, the abundance of a C 28 hopane and a C 27 moretane can be used to correlate oils with specific facies of the generally homogeneous source rock.

Source Rock Evaluation of the Lacustrine Oil Shale Bearing Deposits: Göynük/Bolu,Turkey

“…Studies on the color changes of the organic matter present in this study shows that it ranges from yellow to light brown (thermal alteration scale 3.0 to 4.5), which is also indicative of the immatureearly mature zone, i.e., the initial stages of oil generation. This finding agrees with the T max obtained from the pyrolysis for the Paleocene Kabalar formation (Tables 3, 4 Early maturity is confirmed by the saturate fraction composition (Figure 9), which is typical for an immature-early mature kerogen of algal origin such as the lacustrine Eocene Green River shale of mid-continental USA (Cornford et al, 1983). In particular, the dominance of isoprenoids over n-alkanes (Gilbert et al, 1975) is characteristic of immature-early mature kerogen of this type.…”

Biomarker Characteristics of Crude Oils From the Murzuq Basin, Sw Libya

“…MPI, MA(I)/MA(I+II) and TA(I)/ TA(I+II). Therefore, it is not appropriate to use the Ts/(Ts+Tm) ratio for quantitative estimation of maturity, because facies and depositional environments can also effect this ratio (Cornford et al, 1983;Schou et al, 1985). However, Seifert and Moldowan (1978) recommended that the C 27 Ts/ (Ts+Tm) ratio can be used as an indicator of maturity in samples containing similar source materials.…”

The Albian oceanic anoxic events record in central and northern Tunisia: Geochemical data and paleotectonic controls

“…The quantitative thermal maturity of the OM can be evaluated by using the Ts/ Ts+Tm ratio. However, lithology and depositional environments can also affect this ratio (Cornford et al, 1983;Schou et al, 1985;Moldowan et al, 1986). In our samples, where facies are similar, the relatively high ratio of Ts/Ts+Tm (Table 4) associated with high C29Ts is assumed to reflect the advanced thermal maturity of the Albian OM.…”