As a major research subject in petroleum and natural gas geology, the study of fluid inclusions in oil-gas reservoirs is an important approach to quantitative investigation of the formation of oil and natural gas reservoirs. Geological and geochemical applications and significance for fluid inclusions in oil-gas reservoirs are briefly discussed in the present paper. Fluid inclusions in source rocks and reservoirs are used to discuss the evolution of source rocks, sedimentary environments, matrix type and maturity of organic matter, migration and filling stages of oil and gas. Moreover, four major problems in the study of organic inclusions are pointed out. The following four aspects are considered the main tasks to be solved in the future. (1) Analytic techniques for individual organic inclusions will take a vital role in the research of quantitative oil-gas geology. (2) Geochemical tracer factors for the formation of oil-gas reservoirs can be found based on a comparison research between natural profiles of fluid inclusions and simulation experiments for the fractionation of hydrocarbon fluids in oil-gas transfer. (3) Quantitative researches of the formation of oil and natural gas reservoirs, chemical compositions, isotopic compositions and biomarkers from organic inclusions of different stages in comparison with those of accumulated crude oil and natural gas in specified reservoirs will provide important clues to understanding the genetic type and source of crude oil and natural gas and reconstructing the filling history of reservoirs. (4) The study of hydrocarbon formation of organic inclusions could provide a basis for the theories on hydrocarbon generation in carbonate and origin of deep oil and gas. Results of all these researches are of great significance in deepening the theory of gas-oil reservoirs and boosting the level of oil-gas exploration.