The Reaserch Of Geological Conditions And Driving Mechanism For Gas Accumulation In Midwest Foreland Basins In China

There develops several foreland basins in Midwest China, and all these forelandbasins have the same characteristic-rich gas and poor oil. Dominated by the specialgeological condition, not only conventional gas accumulated in Midwest forelandbasins, but also unconventional gas, such as source-contacting gas, coalbed gas andshale gas, are becoming very important in the gas exploration in future.Because of the multi-structural cycles, there are obvious differences betweentypical foreland basin and Midwest China foreland basin in tectonic characteristic,and the latter belongs to intracontinental reconstructed foreland basin. The sedimentsequences of this basin are almost composed of continental molasses-type sediment,and are short of sea facies sediment under foreland basin. Among the sedimentation,there develops another suit of coal measure strata besides conventional mudstone,shale stone and sand stone.With widespread,large-thickness,III-type organicmatter,this suit of coal strata becomes the dominating factor controlling the gasaccumulation in Midwest China.In Midwest foreland basin,the reservoir physical properties vary with theplace.Most clastic reservior condition is very poor with the characristics of lowporosity, low permeability,even tight rock. In plane, the properties of west basins aresuperior to the middle while in section, shallow layers are superior to deep layers.Influenced by tectonic activity, foreland thrust-faulted belt,adjioning orogenic zonemainly developes structural hydrocarbon reserviors. However,within the basin,lithologic and stratigraphic hydrocarbon reserviors are dominated. In frontal upriftzone, the types of reserviors mainly are block-fault and nose-fault traps.The dynamic condition,on the process of gas accumulation, includesexpansibility, buoyancy, abnormal fluid pressure, tectonic stress, hydrodynamic forceand heating power etc. For conventional gas accumulation in better reserviors,expansibility is the main driving force in primary migration but in secondarymigration the buoyancy is the main driving force. For source-contacting gas in tightreserviors, expansibility is always the main driving force from start to finish both inprimary and secondary migration.Tectonic stress is another important acting force,dominating the migration of gas. The affection michanism of the force for gasmigration can be discribed that stress causees straining in porosity, then affects fluidpressure, finally accelerates gas migration.On the research of tectonic and geological conditions of Midwest foreland basinsin China, it can be concluded that there are three factors dominating the developmentof large and medium size gas fields:high quality coal measure source rock, highintensity hydrocarbon generated center, reservior and sheal assembles suited with coalmeasure formation. Considering the resource quantity, coal-formed gas will be themain prospecting orientation in Midwest foreland basins in China. But consideringexploration replacing, source-contacting gas will be an important prospecting field.