Figures with Comments

 

Figure 1. The Pricaspian sedimentary basin.

• Basin formed by the Late Proterozoic rifting on the East European continent passive margin (D. Feodorov, 1979, 2003).

• The Late Paleozoic the Pricaspian Basin is an analogue of the present day inner and marginal deep-water basins, such as Mediterranean Sea, Black Sea, Caspian Sea (Al-Zhadi, 2004).

• Sedimentary cover overlies the suboceanic basement in the Central Pricaspian Rift, where it has thickness of up to 20-22 km.

• Sediment thickness is reduced to 6-9 km within the Astrakhan-Aktyubin Highs System, where continental type of the basement occurs (D. Feodorov, 2003).

 

Figure 2. The Pricaspian Basin is prolific of oil and gas.

• Bulk oil and gas reserves are concentrated in the giant Paleozoic carbonate buildups within the Astrakhan-Aktyubin Highs System.

• No commercial hydrocarbon fields were discovered along the north-western sector of the Pricaspian Basin.

• The Karachaganak giant reef field discovery (1979) stimulated intensive exploration efforts around the north-western part of the Pricaspian Basin, but no new large carbonate buildups were found.

 

Figure 3. Seismic dynamic-depth cross-section, North-West Pricaspian Basin.

• The Upper Paleozoic subsalt section of the north-western part of Pricaspian Basin is composed by thick basinal mudstone sequences and deep-water condensed carbonates, with no obvious reservoirs.

 

Figure 4. Late Paleozoic depositional systems of the north-western margin, Pricaspian Basin.

• The Pricaspian Basin was a deep-water sea which was confined by shallow-water shelves to the north and to the west during Late Devonian – Early Permian.

• Sea level highstand shelves are the carbonate platforms with high (hundreds meters) and steep (30-60^o) basinal slopes.

• The carbonate platforms grade to thin (tens meters) shaly condensed carbonates of the inner part of Pricaspian Basin.

 

Figure 5. Isolated carbonate-buildups-forming environments.

• Large isolated carbonate buildups formed within the Astrakhan-Aktyubin Highs System (AAHS) during Late Devonian – Middle Carboniferous sea-level highstands.

• The Paleozoic Ryazano-Saratov Trough influenced the north-western part of Pricaspian Basin. Sea floor subsided there most significantly, and deep-water environments followed that subsidence.

• Water depths were too deep for isolated carbonate buildups to form in the North-West Pricaspian Basin.

 

Figure 6. Geologic cross-section along the regional seismic line Bykovo-Aralsor.

• Significant tectonic sea bottom subsidence and starved carbonate deposition during sea-level highstands created great accommodation space for clastic deposition that followed sea-level lowstands.

• Thick basinal delta cone of the Late Bashkirian – Early Moscovian fluvial system partially filled this accommodation space.

 

Figure 7. Paleogeographic map of the Late Bashkirian – Early Moscovian.

• The acreage of the Middle Carboniferous basinal delta cone exceeds 25,000 km². Thickness of its prevailing mudstone sequence is as much as 1.5-1.7 km.

• The delta cone correlates with very thick shaly Middle Carboniferous strata to the south, which was sourced by the Karpinsky Ridge from the southern border of the Pricaspian basin.

• Seismic evidence of delta cone clinoform to the east (Yu. Nikitin, 1992) suggests its grading into the basinal, bottomset, thin condensed deposits.

 

Figure 8. Middle Carboniferous delta cone lithology.

• In borehole logs of the few available wells, the Middle Carboniferous basinal delta cone consists of dark-grey and black mudstones with scarce tight siltstones and sandstones.

• It is common that basinal delta cone mudstones contain predominantly gas source rocks. Core sample pyrolysis results suggest oil window metamorphic stage (Tmax 440^o-469^oC) (B. Soloveov et al., 2002), but only rare gas shows were received in well jets.

 

Figure 9. Mid-Carboniferous coastal-marine plain outside of the Pricaspian Basin’s north-western part.

• The basinal delta cone deposits were sourced by the Late Bashkirian – Early Moscovian alluvial-deltaic system that was developed along Ryazano-Saratov Trough, entering the Pricaspian Basin from the north-west.

• The alluvial-deltaic system was composed of topset, coastal– to shallow-marine-plain sediments that were deposited while sea level changed repeatedly.

• 7-8 parasequences are recognised: lowstand components are sandstone channels, incised in previous highstand topsets and generally extending toward the deep-water Pricaspian Basin (S. Yatskevich et al.,1999).

• Those sandstones (30-75 m) are the reservoirs for large oil and gas deposits outside the Pricaspian Basin.

 

Figure 10. Prediction of sea-bottom sand fans inside the north-western part of the Pricaspian Basin.

• Siliciclastic sediments could bypass the shelf and its slope through the channels and the slope canyons to feed possible basin-floor fans.

• Thick sandstone bodies of basin-floor fans may be present in the lower part of the Middle Carboniferous basinal delta cone not too far from the previous carbonate shelf slope.

 

Figure 11. Seismic evidence of possible submarine fans.

 

Figure 12. Reservoir properties of possible sand fans.

• The deep occurrence of clastics is the negative factor for their reservoir properties, due to compaction, but overpressure may have provided exception.

• Overpressure factor of the Mid-Carboniferous clastics was estimated as 1.3-1.9 within the north-western part of the Pricaspian Basin (A. Zamarenov et al., 1986).

• Thick sandstone bodies, sealed by impermeable mudstones, are assumed to have experienced less than expected compaction and to be naturally fractured in the overpressure zones.

• The above-mentioned allows prediction of the presence of efficient reservoirs, probable Middle Carboniferous sandstones, at greater depths in the inner part of the Pricaspian Basin.

 

Summary

(Figure 13) 

Figure 13. Tectonic setting of Pricaspian Basin (upper); marked basinal subsidence restricted carbonate deposition (middle); possible sand fans present on basin floor (lower).

• Marked subsidence during the Late Paleozoic of the sea floor of the North-West Pricaspian Basin resulted in very deep-water environments, which were adverse to growth of carbonate buildups.

• Significant tectonic subsidence, together with sea-level highstand, starved carbonate deposition but created large accommodation space for subsequent lowstand clastic sedimentation in the North-West Pricaspian Basin.

• Thick basinal delta cone filled the Mid-Carboniferous accommodation space as the proximal component of the Late Bashkirian – Early Moscovian alluvial-deltaic system, which was developed along the Ryazano-Saratov Trough.

• Basin-floor sand fans are predicted in the lower part of the basinal delta cone, which should to be a prospective exploration play for new discoveries within the North-West Pricaspian Basin.

 

References 

Al-Zhadi, R.A., 2004, The Pricaspian Basin – The most important oil-and-gas bearing basin of the North-Western Mediterranean: Almaty, Neft i gas, no. 1, p. 15-23.

Feodorov, D.L., 1979, Sub-salt Paleozoic formations and oil-and-gas content of the East-European platform’s marginal basins: Moscow, Nedra, 171 p.

Feodorov, D.L., 2003, Structure of the basement surface in the Pricaspian Basin. Razvedka i okhrana nedr, no. 2, p. 11-12.

Khutoskyi, M.D., Antipov, M.P., Volozh, Yu.A., and Polyak, B.G., 2004, Temperature field and 3D geothermal model of the Pricaspian Basin: Geotectonica, no. 1, p. 63-72. (Map used in slide 1 was prepared by Yu. Volozh)

Myers, K.J., and Milton, N.J., 2004, Concepts and principles of sequence stratigraphy, in Sequence Stratigraphy, D. Emery and K.J.Myers, eds., Blackwell Science. p. 11-41.

Nikitin, Yu. I., 1992, Seismic-geological zonation of the Pricaspian Basin North-Western part: Nedra Povolzhya i Pricaspiya, Issue 3, Nizhne-Volzhskyi Institute of Geology and Geophysics, Saratov. p. 7-16.

O’Hearn, T., Elliott, S., and Samsonov, A., 2003, Karachaganak Field, Northern Pre-Caspian Basin, Northwestern Kazakhstan. in Giant Oil and Gas Fields of the Decade 1990-1999, Michel T. Halbouty, ed.: AAPG Memoir 78. Tulsa, Oklahoma, USA.

Sheglov, V.B., Yatskevich, S.V., Merkulov, O.I., Mamulina, V.D., and Umnova, L.N., 2005, Reservoir properties of the dubsalt oil-and-gas bearing complexes around Russian sector of the Pricaspian Basin: Nedra Povolzhya i Pricaspiya, Issue 44, Nizhne-Volzhskyi Institute of Geology and Geophysics, Saratov, p. 3-22.

Solovyov, B.A., Ivanov, Yu. A., and Kondratyev, A.N., 2002, Middle Carboniferous clastic complex of the Pricaspian Basin West – Possible object for big oil-and-gas fields exploration: Geologiya nefti I gaza (Oil and Gas Geology), no. 5., p. 2-7.

Yatskevich, S.V., Nikitin, Yu. I., Klimashin, V.P., Shebaldin, V.P., and Postnova, E.V., 1999, Prognostic Models of the Middle Carboniferous – Lower Permian hydrocarbon traps around the Pricaspian Basin North-Western part: Nedra Povolzhya i Pricaspiya, Issue 18, Nizhne-Volzhskyi Institute of Geology and Geophysics, Saratov, p. 17-28.

Zamaryonov, A.K., Shebaldina, M.K., Feodorov, D.L., Yugai T.A., and Yatskevich S.V., 1986, The sedimentation models of the Pricaspian Basin subsalt oil-and-gas bearing complexes: Moscow, Nedra, 137 p.