Devonian Clastic Plays of Astrakhan Arch in Southwest of North Caspian Depression

Internet Geology News Letter No. 9, September 14, 1999

Astrakhan gas-condensate field, the largest such field in Europe, was discovered in 1976 in Middle Carboniferous Bashkirian carbonates on the Astrakhan arch in the southwest part of the North Caspian depression. This high had been mapped by seismic survey in 1968-70. Although most of the North Caspian depression is in Kazakstan, Astrakhan is in Russia.

The sedimentary section that is of interest for oil and gas is divided by Lower Permian salt into two parts: sub-salt (Middle Devonian to Lower Permian) and supra-salt (Upper Permian to Cenozoic).

The Astrakhan arch is the largest structure of the North Caspian oil-gas province. Seismic surveys delineate it clearly on the sub-salt section, where its dimensions are 180 by 120 km (22,000 sq km), and closure is 3 km. Thickness of this sub-salt section is 3.5 km. Depth to the sub-salt complex is more than 3.7 km on the structurally highest parts, and depth to basement is estimated variously as 7.5-8.5 to 9-13 km (Petroleum Geologyu, vol. 31, no. 3, p. 213, 1997).

Exploration in the region did not focus on the section below the Bashkirian Stage, although its favorability was indicated by geophysical surveys and by productivity of this part of the section in other parts of the North Caspian depression. Studies now indicate that a minimum of four potential oil-gas complexes are present on the Astrakhan arch: one in the Lower Carboniferous and three in the Upper and Middle Devonian (Petroleum Geology, vol. 33, no. 4, p. 380, 1999).

The condensate of the productive Bashkirian Stage of Astrakhan field is very sour - high in hydrogen sulfide and carbon dioxide. These unwanted components decrease downward in the section. Underlying favorable but untested Famennian-Tournaisian carbonates are expected to be much lower in these components, and Middle Devonian-Frasnian clastics should be free of them entirely. This would make pools in these Devonian clastics very attractive prizes.

Pressure is a very important factor here, opposing the destructive effect of temperature on oil. Regions are oil-prone where pressures are high, because this forces gas to be dissolved in the oil. On this basis the hydrocarbons in the Devonian clastics are expected to be largely oil, not gas. Besides, these clastics probably interfinger toward the basin interior with Domanik-type cherty-bituminous shales and limestones that are high in sapropelic organic matter.

There is a good basis for expection large, predominantly oil pools to be discovered in the deep Devonian horizons at Astrakhan.

Copyright 1999 James Clarke. You are encouraged to print out these essays and to forward them to others. Astrakhan