Genetic Types of Shallow-Buried Natural Gases and Their Distributions in Sedimentary Basins

Great volumes of shallow-buried (< 2000 m) natural gases have been found in Mesozoic-Cenozoic sedimentary basins in China, which mainly consist of biogases and low-maturity gases. As the two kinds of gases are frequently accumulated in one structure or one horizon, therefore, identification of genetic types of the gases is important for their exploration and development in the basins. Investigation on the gas geochemistry and distribution in different basins, the authors classify the gases into two genetic types, one is biogenetic gas, and the other one is low-maturity gas. The biogenetic gases are subdivided further into two subtypes for their sources, the source rock-derived biogenetic gas and oil derived biogenetic gas. Consideration on burial history of the source rocks, the source rock-derived biogases are divided into primary-buried and secondary-buried biogases, the former is generated from the source rocks in primary burial history, and the latter is from the source rocks in up-rise or secondary burial history. The primary-buried biogases generated from source rocks are mostly distributed in Quaternary basins or modern delta deposits, their accumulations are controlled by reservoir and seal-rock assemblages. The secondary-buried biogas occurs in a basin with down-up-down burial history. The biogas biodegraded from oil is distributed closely with heavy oil pools, and the more the oil biodegraded, the more biogas is accumulated. The low-maturity gases are widely distributed in shallow-buried reservoirs in Meso-Cenozoic basins.