The study area is a belt along the southeast margin of the Siberian craton, extending some 1000 km to the northeast from the area just north of Lake Baykal; width is more or less 200 km. The southwest half of this belt is occupied by the Angara-Lena structural step, and the northeast half by the Nepa-Botuobinsk anteklize.
The composition of the gas dissolved in the formation waters of the Precambrian clastics below the Cambrian salt of the region changes systematically from areas of intake on the border of the basin toward its central part. In this direction the change is from nitrogen to methane-nitrogen to nitrogen-methane to methane. There is an increase in abundance of this gas in this same direction.
The zone of methane gas (more than 75 percent of total gas volume) with high gas saturation of the formation waters extends along the central part of Nepa-Botuobinsk anteklize and farther southwest into the central part of the Angara-Lena structural step. It is subdivided into three sectors.
The first sector is in the central part of the Nepa-Botuobinsk anteklize. In plan it corresponds with an area of slow subsidence and paleo-piezo-minimum, that is, an area of oil-gas accumulation. The second sector is in the western part of the Angara-Lena structural step. It corresponds with a paleo-piezo-minimum controlled by presence of a region of slow subsidence with respect to adjacent areas of downwarping. The third sector is in the southeasten part of the Angara-Lena structural step. Tectonically it is on the northwest border of the Baykal depression and is a main area of oil-gas generation. A stable paleo-piezo- maximum developed in this area.
The zone of nitrogen-methane gas (50-75 percent methane; 25-50 percent nitrogen) encircles the zone of methane gas along the flanks of the Nepa-Botuobinsk anteklize.
The methane-nitrogen zone (50-75 percent nitrogen and 25-50 percent methane) extends along the south margin of the craton and also on the north flank of Brat'sk high.
The zone of nitrogen gas is along the border of the craton. The gas is 90 percent nitrogen, and hydrocarbons are insignificant.
Compositions of the dissolved gases of Nepa-Botuobinsk anteklize and the Angara-Lena structural step are very similar. Since the Nepa-Botuobinsk region is host to oil-gas fields, it can be expected that the central part of Angara-Lena area is also favorable for oil and gas.
All presently known oil and gas fields are located in methane zones.
An important conclusion from this analysis is that zones enriched in hydrocarbons are associated with regions of long and relatively slow subsidence.
Taken from Pavlenko and Obukhov, 1988; digested in
Petroleum Geology, vol. 30, no. 3, 1996, one map showing
distribution of gas zones.
Copyright 2002 James Clarke. You are encouraged to print out
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