Abstract:
The siliceous and calcareous organic-rich shale in the Upper Jurassic-Lower Cretaceous Bazhenov Formation in West Siberian Basin is characterized by high TOC and organic matters of moderate maturity and therefore has been the preferred shale oil exploration target in Russia.Recent study reveals several sets of centimeter to millimeter thick volcanic ash interlayers with yellow fluorescence in this organic-rich shale interval.A large number of volcanic ash-altered minerals such as zeolite are also detected through thin section observation, thus triggering an interest in the investigation of the relationship between volcanic ash and organic matter in this area.This study takes the Bazhenov Formation in the basin as an example to illustrate the influence of volcanic ash on the formation of the organic-rich shale.The composite geological-geochemical analysis on the volcanic ash in the formation shows that the organic-rich shale deposited synchronously with volcanic ash is rich in both nutrients such as silicon and phosphorus and catalytic elements for algae multiplication such as barium, manganese, molybdenum and uranium.It is therefore speculated that materials carried by volcanic ash promoted the mass reproduction or even facilitated a boom of palaeobios during the deposition of the Bazhenov, laying a material foundation for the formation of organic-rich matters.At the same time, a large number of strawberry-like pyrite layers were also developed in the shale overlying the volcanic ash interval, against a strong reducing enviroment of oxyen deficiency after volcanic ash eruption with Co/Ni ratio less than 1 and high S/Fe ratio.The fact that samples from the area with well-developed volcanic ash layers were measured to have higher TOC (over 7%) and maturity (Ro ranging between 0.7% and 1.1%) as well as higher hydrocarbon-generation potentials (almost reaching the peak) than those from adjacent areas without the layers indicates that the elements such as molybdenum and nickel carried by the volcanic ash served to promote the hydrocarbon generation of kerogens during thermocatalysis.The algal laminae interbedded with carbonate laminae in the shale could create plenty of interlayer pores for the migration of oil and gas.The formation of organic-rich shale in the Bazhenov Formation is, to some extent, inherently related to volcanic ash, which probably served to improve the paleo-productivity and preservation conditions for organic matter during the deposition period, promote the early hydrocarbon generation of organic matters, and facilitate the formation of organic-rich interlayers that respond quickly to stimulation operations.
