Distribution and composition of hydrocarbons in bottom sediments of the Shtokman deposit (Barents Sea)

Abstract

The increased interest in investigation of hydrocar� bons (HCs) in Arctic seas is mostly explained by the huge oil and gas potential of the shelf and perspective exploitation of marine deposits. By now 11 oil and gas deposits have been discovered directly on the territory of the Barents Sea [1]. Intensification of oil produc� tion and transportation to the sea should result in an increase in the anthropogenic influence on the envi� ronment. In addition, we should note that HC intro� duction from sedimentary series increases in oil and gas regions [2]. The portion of filtered HCs in the World Ocean is estimated as 0.6 × 10 6 t/year, which is 46% of their total income in seawater (1.3 × 10 6 t/year from all sources in the absence of large accidents on average [3]). Oil filtration on the Russian Arctic shelf was originally described in the 16th century on the bank of the Ukhta River (northern Timan–Pechora area), where pitch lumps were found in sand [4]. Pro� cesses of such kind were later registered in the Chukhi, Bering, Barents, and Kara seas. In accounting for the high oil and gas potential of the Arctic shelf of Russia, we may assume that this natural source provides not less than 50% of the total oil introduction in the Arctic seas. Samples were collected on two polygons in the area of the Shtokman gas–condensate deposit (SGCD) in August 2010 (57th voyage of R/V “Academician Mstislav Keldysh) in order to estimate the level and composition of HCs in bottom sediments. Polygon 1 is located in the water area, where funneled deepenings of a seabed were registered (Fig. 1a); their formation may result either from melting of buried ice (potholes) or gas release on the seabed (pockmarks) [5]. Polygon 2 is located directly in the water area of the deposit (Fig. 1b). Bottom sediments were collected by a box corer and frozen at –18°C. Samples were thawed in the lab� oratory, dried at 50°C, and the fraction of 0.25 mm was separated. HCs were extracted by metylenchloride in an ultrasonic Sapfir bath, and their concentration was analyzed by IR spectrophotometry; the composi� tion of alkanes, by capillary gas–liquid chromatogra� phy; and the concentration and composition of poly� cyclic aromatic hydrocarbons (PAH), by the method of highly effective liquid chromatography. The details of methodological procedures were described in [2, 6].