Origin of biogenic-induced cherts from Permian alkaline saline lake deposits in the NW Junggar Basin, NW China: Implications for hydrocarbon exploration

Abstract

The Permian Fengcheng Formation is composed of alkaline saline lake deposits in NW China and is tectonically part of the Central Asian Orogenic Belt (CAOB). Cherts are common in this formation and contain abundant fossilized microorganisms. These microfossils occur as individual spheres and clots in chert layers (laterally continuous and commonly parallel to the laminae of the host rocks) and nodules (chert grains formed by shrinkage and desiccation of chert layers). Carbonaceous cell walls have been well preserved in chert layers. Based on petrological and geochemical analyses of chert and their host rocks, the mechanisms of cyanobacterial cell silicification and their significance for hydrocarbon exploration have been discussed. A high concentration of dissolved inorganic silica in brine and high pH values are the premise for microorganism blooms and chert deposition. Hot springs may be an important silica source for chert deposition. Hot springs also create extreme conditions for algal blooms in brine with high temperatures, high salinities and toxic and anoxic conditions . Spheroidal cyanobacterial cells were rapidly silicified, and cell structures, e.g., cell walls and inner division patterns (reproduction of cells), were well preserved. Chert with fossilized spheroidal microorganisms can be considered hydrocarbon source rocks.