Concepts on the Generation and Accumulation of Biogenic Gas

Abstract

To help in the search for biogenic gas, parameters affecting its spatial and temporal distribution are delineated here on the basis of recent developments in microbiology and geochemistry. Methane is produced and dissipated at high rates in uppermost sediments. The key to its accumulation lies in the presence of factors which inhibit the metabolism of bacterial methanogens near the surface, and leave them to flourish at greater burial depths. Favourable controlling factors are as follows: a plentiful nutrient supply (mainly carbohydrates), a high SO‐4 content in the bottom water; a post‐oxic sedimentary environment; frequent transgressions and regressions; low temperatures; high pH values; and high depositional rates and pressures. In the course of geological history, biogenic gas could be formed whenever transgressions and regressions have occurred frequently due either to tectonics or Ice Ages; when broad coastal plains were supplied with terrigenous non‐lignin carbohydrates by vast river networks; and when cool climates persisted. Accumulations can be grouped into three major genetical types, i. e. West Siberian (coastal marine facies); Black Sea (transitional marine‐lacustrine facies of restricted basins); and Qaidam (inland brackish lacustrine facies). Some case‐ histories from China and additional information from West Siberia are presented. It seems that many biogenic gasfielh remain undiscovered, and exploration prospects are good.

Evaluation of organic matter abundance of source rocks of biogenic gas: a case from the Quaternary Qigequan Formation Shale, Sanhu Depression, Qaidam Basin

“…First, proteins, polysaccharides, fats, nucleic acids and other polymers are hydrolyzed into monomers and polymers, including peptides, amino acids, sugars, organic acids, and other substances with low molecular weights. Further hydrolysation into alcohols, propionic acid, butyric acid and other products occurs, and these intermediate products are further degraded into acetic acid, formic acid, carbon dioxide and hydrogen (Rice and Claypool, 1981;Gang et al, 1985;Shur, 2002;Colosimo, 2016). Finally, methanogens use carbon dioxide and hydrogen to produce biogenic methane.…”