OIL SOURCE ROCKS IN THE LOWER PLIOCENE DELTAIC-LACUSTRINE SUCCESSIONS IN THE SOUTH CASPIAN BASIN

D. Huseynov¹, E. Aliyeva¹, D. Nummedal², I. Guliyev¹, G. Riley³, J. Friedmann⁴
¹Geological Institute of Azerbaijan National Academy of Sciences, Azerbaijan
²Institute of Energy Research, University of Wyoming, USA
³BP Azerbaijan Resource Development Team, UK
⁴University of Maryland, USA

Summary

Traditionally the principal oil source–rocks in the South Caspian basin (SCB) are considered to be Paleogene-Miocene deposits (Maykop-Diatom). As a result of conducted studies of Lower Pliocene sediments (Productive Series-PS) it had been revealed the oil generation potential of some stratigraphic units of PS. Results of given research are very important for development of further strategy of exploration and resources estimation in the South Caspian region, and shed light on the potential distribution of associated reservoirs.

Main results

South Caspian basin is located in Mediterranean orogenic belt limits. The history of hydrocarbons exploration and production in the basin has more than 150 years. Being one of the oldest oil provinces in the world the South Caspian basin is characterized by the huge accumulations and high density of hydrocarbon resources. The hydrocarbon reservoirs have a wide stratigraphic range, but the majority of oil-gas deposits (about 90%) are concentrated in the Lower Pliocene Productive Series with total production 1,2 x 10⁹ t. The main source-rocks in the basin are considered to be Paleogene-Miocene deposits (Bailey N.J., et al., 1996; Abrams M.A. and Narimanov A.A., 1997; Katz B., et al., 2000; Huseynov D.A., 2000; Guliyev I.S., et al., 2001; etc.). However the principal question is still opened - is the terrigenous deltaic Productive Series sediments deposited by PaleoVolga river in the area of PaleoCaspian (recent South Caspian) only main reservoir in the basin or they are also source-rocks that might charge the Caspian lacustrine reservoir targets?

The thickness of the PS in the basin reaches 7 km. PS represents alternation of sand, clay and silt layers and thickness of sandstone and mudstone units are approximately equal. Productive Series is subdivided into lower (PS₁) and upper (PS₂) parts. The PS1 consists of 5 suites (Gala, PreKirmaki (PK), Kirmaki (KS), PostKirmaki Sand (NKP), PostKirmaki Clay (NKG)), and PS₂ - of 4 suites (Fasila, Balahany, Sabunchi, Surahany.

Geochemical properties, composition and genetic type of organic matter (OM) depend on depositional environment. We studied PS deposits in oil fields located within Absheron peninsula and adjacent offshore area (Fig. 1). Rock-Eval 7 had been used when analyzing of samples. The obtained data prove that OM in sediments of Lower and Upper divisions of PS mostly are represented by III type. It is well correlated with results of study of microelements showing the brackish nature of PS basin. However, deposits of Gala, Kirmaki and, partially, Balakhany suites are characterized by presence of two types of OM, which changes in vertical section. It is connected, we think, with depositional environment changes from proximal to more distal settings. This transition suggests the presence of other source targets offshore, and provides insights into the downdip potential for reservoirs intercalated with these organic-rich mudstones. Specifically, the shift to more distal settings may locally reduce the downdip reservoir potential of traditional fluivo-lacustrine targets within this interval.

Conclusions

The obtained data testifying to good oil-gas generating potential of some Productive Series suites are extremely important. One should note that a long time the generation properties of PS deposits were considered as poor.

It should be pointed out, that analyzed material characterize the marginal parts of the South Caspian oil-gas bearing basin and PS sediments are immature, that is proved by low values of Tmax and production index (PI). According to deep seismic data and results of basin modeling the thickness and content of mudstones gradually increase toward the central part of the basin. One can also expect the facies shift to distal lacustrine environment in deep water part of the South Caspian which together with subsiding of PS sediment into “oil window” assume oil generation not only in Paleogene-Miocene complex, but also in Lower Pliocene series. These new insights should be incorporated into exploration models down-dip regarding the presence of both source and reservoir potential.

References

Abrams M.A. and Narimanov A.A. 1997. Geochemical evaluation of hydrocarbons and their potential sources in the western South Caspian depression, Republic of Azerbaijan: Marine and Petroleum Geology, v.14, p. 451-468.

Bailey, N.J.L., Guliyev, I.S., & Feyzullayev A.A. 1996. Source rocks in the South Caspian. in: AAPG/ASPG research symposium "Oil and gas petroleum systems in rapidly-subsiding basins". Book of abstracts, Baku, Azerbaijan.

Guliyev I.S., Feyzullayev A.A., Tagiyev M.F., 2001. Source potential of the Mesozoic-Cenozoic rocks in the South Caspian Basin and their role in forming the oil accumulations in the Lower Pliocene reservoirs. Petroleum Geoscience, vol. 7, No4, pp. 409-417.

Huseynov D.A., 2000. Origin of oils in the western part of the Kura-South Caspian oil-gas bearing basin. In: Extended Abstracts Book, 62th EAGE Conference and Technical Exhibition, Glasgow, UK.

Katz, B., Richards, D., Long, D., & Lawrence, W. 2000. A new look at the components of the petroleum system of the South Caspian Basin. Petroleum Science and Engineering, 28, 161.

Figure 1. Location map of South Caspian basin and study area.