Oil in the lake of world heritage

Abstract

Lake Baikal is the oldest and deepest lake in the world. It contains approximately 20% (23 · 10 3 km 3 ) of the world’s surface reserves. Since the community of diverse endemic organisms of the lake has not suffered from human impact at its recent stage of development [1], it is included into the UNESCO world heritage list. Unique features of Lake Baikal include natural oil seeps in its shelf zone, which have been known since the 18th century [2]. At oil shows in the middle and south sections of Lake Baikal, oil occurs as bitumen in shore cliffs or rises from the lake bottom as floating spherules of viscous hydrocarbons that make up spills (up to 1.5 m across) in an area of approximately 1 km 2 in summer. In winter, the floating oil is accumulated under ice as bituminous films and inclusions in narrow fissures. The oil composition corresponds to the biodegraded type. Its origin and age were debatable until recently [3]. In 2005, a new natural oil occurrence was discovered in deep-water settings of Lake Baikal that differs from the previously known ones. The discovery poses a problem concerning the role of natural oil occurrences in the ecosystem of the unique basin and its petroleum potential. The new oil occurrence was discovered in Middle Baikal (Cape Gorevoi Utes) owing to satellite observations of a dark spot (~1km across) on the lake surface in the spring of 2003. Observations of 2004 and 2005 showed that such a spot appears in this area every spring. In July 2005, numerous oil spots (up to 1 m in diameter) were recorded on the lake surface in an area of approximately 1 km 2 . Echo sounding revealed an underwater acoustic anomaly in the form of a gas flare ~500 m high. The oil sampled from the lake surface was studied using the gas chromatography/mass spectrometry method (Fig. 1). The samples contained n- alkanes, acyclic isoprenides, and polycyclic aromatic hydrocarbons. Based on the database including characteristics of molecular indicators for 76 genetically different oil fields [4], the ratios of molecular indicators (pristane/phytane 6.7; dibenzothiophene/phenathrene <0/1), and the low sulfur content (<0.08%), we concluded that the samples correspond to oils formed in sediments of deep freshwater basins during the Oligocene‐Early Miocene. This age estimate obtained for the Baikal oil based on molecular indicators is consistent with one hypothesis, according to which the Baikal oil is not older than the Cretaceous and it originates from organic matter buried in a freshwater basin [5]. The bottom sediments were sampled at the center of the gas flare and at its periphery (at a distance of 200 m). The core taken in the central part (0‐40 cm) is composed of dark gray reduced massive clayey‐silty sediment. The Holocene diatomaceous ooze layer typical of Baikal is missing at the surface. The sediment was mixed with oil and saturated with gas. The oil content exceeds 10% of the dry sediment. The second core demonstrates distinct bedding along its entire length. According to biostratigraphic data ( Cyclotella minuta, Aulacoseira baicalensis , and others), the sediment represents the Holocene diatomaceous ooze. The surface sediment (upper 4 cm with a brown color) is oxidized, while the remainder of the core is reduced (gray in color). The interval of 10‐40 cm hosts lenses and interbeds of silt and fine- to medium-grained sand with small brown oil droplets (up to 3 mm across) fringed by small gas bubbles. The oil content in this core interval is approximately 1% of dry sediment. The difference between two sampled cores in terms of oil-and-gas occurrence and lithology is probably explained by different intensities of their influx. The central part of the gas flare is characterized by permanent fluxes of gas, oil, and groundwater, which provide