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Maturity and Depth Occurrence of the Oil and Gas Sources in South Caspian Basin*
A.A. Feyzullayev 1
Search and Discovery Article #40413 (2009)
Posted May 1, 2009
*Adapted from oral presentation at AAPG International Conference and Exhibition, Cape Town, South Africa, October 26-29, 2008
1Geology Institute of the National Academy of Sciences, Baku, Azerbaijan
A number of studies have attempted to estimate the degree of thermal transformation of organic matter (ОM) in the sedimentary rocks of the South Caspian Basin (Wavrek et al., 1996; Abrams and Narimanov, 1997). These studies include attempts of direct estimation of maturity of OM transformation products: oils (Wavrek et al., 1996; Abrams and Narimanov, 1997; Inan et al., 1998; Abdullaev et al., 1998; Guliyev et al., 2000; Кatz et al., 2000; Gurgey, 2003) and HC gases (Кatz et al., 2000). Considering the importance of estimation of maturity of oil and gas in understanding the mechanism of formation of various pools, the goal of this presentation is the generalization of all studies carried out to date for the South Caspian Basin with the purpose of creation of a summary model of vertical zonality of oil and gas formation in this basin.
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Figure Captions
MethodsThe analysis of maturity of organic matter (OM) was based on the optical method (analysis of vitrinite reflectance R0). The methods of a direct estimation of maturity of oil and gas were carried out by using of appropriate isotope-geochemical parameters (Peters and Moldovan, 1993). Biomarker parameters such as the degree of C29 sterane izomerization and methyl - phenanthrene index (MPI-1) were used during the estimation of maturity of oil. The values of these parameters were recalculated in equivalent R0 values on the basis of experimental research results.
The estimation of maturity of gas is based on the experimentally revealed dependence between isotope composition of carbon (ICC) of HC gases and vitrinite reflectance (R0) (Faber, 1987). The forecast of depth of the HC gases source location is given on the basis of this dependence and regularities of change of R0 vs. depth in the South Caspian Basin. The estimations were carried out on ethane because the estimations on methane with relation to the influence of methane of a biochemical origin are less objective.
Results
OM MaturityThe results of the R0 measurements in the South Caspian Basin are partially reflected in publications. However, the most important research was carried out by the Geology Institute of Azerbaijan National Academy of Sciences in cooperation with the foreign oil companies. The results of these studies for Paleogene-Miocene rocks generalized in the form of a bar chart of the R0 values distribution are in Figure 1 A .
According to the bar chart, the R0 modal values varies within the limits of 0.6-0.9% (about 50% of tested samples). In this connection it is possible to conclude that OM of the studied rocks is mainly on the initial stage and peak of maturity. Most probably, the values received are understated as a result of the influence of secondary processes and location of the rocks studied in the elevated marginal part of the basin.
Oil MaturityAccording to Guliyev et al. (2000) about 50% of the oil samples studied have values of maturity at C29 sterane isomerization of 0.63-0.73% Req. Estimations based on the same parameter carried out by Abrams and Narimanov (1997) and Gurgey (2003) set Req values of 0.52-0.81 and 0.56-0.85 accordingly. Estimations at MPI carried out by Wavrek et al. (1996), Inan et al. (1998), Abdullaev et al. (1998), Кatz et al. (2000) and by the author of this presentation vary within the limits of 0.7-0.8%; 0.79-0.87%; 0.56-0.82%; 0.7-0.95% and 0.52-0.96% Req accordingly.
In spite of the fact that the above estimations based on the data of the oil analyses were carried out in various laboratories using various parameters, they, as a whole, coincide with each other, reflecting an initial stage and peak of oil generation.
Peak maturity of oils (on Req) and OM (on R0) as a whole coincide well with each other (see Figure 1 А, B1, B2), confirming objectivity of the calculated Req values.
It is interesting that the maturity of oil seepages related to mud volcanoes coincides with the maturity of Miocene-lower Pliocene reservoir oils. This leads to the conclusion that oils of the mud volcanoes are a product of destruction of oil accumulations in the noted reservoirs.
Maturity of Gases Calculated values of Req of HC gases (at ethane) of fields in the South Caspian Basin (with the reservoirs depth occurrence interval of 501-5778 m) carried out by Katz et al. (2000) varies within the limits of 0.85-1.70% Req. The estimations carried out by the author of this presentation, as a whole, coincide with the estimations of Katz et al. (2000) characterized by Req values of 0.98-1.91%, except in 3 cases (from 56 tested samples) with Req of > 2.2% (Neftchala, wells 867 and 889; Absheron kupesi, well 42).
The maturity of HC gases from mud volcanoes as a whole coincide with the maturity of HC gases of reservoirs varying within the limits of 1.3-2.07% Req (Figure 1 C1 and C2). According to four measurements of maturity of HC gases of hydrates formed in submarine mud volcano craters in a deep-water part of the Southern Caspian sea varies within the limits of 1.47-1.94 % Req (Figure 1 C3). Modal values of maturity of these gases, as well as gases of mud volcanoes and reservoirs, vary in the range of 1.5-2.0% Req. Conclusions
Using all results of estimations of the oil and gas maturity and regularity of change of R0 values with depth revealed for the South Caspian Basin (Wavrek et al. 1996), the summary model of vertical zoning of oil and gas formation (Figure 2) was constructed. According to this model, the generation of petroleum is located within the depth interval of 5-15 km; the interval of generation of oil occurs on the depth of 5-9 km (peak between 7-8 km), and gas in the depth range of 7-15 km (peak between 11-12 km).
It is important to note that the vertical zonality of oil and gas formation do not reflect zonality of their accumulation in reservoirs, which is caused by the process of intensive subvertical migration of HC fluids, having presumably injection character (Feyzullayev and Aliyeva, 2003). The distance of subvertical migration of most volume of oils is, presumably, 4 km (Figure 3 B). It is well known that source rocks and main reservoirs in the South Caspian Basin are related to various stratigraphic complexes, which is once again confirmed by the absence of dependence between depths of generation and accumulation of oils (Figure 3 A).
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