| Forming conditions of the gas reservoirs in medium-deep strata in Bonan sag were researched by using geological, well-logging, seismic data and other research results. The study on gas reservoirs forming conditions could help to understand forming process and mechanism of the gas reservoirs, and to provide theoretical basis for future gas exploration.The genetic types and gas-source rock correlation indicated that there were two dominating kinds of gases including oil-type association gas and high matural gas. The gases mostly derived from source rock of Es4 formation, and partially from Es3 formation. Conducting sand bodies and faults were main migration pathways in Bonan sag. The distribution and characteristics of conducting sand bodies were analyzed by the thickness, the microcosmic permeability and sedimentary facies. By using SGR, shale contents in fault filling materials, active intensity and history, this paper discussed the influences of faults on gas migration and accumulation. As an effective factor in thermal evolution of organic matter, different geotemperatures resulted in different hydrocarbon phases and gas reservoir distribution. Pressure and pressure coefficient of 24 wells were calculated, and the results indicated that the distribution of pressure was different both in vertical and in horizontal. This thesis analyzed the impact of evaporates on temperature field, pressure field, and gas migration and accumulation. Study on hydrocarbon generation-expulsion history and fluid inclusion homogenization temperature showed that hydrocarbon injecting process could be divided into two major stages in which the second one was much more important. Based on the analysis above, this paper summarized five main controlling factors of the gas reservoirs in medium-deep strata in Bonan sag.
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