Geochemistry of the Upper Jurassic-Lower Cretaceous Bazhenov Formation, West Siberia

Abstract

The Bazhenov Formation (Tithonian to Berriasian) occupies about 1 million km 2 in the central part of the West Siberian plate and is buried by 2,000 to 3,000 m of younger sediments. It was formed in a marine basin connected to the world oceans and is composed primarily of siliceous argillites rich in planktonic organic matter (about 8% of total organic carbon on average). Organic-rich sediments have been accumulating for about 5 to 6 million years; they supposedly formed periodically under anoxic conditions. The aggregate mass of organic matter in the Bazhenov Formation is as high as 18 trillion tons. It is known as one of the largest oil sources in the world. About 500 samples from 39 oil wells throughout the formation were analyzed; it was shown that Bazhenov rocks contain more U, Mo, V, Cu, Zn, and Ni than average black shales; moreover, the concentrations of these elements increase toward the center of the paleobasin. Based on the original data on concentrations of several metals in 500 samples, a representative set of 30 samples was selected and analyzed for 43 chemical elements (using atomic absorption, neutron activation, and X-ray fluorescence analyses) as well as for total organic carbon. Using correlation analysis, the elements determined can be divided into three groups: those associated with organic carbon, with aluminosilicate material, or with carbonates. There is almost complete coincidence between these groups in the Bazhenov rocks and in sapropel from the Black Sea: in both instances total organic cargon is accompanied by Mo, Br, Sb, Cd, V, Cu, S, Ni, As, Ag, U, and Se; for rocks from the Bazhenov Formation, this list also includes Zn. It has been suggested that some of the elements extracted primarily from the seawater by dead planktonic material were later released and that these accumulated in sulfides. By including an enormous amount of elements from the world oceans, the Bazhenov Formation became an important factor in the global geochemical balance of the Jurassic period.