Large structures indicated off Turkmenistan

This article concerns offshore Turkmenistan, which occupies the southeastern quadrant of the Caspian Sea, where the government of Turkmenistan plans to open an offshore licensing round, 'Turkmenistan 2000,' in May 2000. Blocks will be offered covering all unlicensed areas (Fig. 1) and will include some highly prospective areas identified on speculative seismic data acquired by Western Geophysical between 1996 and 1998.

Large oil reserves have already been discovered in the offshore and the contiguous onshore parts of the South Caspian basin in Turkmenistan and Azerbaijan. We confidently predict that the conditions for petroleum generation and entrapment will extend over the less explored parts of offshore Turkmenistan.

Geologic setting, evolution

The South Caspian basin (SCB) is occupied by up to 25 km of sediments lying on oceanic crust. The oceanic crust, often referred to as 'Paratethys,' is either a trapped remnant of Early Mesozoic oceanic crust,¹ part of a Mid- to Late Jurassic marginal basin to Tethys,^{2 3} or formed by Cretaceous to Paleogene transtensional 'pull-apart.'⁴

The basin is now surrounded by compressional zones formed initially by Iranian-Transcaucasian collision during the late Eocene/early Oligo-cene⁵ but with main margin deformation and uplift occurring from the Mid Miocene in response to Arabian-Iranian and Indian-Eurasian collision.⁵

The northern boundary of the basin lies to the north of the Apsheron Sill, which links the Greater Caucasus in the west with the Balkan fold/thrust belt and the Kopet Dag in the east. This is a complex zone that has involved northward convergence and subduction of SCB oceanic crust under the Scythian/Turan plate with associated deformation and major shale diapirism. Oblique convergence along this zone has imparted a strike-slip component on the Ashkabat fault, which forms the northern boundary of the Kopet Dag.⁶

Stratigraphy, petroleum geology

The Lower Tertiary is known from drilling in southwestern Caspian and onshore in western Turkmenistan.⁷

It is a marine sequence deposited during the basin's extensional and early compressional history (Fig. 2). This includes the Oligocene-Early Mio-cene Maikopian sequence.

Maikopian sedimentation took place during initial basin isolation as a result of compressional tectonism. This led to the development of euxinic conditions and the deposition of organic-rich shales. Geochemical correlation has shown that marine Maikop and Diatom Suite shales are the most likely source for reservoired oils onshore and offshore Turkmenistan.^{8 9 10}

From the Mid-Miocene deltaic sediments begin to prograde into the basin as a response to compressional deformation and associated marginal uplift. Potential reservoirs are developed in pro-deltaic or delta-slope sandstones of the Diatom Suite and Pontian beds.

Basin filling culminated in the Early Pliocene with the deposition of the Red Series dominated by fluvial/delta plain depositional settings.¹¹ The most important reservoirs in the region are formed by laterally extensive braided channel sands in the Red Series,¹² particularly in the Lower Red Series.

Rising sea levels in the Late Pliocene mark the return to marine conditions and the sedimentation of the Akchagylian and Apsheron 'suites.' Early transgressive marine shales constitute an important seal to reservoirs in the upper part of the Red Series, but reservoir sands are also known in both shallow formations.

Structure and prospectivity

Seismic interpretation has shown a basic WNW-ESE structural zonation offshore Turkmenistan (Fig. 3). The main structural zones are described below:

• Shale diapir zone is characterized by intermediate and high-penetrating shale diapirs forming WNW-ESE-trending anticlinal structures for Red Series reservoirs (Fig. 4). The mobile shale may be originating in overpressured Lower Tertiary sediments (probably Maikopian).³ Shale diapirism was probably triggered by northward-directed compression from the Elborz/Talesh deformed belt on the southern and southwestern margin of the basin.
• Shale swell zone is characterized by higher-level (probably Pontian beds) mobile (overpressured) shales forming a series of swells. The sediments above the shale are deformed as anticlines and listric slides with associated roll-over structures (Figs. 3, 4). They display growth into the extensional faults showing that shale movement was soon after deposition, and during Red Series sedimentation.

This has led to the interpretation that shale movement was triggered by compression above a basal detachment within a ductile shale sequence³ rather than sediment loading. The detachment appears to have ramped up from a deeper level in the shale diapir zone. The boundary between the two zones is formed by a major northward-throwing listric extensional fault against a shale diapiric 'wall' (Fig. 4).

• Southeast thrust zone is characterized by southwesterly verging thrusts affecting section up to the Red Series (Fig. 5). This appears to be a continuation of Kopet Dag structural trends, which are oil-bearing onshore, into the southeastern Caspian.
• Central zone contains thick Red Series deformed in a series of low-amplitude, long-wavelength folds paralleling the general structural trend (Fig. 6). Folding was probably a result of compression allied with deep shale movement, or buckle folding above a detachment zone in overpressured shales.³ Structural growth has occurred from 'Akchagylian' times through to present day.
• Northern zone is a complex structural zone with a prominent WNW-ESE diapiric wall at its center which forms the dominant structural feature of the Apsheron Sill (Fig. 6). Several large oil fields are developed along this trend. Along the southern side of the wall southward-verging low-angle thrusts have been attributed to accretionary tectonics as a response to the northward subduction of SCB oceanic crust. On the northern side structures represent the reactivation (inversion) of the southern margin of the Scythian-Turan platform, a less intense form of that seen in the Kopet Dag in Turkmenistan⁶ and the Greater Caucasus in Dagestan.¹³
• Scythian-Turan Platform is an area where relatively thin Tertiary sediments overlie a Mesozoic (Triassic to Cretaceous) rifted platform sequence. Up to the Lower Jurassic the section is affected by Cimmerian compression and in some places inversion of Triassic rifts. The effects of post-Cimmerian rifting are also recognized (e.g. in the Late Jurassic).

HC generation, entrapment

The occurrence of mud diapirs and volcanoes often with associated oil and/or gas seepages evince to the overpressured nature of parts of the section.

Stratigraphic relationships would suggest that it is organic rich sediments that are becoming overpressured and mobilized, viz. Oligo-Miocene Maik- opian and Miocene shales.³

Rapid sedimentation during the Pliocene combined with compressional tectonics, and probably hydrocarbon generation, has been responsible for excess pressure build-up. The quick, cold sedimentation of the Pliocene has also resulted in depression of the isotherms so that the oil window can still fall within deep Maikopian source rocks.^{9 14}

Oil generation is therefore late, and even structures formed by continued and late shale diapirism can be charged with hydrocarbons.³ The largely unconsolidated upper 10 km of sediments provide excellent pathways for hydrocarbons to migrate from deep source rocks to shallower formations. This occurs through buoyancy, excess pressure, and occasionally faults.^{9 14}

Acknowledgment

This article is presented on behalf of a working group represented by the Ministry of Oil and Gas Industry and Mineral Resources (Minneft) and state concerns Turkmenneft and Turkmengeology, Western Geophysical, and Exploration Consultants Ltd.

References

1. Berberian, M., "The Southern Caspian: A compressional depression floored by a trapped, modified oceanic crust," Canadian Journal of Earth Science, Vol. 20, 1983, pp. 163-183.
2. Zonenshain, L.P., and Le Pichon, X., "Deep basins of the Black Sea and Caspian Sea as remnants of Mesozoic back-arc basins," Tectonophysics, Vol. 123, 1986, pp. 181-211.
3. Devlin, W.J., et al., "South Caspian basin: Young, cool and full of promise," GSA Today, Vol. 9, No. 2, 1999, pp. 1-9.
4. Sengor, A.M.C., "A new model for the late Palaeozoic-Mesozoic tectonic evolution of Iran and implications for Oman," in "The Geology and Tectonics of the Oman Region," Geol. Soc. Spec. Pub. No. 49, 1990, pp. 797-831.
5. Nikishin, A.M., et al., "Scythian platform, Caucasus and Black Sea region: Mesozoic-Cenozoic tectonic history and dynamics," in "Peri-Tethys Memoir 3: Stratigraphy and evolution of Peri-Tethyan platforms," Mem. Mus. Nat. Hist., Vol. 177, 1998, pp. 163-176.
6. Lyberis, N., and Manby, G., "Oblique to orthgonal convergence across the Turan Block in the Post-Miocene," AAPG Bull., Vol. 83, 1999, pp. 1,135-60.
7. Jones, R.W., and Simmons, M.D., "A review of the stratigraphy of Eastern Paratethys (Oligocene-Holocene)," Bull. Nat. Hist. Mus. Lond., Vol. 52, 1996, pp. 25-49.
8. Abrams, M.A., and Narimanov, A.A., "Geochemical evaluation of hydrocarbons and their potential sources in the western South Caspian depression, Republic of Azerbaijan," Marine and Petr. Geol., Vol. 14, 1997, pp. 451-468.
9. Wavrek, D.A., et al., "Maikop/Diatom-Productive Series (!) Petroleum System, South Caspian Basin, Azerbaijan," AAPG annual convention, Salt Lake City abs., 1998.
10. Gürgey, K., "Geochemical characteristics and thermal maturity of oils from the Thrace basin (Western Turkey) and western Turkmenistan," JPG, Vol. 22, 1999, pp. 167-189.
11. Reynolds, A.D. et al., Implications of outcrop geology for reservoirs in the Neogene productive series: Apsheron Peninsula, Azerbaijan," AAPG Bull., Vol. 82, 1998, pp. 25-49.
12. Abdullaev, T., et al., "A reservoir model for the main Pliocene reservoirs of the Bahar field in the Caspian Sea, Azerbaijan," Petr. Geoscience, Vol. 4, 1998, pp. 259-270.
13. Sobornov, K.O., "Structure and petroleum potential of the Dagestan thrust belt, northeastern Caucasus, Russia," Canad. Petr. Geol. Bull., Vol. 42, 1994, pp. 352-364.
14. Tagiyev, M.F., et al., "Geohistory, thermal history and hydrocarbon generation history of the north-west South Caspian basin," Marine and Petr. Geol., Vol. 14., 1997, pp. 363-382.

The Authors

Steve Lawrence is principal geological consultant with Exploration Consultants Ltd. Before joining ECL in 1978 he was operations and exploration geologist for Amoco UK and Cluff Oil Ltd. E-mail: [email protected]

Hoshgeldi Babaev is director of the Ministry of Oil and Gas Industry and Mineral Resources in Ashgabat, Turkmenistan.