Formation of oil- and gas-bearing strata under low-temperature conditions

Considered are paleoclimate and hydrodynamic conditions in sedimentation basins under which accumulation of clayey deposits containing organic matter can be followed by gas hydrate formation. Geological environments that provide for screening of paleogas hydrates and safety of their thermal decay products are assessed. Established are paleobathymetric, geochemical, hydrochemical, and isotopic criteria of hydrate formation in the geological past. A basically new mechanism is discussed for hydrocarbon migration due to thermal destruction of gas hydrates. The performed investigations have revealed that in particular cases clayey strata that traditionally are considered as oil- and gas-bearing mother rocks were at early lithogenesis powerful generators of gaseous hydrocarbons. In the last diagenesis their gas-bearing potential was attributed to the scales of microbial metabolism and gas hydrate formation. At the next catagenesis stages the generating potential assessed by the known methods (Rock-Eval pyrolysis) does not carry any information on hydrate gases because the residual organic matter has not contributed to gas hydrate formation. Generation conditions and regularities in large gas occurrences formed due to thermal decay of paleogas hydrates are discussed using the example of west Siberia and other regions of the Soviet Union; methods are developed to predict zonal and local presence of gas. more » In the near-polar continental regions horizons not containing syngenetic organic matter can be specific generating strata. Of generating potential are thermocatalytic gases that enter from underlying sediments and are transformed in watered and cooled reservoirs into a gas hydrate state. Such gas-bearing source rocks and gas pools can occur beyond oil- and gas-bearing basins. « less